On 4/19/2011 11:06 AM, Steve Turner wrote:
On 4/19/2011 9:51 AM, Smarty wrote:
On 4/19/2011 9:05 AM, Steve Turner wrote:
On 4/18/2011 9:41 PM, Steve Turner wrote:
Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and
whilst
waiting for the replacement transformer to arrive I decided to run
some
innocuous experiments on my other *working* A/C unit. Several days
ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on
the 115V
input side of the transformer, and the unit has been running fine
ever since.
So I know that a 1/4-amp fuse will carry the load on a properly
working system;
what I didn't know for *sure* was if that fuse was rated too high
to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work
installing
connectors and testing the two circuits for continuity. I installed
a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a
1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my
meter up in
series on the secondary side to measure the current draw (I taught
myself how
to do this earlier using my little Dremel tool; it draws about 1/2
amp when
spinning freely). All set, I closed the cover interlock switch to
apply 115V to
the system... The red LED on the circuit board lights up
momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the
current on the
meter.

Some people mentioned possible shorts in the wiring leading to the
thermostat,
so my next move was to to eliminate that as a possibility. Once
again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...7_04012009.pdf




I removed all wires from the thermostat ("YWRGC") connector block
on the
circuit board. The wiring diagram doesn't show it (at least it's
not obvious to
me), but the wires that lead to the external A/C unit also connect
to this
block; I removed them all (after I marked them :-) ). I also
disconnected the
meter from the secondary circuit just to remove that from the
equation. Once
again, I closed the cover interlock switch, and once again the
1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic
board, and
in anticipation of this I already had one on order; it's scheduled
to arrive
tomorrow. I've toyed with the idea of swapping logic boards between
the two
blower units to see if the problem follows the board (I used this
approach to
isolate a similar problem on a Trane unit at my previous residence
about 10
years ago; it helped, one of the boards was bad), but at this point
I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll
be back in
business!

Dammit, I missed a step! One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was
applied to
the input side. I just came back from testing for that, and now I'm
confused
again. I removed both wires from the output side and connected my
meter to
check for voltage, closed the interlock switch, and "piff", the
1/4-amp fuse
(my last one) blew! Apart from the fact that the input voltage comes
to the
transformer via output connectors on the logic board (which are just
tied
directly to input connectors on the board as far as I can see), the
fuse is
blowing and the logic board isn't even a part of the equation...
What do you
make of that?
Steve,

I am (I think) the person who originally suggested getting the inline
fuse
holder and fuses from Radio Shack, and am glad to see you are
pursuing this.
Since you have a disconnected secondary, and a fused primary, and the
transformer blows the 1/4 amp fuse, it certainly sounds like the
primary supply
voltage is wrong. Have you actually measured the primary voltage and
found it
to be about 110 VAC? If so, then the transformer, with no secondary
load, must
be defective if it immediately blows a 1/4 amp fuse with no secondary
load,
unless for some reason your logic board is providing some odd
waveform other
than purely alternating 60 Hz power. Perhaps, for example, some big
DC voltage
is also present?

It is very hard to believe that the several transformers you have
tried are ALL
defective. I am way more suspicious of the supply voltage.

Yes, the supply voltage is correct (actually, it's 120V, which is what
pretty much every other "115V" circuit in the house is receiving). To
be safe this time, I ordered more than one replacement transformer
(they were cheap) and I just hooked another one to a completely
different circuit (on my workbench) with a 1/2-amp fast-acting fuse on
the input side and no load on the secondary, and the fuse held and I
got a constant 26V on the output side. I will be going to procure
some more 1/4-amp fuses (both fast and slow blow) and I will remove
the exact transformer I intend to use from the blower unit (it has all
the right connectors attached to the wires) and attempt the same
experiment on my workbench using both 1/2-amp and 1/4-amp fuses just
to be sure I can completely rule out the transformer. If that works,
I will take that same transformer back to the blower unit and run the
same test there, bypassing the circuit board entirely. More to come...
Sounds good!

On Apr 19, 10:51*am, Smarty wrote:
On 4/19/2011 9:05 AM, Steve Turner wrote:



On 4/18/2011 9:41 PM, Steve Turner wrote:
Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and
whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on
the 115V
input side of the transformer, and the unit has been running fine
ever since.
So I know that a 1/4-amp fuse will carry the load on a properly
working system;
what I didn't know for *sure* was if that fuse was rated too high to
protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work
installing
connectors and testing the two circuits for continuity. I installed a
1/4-amp
fuse in the 115V primary side and (once again, as recommended) a
1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter
up in
series on the secondary side to measure the current draw (I taught
myself how
to do this earlier using my little Dremel tool; it draws about 1/2
amp when
spinning freely). All set, I closed the cover interlock switch to
apply 115V to
the system... The red LED on the circuit board lights up momentarily,
then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the
current on the
meter.

Some people mentioned possible shorts in the wiring leading to the
thermostat,
so my next move was to to eliminate that as a possibility. Once
again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...product/22-166....

I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not
obvious to
me), but the wires that lead to the external A/C unit also connect to
this
block; I removed them all (after I marked them :-) ). I also
disconnected the
meter from the secondary circuit just to remove that from the
equation. Once
again, I closed the cover interlock switch, and once again the
1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic
board, and
in anticipation of this I already had one on order; it's scheduled to
arrive
tomorrow. I've toyed with the idea of swapping logic boards between
the two
blower units to see if the problem follows the board (I used this
approach to
isolate a similar problem on a Trane unit at my previous residence
about 10
years ago; it helped, one of the boards was bad), but at this point
I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be
back in
business!

Dammit, I missed a step! *One thing I forgot to do was verify that I
was getting 24V on the output side of the transformer when 115V was
applied to the input side. *I just came back from testing for that,
and now I'm confused again. *I removed both wires from the output side
and connected my meter to check for voltage, closed the interlock
switch, and "piff", the 1/4-amp fuse (my last one) blew! *Apart from
the fact that the input voltage comes to the transformer via output
connectors on the logic board (which are just tied directly to input
connectors on the board as far as I can see), the fuse is blowing and
the logic board isn't even a part of the equation... *What do you make
of that?

Steve,

I am (I think) the person who originally suggested getting the inline
fuse holder and fuses from Radio Shack, and am glad to see you are
pursuing this. Since you have a disconnected secondary, and a fused
primary, and the transformer blows the 1/4 amp fuse, it certainly sounds
like the primary supply voltage is wrong. Have you actually measured the
primary voltage and found it to be about 110 VAC? If so, then the
transformer, with no secondary load, must be defective if it immediately
blows a 1/4 amp fuse with no secondary load, unless for some reason your
logic board is providing some odd waveform other than purely alternating
60 Hz power. Perhaps, for example, some big DC voltage is also present?

He's talking about it blowing without the transformer connected to
anything
except the AC. And I doubt the logic board provides the power,
typically it
gets it's power from power from the transformer which is wired
directly
to the incoming AC line.


It is very hard to believe that the several transformers you have tried
are ALL defective. I am way more suspicious of the supply voltage.- Hide quoted text -

- Show quoted text -

I believe he said he measured it in the past and it was 120V. Is the
new transformer fastened to the metal cabinet? I'd seperate it from
the cabinet and see if there is any voltage between the two and
see if it still blows the fuse. My first thought though is the fuse
may be close to the limit of the transformer and being fast-blow
it might be blowing just on some initial start-up current.

In a perfect world, two 24 VAC wires hanging out of a
transformer, and check with VOM, you actually get about 26
VAC. In the real world, what happens is that you leave the
VOM leads in the ampere scale, and fry either the
transformer, fuse, or the fuse inside the VOM. Check to see
that the VOM leads are in the correct sockets.

This is one reason when I love the cheapie Harbor Freight
meters, I don't cry as much when I blow a fuse. Fuses for my
Fluke are $11 each.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


"Steve Turner"
wrote in message ...

Dammit, I missed a step! One thing I forgot to do was
verify that I was
getting 24V on the output side of the transformer when 115V
was applied to the
input side. I just came back from testing for that, and now
I'm confused
again. I removed both wires from the output side and
connected my meter to
check for voltage, closed the interlock switch, and "piff",
the 1/4-amp fuse
(my last one) blew! Apart from the fact that the input
voltage comes to the
transformer via output connectors on the logic board (which
are just tied
directly to input connectors on the board as far as I can
see), the fuse is
blowing and the logic board isn't even a part of the
equation... What do you
make of that?

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


wrote in message
...


The first thing I would do is replace the fast
blowing fuses with slow blowing ones. I
thought all along using fast ones was a bad
idea. If its an overload that's causing the
problem, the slow-blow will still fail long
before the transformer. And if it's some
mysterious voltage spike that causing it,
which I doubt, even the fast acting fuse may
not prevent damage. It's possible the fast
acting ones are getting creamed by the initial
start-up current.

I'd also consider useing a fuse temporarily
thats 1.5 or 2X the current expected. The
transformer should be able to handle that
for more than the minute it takes to get a
current measurement. And if you measure
voltage on the secondary, if it's an overload,
I'd expect you'd see less than 24V, as the
excessive load pulls the voltage down.

If the leads on your VOM are still on amps, that provides a
dead short.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


"Steve Turner"
wrote in message ...

Sure, I can try some different fuses (but I have to go make
a run to buy some;
that might not happen until later today). But now with the
connectors removed
from the 24V output side of the transformer, haven't I
removed all the load?
Wouldn't that mean there is something wrong with the input
side of the circuit?

Stormin Mormon wrote:
In a perfect world, two 24 VAC wires hanging out of a
transformer, and check with VOM, you actually get about 26
VAC. In the real world, what happens is that you leave the
VOM leads in the ampere scale, and fry either the
transformer, fuse, or the fuse inside the VOM. Check to see
that the VOM leads are in the correct sockets.

This is one reason when I love the cheapie Harbor Freight
meters, I don't cry as much when I blow a fuse. Fuses for my
Fluke are $11 each.

Hmm,
That is why I use old Simpson 260 in a case like this.
It has pop up mechanical fuse. If it pops you pu**** to reset.
OP is still treating the symptom not even an ich close to digging
out root cause of the problem.Down sream on 24V side there could be
a half cooked relay coil causing overload. Small relays are mounted on
the logic board. Look at the schematic is all that it needs.

(brevity snip)

I'm back. The saga continues.

Ok, I got some more 1/4-amp fast-acting fuses (sorry trader4; neither Radio
Shack nor Home Depot had any slow-blow fuses rated less than 1.5 amps). I took
the transformer out of the blower unit, brought it to my workbench and tested
it there (on a completely different 115V circuit). The 1/4-amp fuse held fine,
and I got a solid 26 volts on the output side. So far so good.

Reinstalled it in the blower unit (screwed it to the frame) and unhooked the
input lines from the logic board (to eliminate it completely from the equation)
and sent them straight into the transformer (with the fuse on the black hot
wire) and with my meter measuring for 24V on the output side. Closed the cover
interlock switch, and "poof"; the fuse blew. That pretty much tells me
something is wrong with the incoming power.

I unhooked the transformer and measured the voltage across the hot and neutral
wires with the interlock switch closed: 120V. I opened the switch, and the
meter still wants to read *something*, like about a volt or so. What the hell?
I metered on the *other* side of the interlock switch: nothing. So I hooked
the transformer back up (it's still bolted to the frame) with the switch
bypassed, and PRESTO; the fuse holds and I have 26V on the output side of the
transformer. Bad interlock switch? I wondered if it was shorting to the
frame, but it's completely encased in plastic. It seems to meter out fine, and
I was going to try to open it up to see if I could find any problem, but it's
completely sealed so I gave up.

Anyway, the switch looks like the culprit, so I turn off the breaker at the
panel, hook everything back up, reconnecting all the wires the way they are
supposed to be, with the exception that the interlock switch is bypassed (not
really comfortable with that) and the 1/4-amp fuse still installed on the hot
line going into the transformer. Turn the breaker back on, and "poof", the
fuse blows again. What the hell? Unhook the transformer output wires from the
logic board to remove any load (and back up to the meter to check for 24V
output), install a new fuse and repeat; the fuse blows AGAIN, and now I'm just
confused. At this point, the "something might be shorted to the frame" theory
is starting to look like a distinct possibility. Thoughts?

On 4/19/2011 1:49 PM, Steve Turner wrote:
(brevity snip)

I'm back. The saga continues.

Ok, I got some more 1/4-amp fast-acting fuses (sorry trader4; neither Radio
Shack nor Home Depot had any slow-blow fuses rated less than 1.5 amps). I took
the transformer out of the blower unit, brought it to my workbench and tested
it there (on a completely different 115V circuit). The 1/4-amp fuse held fine,
and I got a solid 26 volts on the output side. So far so good.

Reinstalled it in the blower unit (screwed it to the frame) and unhooked the
input lines from the logic board (to eliminate it completely from the equation)
and sent them straight into the transformer (with the fuse on the black hot
wire) and with my meter measuring for 24V on the output side. Closed the cover
interlock switch, and "poof"; the fuse blew. That pretty much tells me
something is wrong with the incoming power.

I unhooked the transformer and measured the voltage across the hot and neutral
wires with the interlock switch closed: 120V. I opened the switch, and the
meter still wants to read *something*, like about a volt or so. What the hell?
I metered on the *other* side of the interlock switch: nothing. So I hooked the
transformer back up (it's still bolted to the frame) with the switch bypassed,
and PRESTO; the fuse holds and I have 26V on the output side of the
transformer. Bad interlock switch? I wondered if it was shorting to the frame,
but it's completely encased in plastic. It seems to meter out fine, and I was
going to try to open it up to see if I could find any problem, but it's
completely sealed so I gave up.

Anyway, the switch looks like the culprit, so I turn off the breaker at the
panel, hook everything back up, reconnecting all the wires the way they are
supposed to be, with the exception that the interlock switch is bypassed (not
really comfortable with that) and the 1/4-amp fuse still installed on the hot
line going into the transformer. Turn the breaker back on, and "poof", the fuse
blows again. What the hell? Unhook the transformer output wires from the logic
board to remove any load (and back up to the meter to check for 24V output),
install a new fuse and repeat; the fuse blows AGAIN, and now I'm just confused.
At this point, the "something might be shorted to the frame" theory is starting
to look like a distinct possibility. Thoughts?

One more interesting thing I just discovered. trader4 asked if the transformer
was connected to the frame. I just went back and took it out of the frame, set
it on the plywood floor, then tested it again. Everything was hooked up
normally on the input side (the input wires leading into the logic board
(interlock switch still bypassed), then in turn leading to the transformer),
but I left the output wires unhooked. Applied power (heh, it turns out there
is a local switch in the vicinity to do that, rather than me traipsing to the
breaker box - duh), and the fuse *held* (not the case when the transformer was
installed in the frame). Switch off, connected the meter to the output leads,
switch on; fuse held, 26 volts of output. Switch off, connected the output
leads to the logic board, switch on; momentary red LED, then "poof", the fuse
blows.... Houston, we have a problem.

Ok, now I'm just depressed. I could not reproduce the scenario of the fuse
blowing when the transformer was mounted to the frame. I traced every last bit
of wiring leading to the transformer input; I loosened it from its mounting
clamps where it feeds into the junction box, and I separated all wires so none
were touching each other or the frame, and I tightened the wire nuts. There
are no breaches anywhere, and there just isn't any place where the input wiring
can possibly be shorting to the frame of the blower unit.

I reached the point where the only time the fuse blows is when I plug the 24V
output wires into the logic board, and since the new logic board arrived
earlier today I decided to go ahead and do the swap. Careful labeling of all
wires, and careful reading of the documentation, and I know the board is
installed correctly. This board is a newer model, and it has an interesting
featu

"The 50A55 has only one serviceable part - an automotive type fuse, which
protects the low voltage transformer from damage if its output is
short-circuited. If the fuse has opened up, remove whatever caused the short
circuit and replace the fuse with only a 3 Amp automotive type fuse. If the
fuse is not the cause of the control's problem, replace the entire 50A55
control. There are no other user serviceable parts."

This is the installation manual for the new board:
http://www.emersonclimate.com/Docume.../0037-6265.pdf

So guess what happened when I applied power after installing the board? Yep,
the on-board 3-amp fuse popped. The 1/4-amp fuse I left on the input side did not.

So the more I piddle-fart around, the more apparent it becomes that I have an
electrical problem like none I've personally encountered before. I love fixing
stuff myself (it's how I was raised) and I love the challenge of tracking down
problems, but now I'm growing weary of the chase...

If you're one of those guys waiting in the wings to say "I TOLD you you should
have called in a professional!"... Yeah yeah; save it. :-) I've got some
friends of friends (and family too) that are electricians by trade that I can
lure in with the promise of beer, and if I need to call one in I will be right
by their side shadowing their every move, and I will be *learning* from the
experience, not just throwing some money at a local service company.

On Mon, 18 Apr 2011 21:41:43 -0500, Steve Turner
wrote:

Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V
input side of the transformer, and the unit has been running fine ever since.
So I know that a 1/4-amp fuse will carry the load on a properly working system;
what I didn't know for *sure* was if that fuse was rated too high to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing
connectors and testing the two circuits for continuity. I installed a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter up in
series on the secondary side to measure the current draw (I taught myself how
to do this earlier using my little Dremel tool; it draws about 1/2 amp when
spinning freely). All set, I closed the cover interlock switch to apply 115V
to the system... The red LED on the circuit board lights up momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on
the meter.

Some people mentioned possible shorts in the wiring leading to the thermostat,
so my next move was to to eliminate that as a possibility. Once again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...7_04012009.pdf



Just a note - was going to mention it earlier but didn't - a short in
the thermostat wiring is unlikely to be the cause - all THAT would do
is turn on the heating or cooling. A short to GROUND would likewize
not cause a problem, assuming the secondary side of the transformer is
not grounded.
I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not obvious
to me), but the wires that lead to the external A/C unit also connect to this
block; I removed them all (after I marked them :-) ). I also disconnected the
meter from the secondary circuit just to remove that from the equation. Once
again, I closed the cover interlock switch, and once again the 1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and
in anticipation of this I already had one on order; it's scheduled to arrive
tomorrow. I've toyed with the idea of swapping logic boards between the two
blower units to see if the problem follows the board (I used this approach to
isolate a similar problem on a Trane unit at my previous residence about 10
years ago; it helped, one of the boards was bad), but at this point I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in
business!
I'm betting on a bad board at this point.

On Tue, 19 Apr 2011 08:05:12 -0500, Steve Turner
wrote:

On 4/18/2011 9:41 PM, Steve Turner wrote:
Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V
input side of the transformer, and the unit has been running fine ever since.
So I know that a 1/4-amp fuse will carry the load on a properly working system;
what I didn't know for *sure* was if that fuse was rated too high to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing
connectors and testing the two circuits for continuity. I installed a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter up in
series on the secondary side to measure the current draw (I taught myself how
to do this earlier using my little Dremel tool; it draws about 1/2 amp when
spinning freely). All set, I closed the cover interlock switch to apply 115V to
the system... The red LED on the circuit board lights up momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the
meter.

Some people mentioned possible shorts in the wiring leading to the thermostat,
so my next move was to to eliminate that as a possibility. Once again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...7_04012009.pdf


I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not obvious to
me), but the wires that lead to the external A/C unit also connect to this
block; I removed them all (after I marked them :-) ). I also disconnected the
meter from the secondary circuit just to remove that from the equation. Once
again, I closed the cover interlock switch, and once again the 1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and
in anticipation of this I already had one on order; it's scheduled to arrive
tomorrow. I've toyed with the idea of swapping logic boards between the two
blower units to see if the problem follows the board (I used this approach to
isolate a similar problem on a Trane unit at my previous residence about 10
years ago; it helped, one of the boards was bad), but at this point I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in
business!

Dammit, I missed a step! One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was applied to the
input side. I just came back from testing for that, and now I'm confused
again. I removed both wires from the output side and connected my meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse
(my last one) blew! Apart from the fact that the input voltage comes to the
transformer via output connectors on the logic board (which are just tied
directly to input connectors on the board as far as I can see), the fuse is
blowing and the logic board isn't even a part of the equation... What do you
make of that?
Step #1. Measure the voltage on the primary, BEFORE you connect the
transformer
Step #2 VERIFY it is between 100 and 127 volts.
Step #3 If it is not, FIND OUT WHY.

Step #4 If it IS VERIFY the proper connection of the transformer.

Step #5 - If it is properly connected CHECK THE TRANSFORMER. 3 bad
ones in a row is stretching creduality, but not TOTALLY impossible.

On Apr 19, 2:49*pm, Steve Turner
wrote:
(brevity snip)

I'm back. *The saga continues.

Ok, I got some more 1/4-amp fast-acting fuses (sorry trader4; neither Radio
Shack nor Home Depot had any slow-blow fuses rated less than 1.5 amps). *I took
the transformer out of the blower unit, brought it to my workbench and tested
it there (on a completely different 115V circuit). *The 1/4-amp fuse held fine,
and I got a solid 26 volts on the output side. *So far so good.

What 120V current does the transformer draw when it's on the bench
and not driving anything?


Reinstalled it in the blower unit (screwed it to the frame) and unhooked the
input lines from the logic board (to eliminate it completely from the equation)
and sent them straight into the transformer (with the fuse on the black hot
wire) and with my meter measuring for 24V on the output side. *Closed the cover
interlock switch, and "poof"; the fuse blew. *That pretty much tells me
something is wrong with the incoming power.

Let me make sure I understand this. Normally the 120V goes from
the furnace junction box, through the door switch, then to the
logic board. From there the logic board in turn sends 120V to
the transformer and also gets 24V back from it via the low voltage
connections. Correct?

And what you did above as a test was to instead connect the
transformer
directly so that the 120V goes from junction box, to door switch,
to transformer, with the secondary not connected to anything.

Correct?





I unhooked the transformer and measured the voltage across the hot and neutral
wires with the interlock switch closed: 120V. *I opened the switch, and the
meter still wants to read *something*, like about a volt or so. *What the hell?

That''s probably just due to the high impedance of the meter.


* I metered on the *other* side of the interlock switch: nothing.

Confused here. The other side of the interlock is connected right
to the incoming AC junction box, right? So it should have 120V.


*So I hooked
the transformer back up (it's still bolted to the frame) with the switch
bypassed, and PRESTO; the fuse holds and I have 26V on the output side of the
transformer. *Bad interlock switch? *I wondered if it was shorting to the
frame, but it's completely encased in plastic. *It seems to meter out fine, and
I was going to try to open it up to see if I could find any problem, but it's
completely sealed so I gave up.

Anyway, the switch looks like the culprit, so I turn off the breaker at the
panel, hook everything back up, reconnecting all the wires the way they are
supposed to be, with the exception that the interlock switch is bypassed (not
really comfortable with that) and the 1/4-amp fuse still installed on the hot
line going into the transformer. *Turn the breaker back on, and "poof", the
fuse blows again. *What the hell? *Unhook the transformer output wires from the
logic board to remove any load (and back up to the meter to check for 24V
output), install a new fuse and repeat; the fuse blows AGAIN, and now I'm just
confused. *At this point, the "something might be shorted to the frame" theory
is starting to look like a distinct possibility. *Thoughts?

It is very strange. Even though it may not be the real problem, one
thing
I would do is use bigger fuses. I don't know what the transformer is
rated
at, but using 2X fuses at least for a few minutes to do some testing
should
not burn out the transformer. Then you could at least get it to the
point
you could do some voltage and current measurements, expecially on
the secondary.

On Tue, 19 Apr 2011 06:47:55 -0700 (PDT), "
wrote:

On Apr 19, 9:05Â*am, Steve Turner
wrote:
On 4/18/2011 9:41 PM, Steve Turner wrote:





Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V
input side of the transformer, and the unit has been running fine ever since.
So I know that a 1/4-amp fuse will carry the load on a properly working system;
what I didn't know for *sure* was if that fuse was rated too high to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing
connectors and testing the two circuits for continuity. I installed a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter up in
series on the secondary side to measure the current draw (I taught myself how
to do this earlier using my little Dremel tool; it draws about 1/2 amp when
spinning freely). All set, I closed the cover interlock switch to apply 115V to
the system... The red LED on the circuit board lights up momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the
meter.

Some people mentioned possible shorts in the wiring leading to the thermostat,
so my next move was to to eliminate that as a possibility. Once again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...product/22-166...

I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not obvious to
me), but the wires that lead to the external A/C unit also connect to this
block; I removed them all (after I marked them :-) ). I also disconnected the
meter from the secondary circuit just to remove that from the equation. Once
again, I closed the cover interlock switch, and once again the 1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and
in anticipation of this I already had one on order; it's scheduled to arrive
tomorrow. I've toyed with the idea of swapping logic boards between the two
blower units to see if the problem follows the board (I used this approach to
isolate a similar problem on a Trane unit at my previous residence about 10
years ago; it helped, one of the boards was bad), but at this point I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in
business!

Dammit, I missed a step! Â*One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was applied to the
input side. Â*I just came back from testing for that, and now I'm confused
again. Â*I removed both wires from the output side and connected my meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse
(my last one) blew! Â*Apart from the fact that the input voltage comes to the
transformer via output connectors on the logic board (which are just tied
directly to input connectors on the board as far as I can see), the fuse is
blowing and the logic board isn't even a part of the equation... Â*What do you
make of that?- Hide quoted text -

- Show quoted text -

The first thing I would do is replace the fast blowing fuses with slow
blowing ones. I thought all along using fast ones was a bad idea.
If its an overload that's causing the problem, the slow-blow will
still
fail long before the transformer. And if it's some mysterious
voltage
spike that causing it, which I doubt, even the fast acting fuse may
not
prevent damage. It's possible the fast acting ones are getting
creamed by the initial start-up current.

I'd also consider useing a fuse temporarily thats 1.5 or 2X the
current expected. The transformer should be able to handle that
for more than the minute it takes to get a current measurement.
And if you measure voltage on the secondary, if it's an overload,
I'd expect you'd see less than 24V, as the excessive load
pulls the voltage down.
The ontly thing you are forgetting is the half amp fast blow DID
work on the furnace that works - which is basically identical - so
there is NO REASON the same fuse should not work on this one.
Half amp is already about 50% oversized for the transformer at full
rated output.

On Apr 19, 6:37*pm, Steve Turner
wrote:
Ok, now I'm just depressed. *I could not reproduce the scenario of the fuse
blowing when the transformer was mounted to the frame. *I traced every last bit
of wiring leading to the transformer input; I loosened it from its mounting
clamps where it feeds into the junction box, and I separated all wires so none
were touching each other or the frame, and I tightened the wire nuts. *There
are no breaches anywhere, and there just isn't any place where the input wiring
can possibly be shorting to the frame of the blower unit.

I reached the point where the only time the fuse blows is when I plug the 24V
output wires into the logic board, and since the new logic board arrived
earlier today I decided to go ahead and do the swap. *Careful labeling of all
wires, and careful reading of the documentation, and I know the board is
installed correctly. *This board is a newer model, and it has an interesting
featu

"The 50A55 has only one serviceable part - an automotive type fuse, which
protects the low voltage transformer from damage if its output is
short-circuited. *If the fuse has opened up, remove whatever caused the short
circuit and replace the fuse with only a 3 Amp automotive type fuse. *If the
fuse is not the cause of the control's problem, replace the entire 50A55
control. *There are no other user serviceable parts."


Again, what is the rating of the transformer? The above would
suggest that
it's at least 72VA and that in turn means that the 1/4 amp fuse on the
primary is way to small. Combine that with that it's fast acting and
that
could be why you're seeing primary fuses blow, adding to the
confusion.
If they have a 3A on the 24V side, I'd put a 3/4 amp one on the
primary,
or maybe even a 1 amp temprarily so you can get the thing debugged.
Why the 1/4 primary would work on the bench, but not with the same
transformer with no load in the furnace, remains a mystery.

At this point, I'd get the bigger fuse in the primary and hopefully
it
will stop blowing. Then I'd
be looking for what else the logic board drives with
24V and start disconnecting those wires one by one until it stops
blowing fuses. My best guess would be that something else it
supplies with 24v, ie contactor relay, etc is bad, or that a 24v wire
is shorting out somewhere. Did you try disconnecting the thermostat
wires, for example?





This is the installation manual for the new board:http://www.emersonclimate.com/Docume...nstruction_she...

So guess what happened when I applied power after installing the board? *Yep,
the on-board 3-amp fuse popped. *The 1/4-amp fuse I left on the input side did not.

So the more I piddle-fart around, the more apparent it becomes that I have an
electrical problem like none I've personally encountered before. *I love fixing
stuff myself (it's how I was raised) and I love the challenge of tracking down
problems, but now I'm growing weary of the chase...

If you're one of those guys waiting in the wings to say "I TOLD you you should
have called in a professional!"... *Yeah yeah; save it. *:-) *I've got some
friends of friends (and family too) that are electricians by trade that I can
lure in with the promise of beer, and if I need to call one in I will be right
by their side shadowing their every move, and I will be *learning* from the
experience, not just throwing some money at a local service company.

I think if you called a professional you could very well be at the
same
point, possibly with an even bigger bill for other parts and certainly
lots of labor.

On Apr 19, 8:06*pm, wrote:
On Tue, 19 Apr 2011 06:47:55 -0700 (PDT), "





wrote:
On Apr 19, 9:05*am, Steve Turner
wrote:
On 4/18/2011 9:41 PM, Steve Turner wrote:

Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V
input side of the transformer, and the unit has been running fine ever since.
So I know that a 1/4-amp fuse will carry the load on a properly working system;
what I didn't know for *sure* was if that fuse was rated too high to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing
connectors and testing the two circuits for continuity. I installed a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter up in
series on the secondary side to measure the current draw (I taught myself how
to do this earlier using my little Dremel tool; it draws about 1/2 amp when
spinning freely). All set, I closed the cover interlock switch to apply 115V to
the system... The red LED on the circuit board lights up momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the
meter.

Some people mentioned possible shorts in the wiring leading to the thermostat,
so my next move was to to eliminate that as a possibility. Once again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...product/22-166....

I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not obvious to
me), but the wires that lead to the external A/C unit also connect to this
block; I removed them all (after I marked them :-) ). I also disconnected the
meter from the secondary circuit just to remove that from the equation. Once
again, I closed the cover interlock switch, and once again the 1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and
in anticipation of this I already had one on order; it's scheduled to arrive
tomorrow. I've toyed with the idea of swapping logic boards between the two
blower units to see if the problem follows the board (I used this approach to
isolate a similar problem on a Trane unit at my previous residence about 10
years ago; it helped, one of the boards was bad), but at this point I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in
business!

Dammit, I missed a step! *One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was applied to the
input side. *I just came back from testing for that, and now I'm confused
again. *I removed both wires from the output side and connected my meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse
(my last one) blew! *Apart from the fact that the input voltage comes to the
transformer via output connectors on the logic board (which are just tied
directly to input connectors on the board as far as I can see), the fuse is
blowing and the logic board isn't even a part of the equation... *What do you
make of that?- Hide quoted text -

- Show quoted text -

The first thing I would do is replace the fast blowing fuses with slow
blowing ones. *I thought all along using fast ones was a bad idea.
If its an overload that's causing the problem, the slow-blow will
still
fail long before the transformer. * And if it's some mysterious
voltage
spike that causing it, which I doubt, even the fast acting fuse may
not
prevent damage. *It's possible the fast acting ones are getting
creamed by the initial start-up current.

I'd also consider useing a fuse temporarily thats 1.5 or 2X the
current expected. *The transformer should be able to handle that
for more than the minute it takes to get a current measurement.
And if you measure voltage on the secondary, if it's an overload,
I'd expect you'd see less than 24V, as the excessive load
pulls the voltage down.

* The ontly thing you are forgetting is the half amp fast blow DID
work on the furnace that works - which is basically identical - so
there is NO REASON the same fuse should not work on this one.
Half amp is already about 50% oversized for the transformer at full
rated output.- Hide quoted text -

- Show quoted text -

But he's using a 1/4 amp fuse, not 1/2 amp.
He just posted further down in this thread that the new logic board
has a 3A 24V fuse on it. In which case, it would seem his 1/4
amp primary fuse is too small. I don;t see the harm in at
least temporarily going to a larger fuse so he can get the
damn thing going enough to take some measurements.

On Tue, 19 Apr 2011 10:51:00 -0400, Smarty wrote:

On 4/19/2011 9:05 AM, Steve Turner wrote:
On 4/18/2011 9:41 PM, Steve Turner wrote:
Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and
whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on
the 115V
input side of the transformer, and the unit has been running fine
ever since.
So I know that a 1/4-amp fuse will carry the load on a properly
working system;
what I didn't know for *sure* was if that fuse was rated too high to
protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work
installing
connectors and testing the two circuits for continuity. I installed a
1/4-amp
fuse in the 115V primary side and (once again, as recommended) a
1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter
up in
series on the secondary side to measure the current draw (I taught
myself how
to do this earlier using my little Dremel tool; it draws about 1/2
amp when
spinning freely). All set, I closed the cover interlock switch to
apply 115V to
the system... The red LED on the circuit board lights up momentarily,
then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the
current on the
meter.

Some people mentioned possible shorts in the wiring leading to the
thermostat,
so my next move was to to eliminate that as a possibility. Once
again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...7_04012009.pdf



I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not
obvious to
me), but the wires that lead to the external A/C unit also connect to
this
block; I removed them all (after I marked them :-) ). I also
disconnected the
meter from the secondary circuit just to remove that from the
equation. Once
again, I closed the cover interlock switch, and once again the
1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic
board, and
in anticipation of this I already had one on order; it's scheduled to
arrive
tomorrow. I've toyed with the idea of swapping logic boards between
the two
blower units to see if the problem follows the board (I used this
approach to
isolate a similar problem on a Trane unit at my previous residence
about 10
years ago; it helped, one of the boards was bad), but at this point
I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be
back in
business!

Dammit, I missed a step! One thing I forgot to do was verify that I
was getting 24V on the output side of the transformer when 115V was
applied to the input side. I just came back from testing for that,
and now I'm confused again. I removed both wires from the output side
and connected my meter to check for voltage, closed the interlock
switch, and "piff", the 1/4-amp fuse (my last one) blew! Apart from
the fact that the input voltage comes to the transformer via output
connectors on the logic board (which are just tied directly to input
connectors on the board as far as I can see), the fuse is blowing and
the logic board isn't even a part of the equation... What do you make
of that?
Steve,

I am (I think) the person who originally suggested getting the inline
fuse holder and fuses from Radio Shack, and am glad to see you are
pursuing this. Since you have a disconnected secondary, and a fused
primary, and the transformer blows the 1/4 amp fuse, it certainly sounds
like the primary supply voltage is wrong. Have you actually measured the
primary voltage and found it to be about 110 VAC? If so, then the
transformer, with no secondary load, must be defective if it immediately
blows a 1/4 amp fuse with no secondary load, unless for some reason your
logic board is providing some odd waveform other than purely alternating
60 Hz power. Perhaps, for example, some big DC voltage is also present?

It is very hard to believe that the several transformers you have tried
are ALL defective. I am way more suspicious of the supply voltage.

Except the logic board is only on the secondary and does not provide
power to the primary.

He DOES need to check the primary voltage, and also check for DC.

On Tue, 19 Apr 2011 10:06:40 -0500, Steve Turner
wrote:

On 4/19/2011 9:51 AM, Smarty wrote:
On 4/19/2011 9:05 AM, Steve Turner wrote:
On 4/18/2011 9:41 PM, Steve Turner wrote:
Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V
input side of the transformer, and the unit has been running fine ever since.
So I know that a 1/4-amp fuse will carry the load on a properly working system;
what I didn't know for *sure* was if that fuse was rated too high to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing
connectors and testing the two circuits for continuity. I installed a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter up in
series on the secondary side to measure the current draw (I taught myself how
to do this earlier using my little Dremel tool; it draws about 1/2 amp when
spinning freely). All set, I closed the cover interlock switch to apply 115V to
the system... The red LED on the circuit board lights up momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the
meter.

Some people mentioned possible shorts in the wiring leading to the thermostat,
so my next move was to to eliminate that as a possibility. Once again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...7_04012009.pdf



I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not obvious to
me), but the wires that lead to the external A/C unit also connect to this
block; I removed them all (after I marked them :-) ). I also disconnected the
meter from the secondary circuit just to remove that from the equation. Once
again, I closed the cover interlock switch, and once again the 1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and
in anticipation of this I already had one on order; it's scheduled to arrive
tomorrow. I've toyed with the idea of swapping logic boards between the two
blower units to see if the problem follows the board (I used this approach to
isolate a similar problem on a Trane unit at my previous residence about 10
years ago; it helped, one of the boards was bad), but at this point I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in
business!

Dammit, I missed a step! One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was applied to
the input side. I just came back from testing for that, and now I'm confused
again. I removed both wires from the output side and connected my meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse
(my last one) blew! Apart from the fact that the input voltage comes to the
transformer via output connectors on the logic board (which are just tied
directly to input connectors on the board as far as I can see), the fuse is
blowing and the logic board isn't even a part of the equation... What do you
make of that?
Steve,

I am (I think) the person who originally suggested getting the inline fuse
holder and fuses from Radio Shack, and am glad to see you are pursuing this.
Since you have a disconnected secondary, and a fused primary, and the
transformer blows the 1/4 amp fuse, it certainly sounds like the primary supply
voltage is wrong. Have you actually measured the primary voltage and found it
to be about 110 VAC? If so, then the transformer, with no secondary load, must
be defective if it immediately blows a 1/4 amp fuse with no secondary load,
unless for some reason your logic board is providing some odd waveform other
than purely alternating 60 Hz power. Perhaps, for example, some big DC voltage
is also present?

It is very hard to believe that the several transformers you have tried are ALL
defective. I am way more suspicious of the supply voltage.

Yes, the supply voltage is correct (actually, it's 120V, which is what pretty
much every other "115V" circuit in the house is receiving). To be safe this
time, I ordered more than one replacement transformer (they were cheap) and I
just hooked another one to a completely different circuit (on my workbench)
with a 1/2-amp fast-acting fuse on the input side and no load on the secondary,
and the fuse held and I got a constant 26V on the output side. I will be going
to procure some more 1/4-amp fuses (both fast and slow blow) and I will remove
the exact transformer I intend to use from the blower unit (it has all the
right connectors attached to the wires) and attempt the same experiment on my
workbench using both 1/2-amp and 1/4-amp fuses just to be sure I can completely
rule out the transformer. If that works, I will take that same transformer
back to the blower unit and run the same test there, bypassing the circuit
board entirely. More to come...
Connect the transformer that tested OK on the bench to the furnace
unit and see what happens - - - - - .

On Tue, 19 Apr 2011 10:50:49 -0600, Tony Hwang
wrote:



Stormin Mormon wrote:
In a perfect world, two 24 VAC wires hanging out of a
transformer, and check with VOM, you actually get about 26
VAC. In the real world, what happens is that you leave the
VOM leads in the ampere scale, and fry either the
transformer, fuse, or the fuse inside the VOM. Check to see
that the VOM leads are in the correct sockets.

This is one reason when I love the cheapie Harbor Freight
meters, I don't cry as much when I blow a fuse. Fuses for my
Fluke are $11 each.

Hmm,
That is why I use old Simpson 260 in a case like this.
It has pop up mechanical fuse. If it pops you pu**** to reset.
OP is still treating the symptom not even an ich close to digging
out root cause of the problem.Down sream on 24V side there could be
a half cooked relay coil causing overload. Small relays are mounted on
the logic board. Look at the schematic is all that it needs.
Except there is NOTHING DOWNSTREAM with the secondary disconnected.

On Apr 19, 8:22*pm, wrote:
On Tue, 19 Apr 2011 10:51:00 -0400, Smarty wrote:
On 4/19/2011 9:05 AM, Steve Turner wrote:
On 4/18/2011 9:41 PM, Steve Turner wrote:
Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and
whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on
the 115V
input side of the transformer, and the unit has been running fine
ever since.
So I know that a 1/4-amp fuse will carry the load on a properly
working system;
what I didn't know for *sure* was if that fuse was rated too high to
protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work
installing
connectors and testing the two circuits for continuity. I installed a
1/4-amp
fuse in the 115V primary side and (once again, as recommended) a
1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter
up in
series on the secondary side to measure the current draw (I taught
myself how
to do this earlier using my little Dremel tool; it draws about 1/2
amp when
spinning freely). All set, I closed the cover interlock switch to
apply 115V to
the system... The red LED on the circuit board lights up momentarily,
then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the
current on the
meter.

Some people mentioned possible shorts in the wiring leading to the
thermostat,
so my next move was to to eliminate that as a possibility. Once
again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...product/22-166....

I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not
obvious to
me), but the wires that lead to the external A/C unit also connect to
this
block; I removed them all (after I marked them :-) ). I also
disconnected the
meter from the secondary circuit just to remove that from the
equation. Once
again, I closed the cover interlock switch, and once again the
1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic
board, and
in anticipation of this I already had one on order; it's scheduled to
arrive
tomorrow. I've toyed with the idea of swapping logic boards between
the two
blower units to see if the problem follows the board (I used this
approach to
isolate a similar problem on a Trane unit at my previous residence
about 10
years ago; it helped, one of the boards was bad), but at this point
I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be
back in
business!

Dammit, I missed a step! *One thing I forgot to do was verify that I
was getting 24V on the output side of the transformer when 115V was
applied to the input side. *I just came back from testing for that,
and now I'm confused again. *I removed both wires from the output side
and connected my meter to check for voltage, closed the interlock
switch, and "piff", the 1/4-amp fuse (my last one) blew! *Apart from
the fact that the input voltage comes to the transformer via output
connectors on the logic board (which are just tied directly to input
connectors on the board as far as I can see), the fuse is blowing and
the logic board isn't even a part of the equation... *What do you make
of that?
Steve,

I am (I think) the person who originally suggested getting the inline
fuse holder and fuses from Radio Shack, and am glad to see you are
pursuing this. Since you have a disconnected secondary, and a fused
primary, and the transformer blows the 1/4 amp fuse, it certainly sounds
like the primary supply voltage is wrong. Have you actually measured the
primary voltage and found it to be about 110 VAC? If so, then the
transformer, with no secondary load, must be defective if it immediately
blows a 1/4 amp fuse with no secondary load, unless for some reason your
logic board is providing some odd waveform other than purely alternating
60 Hz power. Perhaps, for example, some big DC voltage is also present?

It is very hard to believe that the several transformers you have tried
are ALL defective. I am way more suspicious of the supply voltage.

Except the logic board is only on the secondary and does not provide
power to the primary.

He DOES need to check the primary voltage, and also check for DC.- Hide quoted text -

- Show quoted text -

He has checked the primary voltage and reported that it's 120v.
The DC component on the primary is an interesting possibility,
but where that would be coming from isn't clear.
However he also reported that with the thing hooked up to
the new logic board, which has a 3A 24V fuse on it, that
fuse blew. DC on the primary wouldn't be blowing the
3A secondary fuse. One thing is for sure, with a 3A secondary
fuse he should go up to at least a 3/4 amp, if not 1 amp
fuse on the primary for testing so that he can at least take some
measurements.

On Tue, 19 Apr 2011 13:49:05 -0500, Steve Turner
wrote:

(brevity snip)

I'm back. The saga continues.

Ok, I got some more 1/4-amp fast-acting fuses (sorry trader4; neither Radio
Shack nor Home Depot had any slow-blow fuses rated less than 1.5 amps). I took
the transformer out of the blower unit, brought it to my workbench and tested
it there (on a completely different 115V circuit). The 1/4-amp fuse held fine,
and I got a solid 26 volts on the output side. So far so good.

Reinstalled it in the blower unit (screwed it to the frame) and unhooked the
input lines from the logic board (to eliminate it completely from the equation)
and sent them straight into the transformer (with the fuse on the black hot
wire) and with my meter measuring for 24V on the output side. Closed the cover
interlock switch, and "poof"; the fuse blew. That pretty much tells me
something is wrong with the incoming power.

I unhooked the transformer and measured the voltage across the hot and neutral
wires with the interlock switch closed: 120V. I opened the switch, and the
meter still wants to read *something*, like about a volt or so. What the hell?
I metered on the *other* side of the interlock switch: nothing. So I hooked
the transformer back up (it's still bolted to the frame) with the switch
bypassed, and PRESTO; the fuse holds and I have 26V on the output side of the
transformer. Bad interlock switch? I wondered if it was shorting to the
frame, but it's completely encased in plastic. It seems to meter out fine, and
I was going to try to open it up to see if I could find any problem, but it's
completely sealed so I gave up.

Anyway, the switch looks like the culprit, so I turn off the breaker at the
panel, hook everything back up, reconnecting all the wires the way they are
supposed to be, with the exception that the interlock switch is bypassed (not
really comfortable with that) and the 1/4-amp fuse still installed on the hot
line going into the transformer. Turn the breaker back on, and "poof", the
fuse blows again. What the hell? Unhook the transformer output wires from the
logic board to remove any load (and back up to the meter to check for 24V
output), install a new fuse and repeat; the fuse blows AGAIN, and now I'm just
confused. At this point, the "something might be shorted to the frame" theory
is starting to look like a distinct possibility. Thoughts?

Connect the wires to the transformer, without the transformer screwed
down (or contacting the "ground" of the furnace), and check for
voltage between the transformer frame and the furnace frame (ground).
There should be less than a volt (ideally, nothing, but a digital
meter can read phantom voltage). If there is no voltage, the PRIMARY
is not grounding. Measure voltage from each secondary wire to the
transformer case. There should be NONE. (Again, a small "phantom" may
appear, but touching both leads with your fingers at the same time
should eliminate that)

When testing the primary voltage, you ARE closing the lockout switch,
right?? And measuring from black to white?

How about measuring from black to ground, and white to ground, with
and without the lockout switch closed??

To the op.

here is another troubleshooting tip..

Since you have wired a fuse in series with the pri input and they blow, you
can try the next thing,

instead of the fuse, use an ordinary incendescent light bulb (Not a CFL) .
Wire a 40W light bulb in series instead of the fuse. (or if you prefer, wire
it in series with the fuse too.

If there is a problem, the bulb will light bright, if all is well it may
glow a bit.

With this in place, you can carefuly try all modes and move things etc,
anytime the bulb goes bright, you have a problem.

Wire it carfully so that there isno chance for the leads to short to
anything else.

You may need to use a 60 Watt or more. Note that no matter what else
happens, no more then 1/2 Amp can flow through a 60 Watt bulb long term.
And even if there is a fault, 1/2 Amp will not fry anything in less than a
few minutes.

Mark


This is

On 4/19/2011 7:12 PM, wrote:
On Apr 19, 6:37 pm, Steve
wrote:
Ok, now I'm just depressed. I could not reproduce the scenario of the fuse
blowing when the transformer was mounted to the frame. I traced every last bit
of wiring leading to the transformer input; I loosened it from its mounting
clamps where it feeds into the junction box, and I separated all wires so none
were touching each other or the frame, and I tightened the wire nuts. There
are no breaches anywhere, and there just isn't any place where the input wiring
can possibly be shorting to the frame of the blower unit.

I reached the point where the only time the fuse blows is when I plug the 24V
output wires into the logic board, and since the new logic board arrived
earlier today I decided to go ahead and do the swap. Careful labeling of all
wires, and careful reading of the documentation, and I know the board is
installed correctly. This board is a newer model, and it has an interesting
featu

"The 50A55 has only one serviceable part - an automotive type fuse, which
protects the low voltage transformer from damage if its output is
short-circuited. If the fuse has opened up, remove whatever caused the short
circuit and replace the fuse with only a 3 Amp automotive type fuse. If the
fuse is not the cause of the control's problem, replace the entire 50A55
control. There are no other user serviceable parts."


Again, what is the rating of the transformer? The above would
suggest that
it's at least 72VA and that in turn means that the 1/4 amp fuse on the
primary is way to small. Combine that with that it's fast acting and
that
could be why you're seeing primary fuses blow, adding to the
confusion.
If they have a 3A on the 24V side, I'd put a 3/4 amp one on the
primary,
or maybe even a 1 amp temprarily so you can get the thing debugged.
Why the 1/4 primary would work on the bench, but not with the same
transformer with no load in the furnace, remains a mystery.

At this point, I'd get the bigger fuse in the primary and hopefully
it
will stop blowing. Then I'd
be looking for what else the logic board drives with
24V and start disconnecting those wires one by one until it stops
blowing fuses. My best guess would be that something else it
supplies with 24v, ie contactor relay, etc is bad, or that a 24v wire
is shorting out somewhere. Did you try disconnecting the thermostat
wires, for example?

AHA! - progress. Yes, I did do that, but what I did not do was to also remove
the compressor contactor wires when I hooked them back up. One of those (a
white wire) connects to the "C" terminal on the thermostat block, the other (a
red wire) connects to the "Y" terminal (along with the yellow wire leading to
the thermostat). I disconnected those white and red wires (leaving the others
from the thermostat connected) and BINGO; the board comes up, the relays pick
and the blower motor comes on! Obviously there is a problem in the wiring
leading to the compressor, but now at least I know where the hell the problem
IS. Thanks for all the help!

On Apr 19, 7:40*pm, wrote:
..com/webcache/un/furnaces%20%28furn%29/product/22-166...

Just a note *- was going to mention it earlier but didn't - a short in
the thermostat wiring is unlikely to be the cause - all THAT would do
is turn on the heating or cooling. A short to GROUND would likewize
not cause a problem, assuming the secondary side of the transformer is
not grounded.



True of many common thermostats, but not all thermotats. My
Honeywell VisionPro, for example, has a backlight which you
can power all the time by using a common wire from the
furnace. Bad thermostat or short in the wire and you have
an overload on the 24V transformer.

On 4/19/2011 7:45 PM, Steve Turner wrote:
On 4/19/2011 7:12 PM, wrote:
On Apr 19, 6:37 pm, Steve
wrote:
Ok, now I'm just depressed. I could not reproduce the scenario of the fuse
blowing when the transformer was mounted to the frame. I traced every last bit
of wiring leading to the transformer input; I loosened it from its mounting
clamps where it feeds into the junction box, and I separated all wires so none
were touching each other or the frame, and I tightened the wire nuts. There
are no breaches anywhere, and there just isn't any place where the input wiring
can possibly be shorting to the frame of the blower unit.

I reached the point where the only time the fuse blows is when I plug the 24V
output wires into the logic board, and since the new logic board arrived
earlier today I decided to go ahead and do the swap. Careful labeling of all
wires, and careful reading of the documentation, and I know the board is
installed correctly. This board is a newer model, and it has an interesting
featu

"The 50A55 has only one serviceable part - an automotive type fuse, which
protects the low voltage transformer from damage if its output is
short-circuited. If the fuse has opened up, remove whatever caused the short
circuit and replace the fuse with only a 3 Amp automotive type fuse. If the
fuse is not the cause of the control's problem, replace the entire 50A55
control. There are no other user serviceable parts."


Again, what is the rating of the transformer? The above would
suggest that
it's at least 72VA and that in turn means that the 1/4 amp fuse on the
primary is way to small. Combine that with that it's fast acting and
that
could be why you're seeing primary fuses blow, adding to the
confusion.
If they have a 3A on the 24V side, I'd put a 3/4 amp one on the
primary,
or maybe even a 1 amp temprarily so you can get the thing debugged.
Why the 1/4 primary would work on the bench, but not with the same
transformer with no load in the furnace, remains a mystery.

At this point, I'd get the bigger fuse in the primary and hopefully
it
will stop blowing. Then I'd
be looking for what else the logic board drives with
24V and start disconnecting those wires one by one until it stops
blowing fuses. My best guess would be that something else it
supplies with 24v, ie contactor relay, etc is bad, or that a 24v wire
is shorting out somewhere. Did you try disconnecting the thermostat
wires, for example?

AHA! - progress. Yes, I did do that, but what I did not do was to also remove
the compressor contactor wires when I hooked them back up. One of those (a
white wire) connects to the "C" terminal on the thermostat block, the other (a
red wire) connects to the "Y" terminal (along with the yellow wire leading to
the thermostat). I disconnected those white and red wires (leaving the others
from the thermostat connected) and BINGO; the board comes up, the relays pick
and the blower motor comes on! Obviously there is a problem in the wiring
leading to the compressor, but now at least I know where the hell the problem
IS. Thanks for all the help!

And.... BINGO. I followed the two wires from the thermostat block which
clearly led to the outside compressor unit. However, when I popped the service
cover on the outside unit, I saw no evidence of those two wires. It was only
when I peeked with a flashlight that I could see they were hidden by a
secondary access panel (which I've yet to remove), but I *could* see they were
connected to larger yellow and blue wires that led directly to the compressor
relay. I concluded that the 24V signal was not able to pick the relay for some
reason, but actuating it by hand (with a screwdriver, actually) proved that the
contactor moves freely (no binding), and the unit comes on when the relay is
manually picked. Hmm. I started moving and jiggling those wires and I could
tell they had fallen down into the unit and were probably resting on something.
When I pulled them upwards and peeked some more, I could clearly see that the
larger yellow and blue wires were completely melted through! It looks like
they had come to rest on some portion of the condenser coils. It's dark now so
I can't do much to repair them, but hopefully with the light of day and some
time I'll be back in business!

On Tue, 19 Apr 2011 20:41:21 -0500, Steve Turner
wrote:


When I pulled them upwards and peeked some more, I could clearly see that the
larger yellow and blue wires were completely melted through! It looks like
they had come to rest on some portion of the condenser coils. It's dark now so
I can't do much to repair them, but hopefully with the light of day and some
time I'll be back in business!

I fully expect a fairy tale happy ending here.
And with babies- twin control boards!
You've brought AHR together as one big happy family.
Thanks!

--Vic

On 4/19/2011 6:41 PM Steve Turner spake thus:

[...]

And.... BINGO. I followed the two wires from the thermostat block which
clearly led to the outside compressor unit. However, when I popped the service
cover on the outside unit, I saw no evidence of those two wires. It was only
when I peeked with a flashlight that I could see they were hidden by a
secondary access panel (which I've yet to remove), but I *could* see they were
connected to larger yellow and blue wires that led directly to the compressor
relay. I concluded that the 24V signal was not able to pick the relay for some
reason, but actuating it by hand (with a screwdriver, actually) proved that the
contactor moves freely (no binding), and the unit comes on when the relay is
manually picked. Hmm. I started moving and jiggling those wires and I could
tell they had fallen down into the unit and were probably resting on something.
When I pulled them upwards and peeked some more, I could clearly see that the
larger yellow and blue wires were completely melted through! It looks like
they had come to rest on some portion of the condenser coils.

And thus endeth our saga. Pretty exciting conclusion there. Seriously; I
was starting to wonder if you'd ever get this one figured out. This post
ought to be put on a plaque somewhere. Maybe print it up like a
certificate and post it next to your furnace?

So much for those wild-ass guesses in the thread ("could be a magnet
causing a saturated transformer core", etc., etc.)


--
The current state of literacy in our advanced civilization:

yo
wassup
nuttin
wan2 hang
k
where
here
k
l8tr
by

- from Usenet (what's *that*?)

On Tue, 19 Apr 2011 17:17:59 -0700 (PDT), "
wrote:

On Apr 19, 8:06Â*pm, wrote:
On Tue, 19 Apr 2011 06:47:55 -0700 (PDT), "





wrote:
On Apr 19, 9:05Â*am, Steve Turner
wrote:
On 4/18/2011 9:41 PM, Steve Turner wrote:

Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V
input side of the transformer, and the unit has been running fine ever since.
So I know that a 1/4-amp fuse will carry the load on a properly working system;
what I didn't know for *sure* was if that fuse was rated too high to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing
connectors and testing the two circuits for continuity. I installed a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter up in
series on the secondary side to measure the current draw (I taught myself how
to do this earlier using my little Dremel tool; it draws about 1/2 amp when
spinning freely). All set, I closed the cover interlock switch to apply 115V to
the system... The red LED on the circuit board lights up momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the
meter.

Some people mentioned possible shorts in the wiring leading to the thermostat,
so my next move was to to eliminate that as a possibility. Once again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...product/22-166...

I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not obvious to
me), but the wires that lead to the external A/C unit also connect to this
block; I removed them all (after I marked them :-) ). I also disconnected the
meter from the secondary circuit just to remove that from the equation. Once
again, I closed the cover interlock switch, and once again the 1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and
in anticipation of this I already had one on order; it's scheduled to arrive
tomorrow. I've toyed with the idea of swapping logic boards between the two
blower units to see if the problem follows the board (I used this approach to
isolate a similar problem on a Trane unit at my previous residence about 10
years ago; it helped, one of the boards was bad), but at this point I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in
business!

Dammit, I missed a step! Â*One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was applied to the
input side. Â*I just came back from testing for that, and now I'm confused
again. Â*I removed both wires from the output side and connected my meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse
(my last one) blew! Â*Apart from the fact that the input voltage comes to the
transformer via output connectors on the logic board (which are just tied
directly to input connectors on the board as far as I can see), the fuse is
blowing and the logic board isn't even a part of the equation... Â*What do you
make of that?- Hide quoted text -

- Show quoted text -

The first thing I would do is replace the fast blowing fuses with slow
blowing ones. Â*I thought all along using fast ones was a bad idea.
If its an overload that's causing the problem, the slow-blow will
still
fail long before the transformer. Â* And if it's some mysterious
voltage
spike that causing it, which I doubt, even the fast acting fuse may
not
prevent damage. Â*It's possible the fast acting ones are getting
creamed by the initial start-up current.

I'd also consider useing a fuse temporarily thats 1.5 or 2X the
current expected. Â*The transformer should be able to handle that
for more than the minute it takes to get a current measurement.
And if you measure voltage on the secondary, if it's an overload,
I'd expect you'd see less than 24V, as the excessive load
pulls the voltage down.

Â* The ontly thing you are forgetting is the half amp fast blow DID
work on the furnace that works - which is basically identical - so
there is NO REASON the same fuse should not work on this one.
Half amp is already about 50% oversized for the transformer at full
rated output.- Hide quoted text -

- Show quoted text -

But he's using a 1/4 amp fuse, not 1/2 amp.
He just posted further down in this thread that the new logic board
has a 3A 24V fuse on it. In which case, it would seem his 1/4
amp primary fuse is too small. I don;t see the harm in at
least temporarily going to a larger fuse so he can get the
damn thing going enough to take some measurements.


Missed th 1/4 amp - yes, I'd go to 1/2, or possibly 3/4.

Thetransformer is 40va, from previous posts. That's 1/3 amp at full
load. It SHOULD hold on 1/4 amp with no load, but he SHOULD fuse it at
1/2 amp.

On 4/19/2011 9:05 PM, Vic Smith wrote:
On Tue, 19 Apr 2011 20:41:21 -0500, Steve Turner
wrote:


When I pulled them upwards and peeked some more, I could clearly see that the
larger yellow and blue wires were completely melted through! It looks like
they had come to rest on some portion of the condenser coils. It's dark now so
I can't do much to repair them, but hopefully with the light of day and some
time I'll be back in business!

I fully expect a fairy tale happy ending here.
And with babies- twin control boards!
You've brought AHR together as one big happy family.
Thanks!

--Vic

I was certainly surprised and thankful for all the participation. Glad I could
provide some entertainment. :-)

Yeah, I think I mentioned elsewhere that in a previous home I also had Trane
A/C units with White Rodgers control boards and one of them went bad. It
certainly won't hurt to have an extra on hand.

On 4/19/2011 9:11 PM, David Nebenzahl wrote:
On 4/19/2011 6:41 PM Steve Turner spake thus:

[...]

And.... BINGO. I followed the two wires from the thermostat block which
clearly led to the outside compressor unit. However, when I popped the
service cover on the outside unit, I saw no evidence of those two wires. It
was only when I peeked with a flashlight that I could see they were hidden by
a secondary access panel (which I've yet to remove), but I *could* see they
were connected to larger yellow and blue wires that led directly to the
compressor relay. I concluded that the 24V signal was not able to pick the
relay for some reason, but actuating it by hand (with a screwdriver,
actually) proved that the contactor moves freely (no binding), and the unit
comes on when the relay is manually picked. Hmm. I started moving and
jiggling those wires and I could tell they had fallen down into the unit and
were probably resting on something. When I pulled them upwards and peeked
some more, I could clearly see that the larger yellow and blue wires were
completely melted through! It looks like they had come to rest on some
portion of the condenser coils.

And thus endeth our saga. Pretty exciting conclusion there. Seriously; I was
starting to wonder if you'd ever get this one figured out. This post ought to
be put on a plaque somewhere. Maybe print it up like a certificate and post it
next to your furnace?

Yeah, no kidding! A simple damn problem that was one *hell* of a bugger to find.

So much for those wild-ass guesses in the thread ("could be a magnet causing a
saturated transformer core", etc., etc.)

Well I tried to stay on some sort of course that at least made sense to me, but
I certainly appreciate all the input no matter how far off base some of it
turned out to be. :-)

On Apr 19, 10:31*pm, wrote:
On Tue, 19 Apr 2011 17:17:59 -0700 (PDT), "





wrote:
On Apr 19, 8:06*pm, wrote:
On Tue, 19 Apr 2011 06:47:55 -0700 (PDT), "

wrote:
On Apr 19, 9:05*am, Steve Turner
wrote:
On 4/18/2011 9:41 PM, Steve Turner wrote:

Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V
input side of the transformer, and the unit has been running fine ever since.
So I know that a 1/4-amp fuse will carry the load on a properly working system;
what I didn't know for *sure* was if that fuse was rated too high to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing
connectors and testing the two circuits for continuity. I installed a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter up in
series on the secondary side to measure the current draw (I taught myself how
to do this earlier using my little Dremel tool; it draws about 1/2 amp when
spinning freely). All set, I closed the cover interlock switch to apply 115V to
the system... The red LED on the circuit board lights up momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the
meter.

Some people mentioned possible shorts in the wiring leading to the thermostat,
so my next move was to to eliminate that as a possibility. Once again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...product/22-166...

I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not obvious to
me), but the wires that lead to the external A/C unit also connect to this
block; I removed them all (after I marked them :-) ). I also disconnected the
meter from the secondary circuit just to remove that from the equation. Once
again, I closed the cover interlock switch, and once again the 1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and
in anticipation of this I already had one on order; it's scheduled to arrive
tomorrow. I've toyed with the idea of swapping logic boards between the two
blower units to see if the problem follows the board (I used this approach to
isolate a similar problem on a Trane unit at my previous residence about 10
years ago; it helped, one of the boards was bad), but at this point I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in
business!

Dammit, I missed a step! *One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was applied to the
input side. *I just came back from testing for that, and now I'm confused
again. *I removed both wires from the output side and connected my meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse
(my last one) blew! *Apart from the fact that the input voltage comes to the
transformer via output connectors on the logic board (which are just tied
directly to input connectors on the board as far as I can see), the fuse is
blowing and the logic board isn't even a part of the equation... *What do you
make of that?- Hide quoted text -

- Show quoted text -

The first thing I would do is replace the fast blowing fuses with slow
blowing ones. *I thought all along using fast ones was a bad idea.
If its an overload that's causing the problem, the slow-blow will
still
fail long before the transformer. * And if it's some mysterious
voltage
spike that causing it, which I doubt, even the fast acting fuse may
not
prevent damage. *It's possible the fast acting ones are getting
creamed by the initial start-up current.

I'd also consider useing a fuse temporarily thats 1.5 or 2X the
current expected. *The transformer should be able to handle that
for more than the minute it takes to get a current measurement.
And if you measure voltage on the secondary, if it's an overload,
I'd expect you'd see less than 24V, as the excessive load
pulls the voltage down.

* The ontly thing you are forgetting is the half amp fast blow DID
work on the furnace that works - which is basically identical - so
there is NO REASON the same fuse should not work on this one.
Half amp is already about 50% oversized for the transformer at full
rated output.- Hide quoted text -

- Show quoted text -

But he's using a 1/4 amp fuse, not 1/2 amp.
He just posted further down in this thread that the new logic board
has a 3A 24V fuse on it. *In which case, it would seem his 1/4
amp primary fuse is too small. *I don;t see the harm in at
least temporarily going to a larger fuse so he can get the
damn thing going enough to take some measurements.

Missed th 1/4 amp - yes, I'd go to 1/2, or possibly 3/4.

Thetransformer is 40va, from previous posts. That's *1/3 amp at full
load. It SHOULD hold on 1/4 amp with no load, but he SHOULD fuse it at
1/2 amp.- Hide quoted text -


And obviously it actually won't hold at 1/4 amp at even
ZERO load. That caused most of the confusion near the
end of the story, where the transformer blew the fuse repeatedly
at the furnace without any load at all attached to it. I thought
from the beginning it was a bad idea to use fast-acting fuses
close to the limit, at least for testing purposes, to be
able to get it running for current measurements.

On 4/19/2011 8:41 PM, Steve Turner wrote:
On 4/19/2011 7:45 PM, Steve Turner wrote:
On 4/19/2011 7:12 PM, wrote:
On Apr 19, 6:37 pm, Steve
wrote:
Ok, now I'm just depressed. I could not reproduce the scenario of the fuse
blowing when the transformer was mounted to the frame. I traced every last bit
of wiring leading to the transformer input; I loosened it from its mounting
clamps where it feeds into the junction box, and I separated all wires so none
were touching each other or the frame, and I tightened the wire nuts. There
are no breaches anywhere, and there just isn't any place where the input
wiring
can possibly be shorting to the frame of the blower unit.

I reached the point where the only time the fuse blows is when I plug the 24V
output wires into the logic board, and since the new logic board arrived
earlier today I decided to go ahead and do the swap. Careful labeling of all
wires, and careful reading of the documentation, and I know the board is
installed correctly. This board is a newer model, and it has an interesting
featu

"The 50A55 has only one serviceable part - an automotive type fuse, which
protects the low voltage transformer from damage if its output is
short-circuited. If the fuse has opened up, remove whatever caused the short
circuit and replace the fuse with only a 3 Amp automotive type fuse. If the
fuse is not the cause of the control's problem, replace the entire 50A55
control. There are no other user serviceable parts."


Again, what is the rating of the transformer? The above would
suggest that
it's at least 72VA and that in turn means that the 1/4 amp fuse on the
primary is way to small. Combine that with that it's fast acting and
that
could be why you're seeing primary fuses blow, adding to the
confusion.
If they have a 3A on the 24V side, I'd put a 3/4 amp one on the
primary,
or maybe even a 1 amp temprarily so you can get the thing debugged.
Why the 1/4 primary would work on the bench, but not with the same
transformer with no load in the furnace, remains a mystery.

At this point, I'd get the bigger fuse in the primary and hopefully
it
will stop blowing. Then I'd
be looking for what else the logic board drives with
24V and start disconnecting those wires one by one until it stops
blowing fuses. My best guess would be that something else it
supplies with 24v, ie contactor relay, etc is bad, or that a 24v wire
is shorting out somewhere. Did you try disconnecting the thermostat
wires, for example?

AHA! - progress. Yes, I did do that, but what I did not do was to also remove
the compressor contactor wires when I hooked them back up. One of those (a
white wire) connects to the "C" terminal on the thermostat block, the other (a
red wire) connects to the "Y" terminal (along with the yellow wire leading to
the thermostat). I disconnected those white and red wires (leaving the others
from the thermostat connected) and BINGO; the board comes up, the relays pick
and the blower motor comes on! Obviously there is a problem in the wiring
leading to the compressor, but now at least I know where the hell the problem
IS. Thanks for all the help!

And.... BINGO. I followed the two wires from the thermostat block which clearly
led to the outside compressor unit. However, when I popped the service cover on
the outside unit, I saw no evidence of those two wires. It was only when I
peeked with a flashlight that I could see they were hidden by a secondary
access panel (which I've yet to remove), but I *could* see they were connected
to larger yellow and blue wires that led directly to the compressor relay. I
concluded that the 24V signal was not able to pick the relay for some reason,
but actuating it by hand (with a screwdriver, actually) proved that the
contactor moves freely (no binding), and the unit comes on when the relay is
manually picked. Hmm. I started moving and jiggling those wires and I could
tell they had fallen down into the unit and were probably resting on something.
When I pulled them upwards and peeked some more, I could clearly see that the
larger yellow and blue wires were completely melted through! It looks like they
had come to rest on some portion of the condenser coils. It's dark now so I
can't do much to repair them, but hopefully with the light of day and some time
I'll be back in business!

The culprits:

http://www.flickr.com/photos/bbqboye...in/photostream

Not sure how it happened, but those two wires are completely fused together. I
cut away the nasty business, reconnected them to the signal wires from the
logic board and positioned them so they can't ever argue with each other again.
The air conditioner is happily plodding along, and everyone lived happily
ever after.

On 4/20/2011 6:38 AM, wrote:
On Apr 19, 10:31 pm, wrote:
On Tue, 19 Apr 2011 17:17:59 -0700 (PDT), "





wrote:
On Apr 19, 8:06 pm, wrote:
On Tue, 19 Apr 2011 06:47:55 -0700 (PDT), "

wrote:
On Apr 19, 9:05 am, Steve
wrote:
On 4/18/2011 9:41 PM, Steve Turner wrote:

Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V
input side of the transformer, and the unit has been running fine ever since.
So I know that a 1/4-amp fuse will carry the load on a properly working system;
what I didn't know for *sure* was if that fuse was rated too high to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing
connectors and testing the two circuits for continuity. I installed a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter up in
series on the secondary side to measure the current draw (I taught myself how
to do this earlier using my little Dremel tool; it draws about 1/2 amp when
spinning freely). All set, I closed the cover interlock switch to apply 115V to
the system... The red LED on the circuit board lights up momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the
meter.

Some people mentioned possible shorts in the wiring leading to the thermostat,
so my next move was to to eliminate that as a possibility. Once again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...product/22-166...

I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not obvious to
me), but the wires that lead to the external A/C unit also connect to this
block; I removed them all (after I marked them :-) ). I also disconnected the
meter from the secondary circuit just to remove that from the equation. Once
again, I closed the cover interlock switch, and once again the 1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and
in anticipation of this I already had one on order; it's scheduled to arrive
tomorrow. I've toyed with the idea of swapping logic boards between the two
blower units to see if the problem follows the board (I used this approach to
isolate a similar problem on a Trane unit at my previous residence about 10
years ago; it helped, one of the boards was bad), but at this point I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in
business!

Dammit, I missed a step! One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was applied to the
input side. I just came back from testing for that, and now I'm confused
again. I removed both wires from the output side and connected my meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse
(my last one) blew! Apart from the fact that the input voltage comes to the
transformer via output connectors on the logic board (which are just tied
directly to input connectors on the board as far as I can see), the fuse is
blowing and the logic board isn't even a part of the equation... What do you
make of that?- Hide quoted text -

- Show quoted text -

The first thing I would do is replace the fast blowing fuses with slow
blowing ones. I thought all along using fast ones was a bad idea.
If its an overload that's causing the problem, the slow-blow will
still
fail long before the transformer. And if it's some mysterious
voltage
spike that causing it, which I doubt, even the fast acting fuse may
not
prevent damage. It's possible the fast acting ones are getting
creamed by the initial start-up current.

I'd also consider useing a fuse temporarily thats 1.5 or 2X the
current expected. The transformer should be able to handle that
for more than the minute it takes to get a current measurement.
And if you measure voltage on the secondary, if it's an overload,
I'd expect you'd see less than 24V, as the excessive load
pulls the voltage down.

The ontly thing you are forgetting is the half amp fast blow DID
work on the furnace that works - which is basically identical - so
there is NO REASON the same fuse should not work on this one.
Half amp is already about 50% oversized for the transformer at full
rated output.- Hide quoted text -

- Show quoted text -

But he's using a 1/4 amp fuse, not 1/2 amp.
He just posted further down in this thread that the new logic board
has a 3A 24V fuse on it. In which case, it would seem his 1/4
amp primary fuse is too small. I don;t see the harm in at
least temporarily going to a larger fuse so he can get the
damn thing going enough to take some measurements.

Missed th 1/4 amp - yes, I'd go to 1/2, or possibly 3/4.

Thetransformer is 40va, from previous posts. That's 1/3 amp at full
load. It SHOULD hold on 1/4 amp with no load, but he SHOULD fuse it at
1/2 amp.- Hide quoted text -


And obviously it actually won't hold at 1/4 amp at even
ZERO load. That caused most of the confusion near the
end of the story, where the transformer blew the fuse repeatedly
at the furnace without any load at all attached to it. I thought
from the beginning it was a bad idea to use fast-acting fuses
close to the limit, at least for testing purposes, to be
able to get it running for current measurements.

Yes you are correct; it did cause some confusion, and a bit of tail chasing on
my part. I would like to have tried the slow-blow fuses, but I couldn't *find*
any rated anything less than 2 or 3 amps, and I tried two different Home
Depots, two different Radio Shacks, and a Walmart.

On Wed, 20 Apr 2011 04:38:04 -0700 (PDT), "
wrote:

On Apr 19, 10:31Â*pm, wrote:
On Tue, 19 Apr 2011 17:17:59 -0700 (PDT), "





wrote:
On Apr 19, 8:06Â*pm, wrote:
On Tue, 19 Apr 2011 06:47:55 -0700 (PDT), "

wrote:
On Apr 19, 9:05Â*am, Steve Turner
wrote:
On 4/18/2011 9:41 PM, Steve Turner wrote:

Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V
input side of the transformer, and the unit has been running fine ever since.
So I know that a 1/4-amp fuse will carry the load on a properly working system;
what I didn't know for *sure* was if that fuse was rated too high to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing
connectors and testing the two circuits for continuity. I installed a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter up in
series on the secondary side to measure the current draw (I taught myself how
to do this earlier using my little Dremel tool; it draws about 1/2 amp when
spinning freely). All set, I closed the cover interlock switch to apply 115V to
the system... The red LED on the circuit board lights up momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the
meter.

Some people mentioned possible shorts in the wiring leading to the thermostat,
so my next move was to to eliminate that as a possibility. Once again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...product/22-166...

I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not obvious to
me), but the wires that lead to the external A/C unit also connect to this
block; I removed them all (after I marked them :-) ). I also disconnected the
meter from the secondary circuit just to remove that from the equation. Once
again, I closed the cover interlock switch, and once again the 1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and
in anticipation of this I already had one on order; it's scheduled to arrive
tomorrow. I've toyed with the idea of swapping logic boards between the two
blower units to see if the problem follows the board (I used this approach to
isolate a similar problem on a Trane unit at my previous residence about 10
years ago; it helped, one of the boards was bad), but at this point I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in
business!

Dammit, I missed a step! Â*One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was applied to the
input side. Â*I just came back from testing for that, and now I'm confused
again. Â*I removed both wires from the output side and connected my meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse
(my last one) blew! Â*Apart from the fact that the input voltage comes to the
transformer via output connectors on the logic board (which are just tied
directly to input connectors on the board as far as I can see), the fuse is
blowing and the logic board isn't even a part of the equation... Â*What do you
make of that?- Hide quoted text -

- Show quoted text -

The first thing I would do is replace the fast blowing fuses with slow
blowing ones. Â*I thought all along using fast ones was a bad idea.
If its an overload that's causing the problem, the slow-blow will
still
fail long before the transformer. Â* And if it's some mysterious
voltage
spike that causing it, which I doubt, even the fast acting fuse may
not
prevent damage. Â*It's possible the fast acting ones are getting
creamed by the initial start-up current.

I'd also consider useing a fuse temporarily thats 1.5 or 2X the
current expected. Â*The transformer should be able to handle that
for more than the minute it takes to get a current measurement.
And if you measure voltage on the secondary, if it's an overload,
I'd expect you'd see less than 24V, as the excessive load
pulls the voltage down.

Â* The ontly thing you are forgetting is the half amp fast blow DID
work on the furnace that works - which is basically identical - so
there is NO REASON the same fuse should not work on this one.
Half amp is already about 50% oversized for the transformer at full
rated output.- Hide quoted text -

- Show quoted text -

But he's using a 1/4 amp fuse, not 1/2 amp.
He just posted further down in this thread that the new logic board
has a 3A 24V fuse on it. Â*In which case, it would seem his 1/4
amp primary fuse is too small. Â*I don;t see the harm in at
least temporarily going to a larger fuse so he can get the
damn thing going enough to take some measurements.

Missed th 1/4 amp - yes, I'd go to 1/2, or possibly 3/4.

Thetransformer is 40va, from previous posts. That's Â*1/3 amp at full
load. It SHOULD hold on 1/4 amp with no load, but he SHOULD fuse it at
1/2 amp.- Hide quoted text -


And obviously it actually won't hold at 1/4 amp at even
ZERO load. That caused most of the confusion near the
end of the story, where the transformer blew the fuse repeatedly
at the furnace without any load at all attached to it. I thought
from the beginning it was a bad idea to use fast-acting fuses
close to the limit, at least for testing purposes, to be
able to get it running for current measurements.
Ends up it was not "without a load" - just with the BOARD
disconnected. He still had wires connected - that went to the AC unit,
where the wires were shorted.

On Apr 19, 12:50*pm, Tony Hwang wrote:
Stormin Mormon wrote:
In a perfect world, two 24 VAC wires hanging out of a
transformer, and check with VOM, you actually get about 26
VAC. In the real world, what happens is that you leave the
VOM leads in the ampere scale, and fry either the
transformer, fuse, or the fuse inside the VOM. Check to see
that the VOM leads are in the correct sockets.

This is one reason when I love the cheapie Harbor Freight
meters, I don't cry as much when I blow a fuse. Fuses for my
Fluke are $11 each.

Hmm,
That is why I use old Simpson 260 in a case like this.
It has pop up mechanical fuse. If it pops you pu**** to reset.
OP is still treating the symptom not even an ich close to digging
out root cause of the problem.Down sream on 24V side there could be
a half cooked relay coil causing overload. Small relays are mounted on
the logic board. Look at the schematic is all that it needs.

What series 260? Mine uses fuses that are similar to and only
slightly cheaper than the Fluke fuses. I ended up ordering from
McMaster-Carr after x-ref'ing part nos. and now I have a couple extras
of both but they are Not Cheap.

What I really hated to see was when I bought my 260 (used) the fuses
in it were completely the wrong size... yeah, that's a great idea,
let's fry an expensive meter next time you have a brain fart because
you're too damn cheap to buy the right fuse. (or kill yourself,
potentially, although that's less likely.)

nate

On Apr 20, 6:02*pm, wrote:
On Wed, 20 Apr 2011 04:38:04 -0700 (PDT), "





wrote:
On Apr 19, 10:31*pm, wrote:
On Tue, 19 Apr 2011 17:17:59 -0700 (PDT), "

wrote:
On Apr 19, 8:06*pm, wrote:
On Tue, 19 Apr 2011 06:47:55 -0700 (PDT), "

wrote:
On Apr 19, 9:05*am, Steve Turner
wrote:
On 4/18/2011 9:41 PM, Steve Turner wrote:

Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V
input side of the transformer, and the unit has been running fine ever since.
So I know that a 1/4-amp fuse will carry the load on a properly working system;
what I didn't know for *sure* was if that fuse was rated too high to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing
connectors and testing the two circuits for continuity. I installed a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter up in
series on the secondary side to measure the current draw (I taught myself how
to do this earlier using my little Dremel tool; it draws about 1/2 amp when
spinning freely). All set, I closed the cover interlock switch to apply 115V to
the system... The red LED on the circuit board lights up momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the
meter.

Some people mentioned possible shorts in the wiring leading to the thermostat,
so my next move was to to eliminate that as a possibility. Once again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...product/22-166...

I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not obvious to
me), but the wires that lead to the external A/C unit also connect to this
block; I removed them all (after I marked them :-) ). I also disconnected the
meter from the secondary circuit just to remove that from the equation. Once
again, I closed the cover interlock switch, and once again the 1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and
in anticipation of this I already had one on order; it's scheduled to arrive
tomorrow. I've toyed with the idea of swapping logic boards between the two
blower units to see if the problem follows the board (I used this approach to
isolate a similar problem on a Trane unit at my previous residence about 10
years ago; it helped, one of the boards was bad), but at this point I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in
business!

Dammit, I missed a step! *One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was applied to the
input side. *I just came back from testing for that, and now I'm confused
again. *I removed both wires from the output side and connected my meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse
(my last one) blew! *Apart from the fact that the input voltage comes to the
transformer via output connectors on the logic board (which are just tied
directly to input connectors on the board as far as I can see), the fuse is
blowing and the logic board isn't even a part of the equation... *What do you
make of that?- Hide quoted text -

- Show quoted text -

The first thing I would do is replace the fast blowing fuses with slow
blowing ones. *I thought all along using fast ones was a bad idea.
If its an overload that's causing the problem, the slow-blow will
still
fail long before the transformer. * And if it's some mysterious
voltage
spike that causing it, which I doubt, even the fast acting fuse may
not
prevent damage. *It's possible the fast acting ones are getting
creamed by the initial start-up current.

I'd also consider useing a fuse temporarily thats 1.5 or 2X the
current expected. *The transformer should be able to handle that
for more than the minute it takes to get a current measurement.
And if you measure voltage on the secondary, if it's an overload,
I'd expect you'd see less than 24V, as the excessive load
pulls the voltage down.

* The ontly thing you are forgetting is the half amp fast blow DID
work on the furnace that works - which is basically identical - so
there is NO REASON the same fuse should not work on this one.
Half amp is already about 50% oversized for the transformer at full
rated output.- Hide quoted text -

- Show quoted text -

But he's using a 1/4 amp fuse, not 1/2 amp.
He just posted further down in this thread that the new logic board
has a 3A 24V fuse on it. *In which case, it would seem his 1/4
amp primary fuse is too small. *I don;t see the harm in at
least temporarily going to a larger fuse so he can get the
damn thing going enough to take some measurements.

Missed th 1/4 amp - yes, I'd go to 1/2, or possibly 3/4.

Thetransformer is 40va, from previous posts. That's *1/3 amp at full
load. It SHOULD hold on 1/4 amp with no load, but he SHOULD fuse it at
1/2 amp.- Hide quoted text -

And obviously it actually won't hold at 1/4 amp at even
ZERO load. *That caused most of the confusion near the
end of the story, where the transformer blew the fuse repeatedly
at the furnace without any load at all attached to it. *I thought
from the beginning it was a bad idea to use fast-acting fuses
close to the limit, at least for testing purposes, to be
able to get it running for current measurements.

* Ends up it was not "without a load" - just with the BOARD
disconnected. He still had wires connected - that went to the AC unit,
where the wires were shorted.- Hide quoted text -

- Show quoted text -

Wrong again. He made it clear he had the transformer installed in the
furnace with only the primary side connected and the secondary side
open and it still was blowing fuses. That apparently was because he
was using the 1/4 amp fast acting fuses. Your statement makes no
sense because the transformer supplies 24V to the logic board, which
in turn supplies 24V to the shorted AC contactor circuit when it's
time
to turn the AC condenser unit on.

He clearly stated:

"One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was
applied to the
input side. I just came back from testing for that, and now I'm
confused
again. I removed both wires from the output side and connected my
meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-
amp fuse
(my last one) blew! "

You really need to follow the thread more carefully.

On Thu, 21 Apr 2011 05:17:58 -0700 (PDT), "
wrote:

On Apr 20, 6:02Â*pm, wrote:
On Wed, 20 Apr 2011 04:38:04 -0700 (PDT), "





wrote:
On Apr 19, 10:31Â*pm, wrote:
On Tue, 19 Apr 2011 17:17:59 -0700 (PDT), "

wrote:
On Apr 19, 8:06Â*pm, wrote:
On Tue, 19 Apr 2011 06:47:55 -0700 (PDT), "

wrote:
On Apr 19, 9:05Â*am, Steve Turner
wrote:
On 4/18/2011 9:41 PM, Steve Turner wrote:

Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst
waiting for the replacement transformer to arrive I decided to run some
innocuous experiments on my other *working* A/C unit. Several days ago I
installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V
input side of the transformer, and the unit has been running fine ever since.
So I know that a 1/4-amp fuse will carry the load on a properly working system;
what I didn't know for *sure* was if that fuse was rated too high to protect a
transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing
connectors and testing the two circuits for continuity. I installed a 1/4-amp
fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse
(both fast acting) on the 24V secondary side. I also hooked my meter up in
series on the secondary side to measure the current draw (I taught myself how
to do this earlier using my little Dremel tool; it draws about 1/2 amp when
spinning freely). All set, I closed the cover interlock switch to apply 115V to
the system... The red LED on the circuit board lights up momentarily, then
"piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the
meter.

Some people mentioned possible shorts in the wiring leading to the thermostat,
so my next move was to to eliminate that as a possibility. Once again, the
wiring diagrams can be found he

http://www.trane.com/webcache/un/fur...product/22-166...

I removed all wires from the thermostat ("YWRGC") connector block on the
circuit board. The wiring diagram doesn't show it (at least it's not obvious to
me), but the wires that lead to the external A/C unit also connect to this
block; I removed them all (after I marked them :-) ). I also disconnected the
meter from the secondary circuit just to remove that from the equation. Once
again, I closed the cover interlock switch, and once again the 1/4-amp fuse on
the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and
in anticipation of this I already had one on order; it's scheduled to arrive
tomorrow. I've toyed with the idea of swapping logic boards between the two
blower units to see if the problem follows the board (I used this approach to
isolate a similar problem on a Trane unit at my previous residence about 10
years ago; it helped, one of the boards was bad), but at this point I'm calling
it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in
business!

Dammit, I missed a step! Â*One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was applied to the
input side. Â*I just came back from testing for that, and now I'm confused
again. Â*I removed both wires from the output side and connected my meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse
(my last one) blew! Â*Apart from the fact that the input voltage comes to the
transformer via output connectors on the logic board (which are just tied
directly to input connectors on the board as far as I can see), the fuse is
blowing and the logic board isn't even a part of the equation... Â*What do you
make of that?- Hide quoted text -

- Show quoted text -

The first thing I would do is replace the fast blowing fuses with slow
blowing ones. Â*I thought all along using fast ones was a bad idea.
If its an overload that's causing the problem, the slow-blow will
still
fail long before the transformer. Â* And if it's some mysterious
voltage
spike that causing it, which I doubt, even the fast acting fuse may
not
prevent damage. Â*It's possible the fast acting ones are getting
creamed by the initial start-up current.

I'd also consider useing a fuse temporarily thats 1.5 or 2X the
current expected. Â*The transformer should be able to handle that
for more than the minute it takes to get a current measurement.
And if you measure voltage on the secondary, if it's an overload,
I'd expect you'd see less than 24V, as the excessive load
pulls the voltage down.

Â* The ontly thing you are forgetting is the half amp fast blow DID
work on the furnace that works - which is basically identical - so
there is NO REASON the same fuse should not work on this one.
Half amp is already about 50% oversized for the transformer at full
rated output.- Hide quoted text -

- Show quoted text -

But he's using a 1/4 amp fuse, not 1/2 amp.
He just posted further down in this thread that the new logic board
has a 3A 24V fuse on it. Â*In which case, it would seem his 1/4
amp primary fuse is too small. Â*I don;t see the harm in at
least temporarily going to a larger fuse so he can get the
damn thing going enough to take some measurements.

Missed th 1/4 amp - yes, I'd go to 1/2, or possibly 3/4.

Thetransformer is 40va, from previous posts. That's Â*1/3 amp at full
load. It SHOULD hold on 1/4 amp with no load, but he SHOULD fuse it at
1/2 amp.- Hide quoted text -

And obviously it actually won't hold at 1/4 amp at even
ZERO load. Â*That caused most of the confusion near the
end of the story, where the transformer blew the fuse repeatedly
at the furnace without any load at all attached to it. Â*I thought
from the beginning it was a bad idea to use fast-acting fuses
close to the limit, at least for testing purposes, to be
able to get it running for current measurements.

Â* Ends up it was not "without a load" - just with the BOARD
disconnected. He still had wires connected - that went to the AC unit,
where the wires were shorted.- Hide quoted text -

- Show quoted text -

Wrong again. He made it clear he had the transformer installed in the
furnace with only the primary side connected and the secondary side
open and it still was blowing fuses. That apparently was because he
was using the 1/4 amp fast acting fuses. Your statement makes no
sense because the transformer supplies 24V to the logic board, which
in turn supplies 24V to the shorted AC contactor circuit when it's
time
to turn the AC condenser unit on.

He clearly stated:

"One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was
applied to the
input side. I just came back from testing for that, and now I'm
confused
again. I removed both wires from the output side and connected my
meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-
amp fuse
(my last one) blew! "

You really need to follow the thread more carefully.


OK. Why did it work on the bench?????
He was so vague with what he was doing/checking that NOTHING really
made sense.

Main thing is, he found the problem and FIXED it without blowing more
smoke.

Half the responses to the problem WERE blowing smoke.

On Apr 21, 9:35*pm, wrote:
On Thu, 21 Apr 2011 05:17:58 -0700 (PDT), "




* The ontly thing you are forgetting is the half amp fast blow DID
work on the furnace that works - which is basically identical - so
there is NO REASON the same fuse should not work on this one.
Half amp is already about 50% oversized for the transformer at full
rated output.- Hide quoted text -

- Show quoted text -

But he's using a 1/4 amp fuse, not 1/2 amp.
He just posted further down in this thread that the new logic board
has a 3A 24V fuse on it. *In which case, it would seem his 1/4
amp primary fuse is too small. *I don;t see the harm in at
least temporarily going to a larger fuse so he can get the
damn thing going enough to take some measurements.

Missed th 1/4 amp - yes, I'd go to 1/2, or possibly 3/4.

Thetransformer is 40va, from previous posts. That's *1/3 amp at full
load. It SHOULD hold on 1/4 amp with no load, but he SHOULD fuse it at
1/2 amp.- Hide quoted text -

And obviously it actually won't hold at 1/4 amp at even
ZERO load. *That caused most of the confusion near the
end of the story, where the transformer blew the fuse repeatedly
at the furnace without any load at all attached to it. *I thought
from the beginning it was a bad idea to use fast-acting fuses
close to the limit, at least for testing purposes, to be
able to get it running for current measurements.

* Ends up it was not "without a load" - just with the BOARD
disconnected. He still had wires connected - that went to the AC unit,
where the wires were shorted.- Hide quoted text -

- Show quoted text -

Wrong again. *He made it clear he had the transformer installed in the
furnace with only the primary side connected *and the secondary side
open and it still was blowing fuses. * That apparently was because he
was using the 1/4 amp fast acting fuses. * Your statement makes no
sense because the transformer supplies 24V to the logic board, which
in turn supplies 24V to the shorted AC contactor circuit when it's
time
to turn the AC condenser unit on.

He clearly stated:

"One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was
applied to the
input side. *I just came back from testing for that, and now I'm
confused
again. *I removed both wires from the output side and connected my
meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-
amp fuse
(my last one) blew! "

You really need to follow the thread more carefully.

OK. Why did it work on the bench?????

Probably for the same reason the 1/4 amp fuse held sometimes
on the furnace with just the transformer connected. It was
marginal, so sometimes the inital current caused it to blow and
other times it just barely held.


He was so vague with what he was doing/checking that NOTHING really
made sense.

I didn't have any problems following what he was saying and
telling us he was doing. For example, from the very start
he posted the wiring schematic. Taking a look at that it's
obvious, for example, that with the logic board removed
as he described there was no connection to the shorted
AC contactor circuit. Yet there you were, long into the
discusssion, claiming that the shorted AC contactor
circuit was still involved with the logic board removed.



Main thing is, he found the problem and FIXED it without blowing more
smoke.

Half the responses to the problem WERE blowing smoke.- Hide quoted text -

- Show quoted text -

Yes, like a few of yours.

On 4/22/2011 9:55 AM, wrote:
On Apr 21, 9:35 pm, wrote:
On Thu, 21 Apr 2011 05:17:58 -0700 (PDT), "




The ontly thing you are forgetting is the half amp fast blow DID
work on the furnace that works - which is basically identical - so
there is NO REASON the same fuse should not work on this one.
Half amp is already about 50% oversized for the transformer at full
rated output.- Hide quoted text -

- Show quoted text -

But he's using a 1/4 amp fuse, not 1/2 amp.
He just posted further down in this thread that the new logic board
has a 3A 24V fuse on it. In which case, it would seem his 1/4
amp primary fuse is too small. I don;t see the harm in at
least temporarily going to a larger fuse so he can get the
damn thing going enough to take some measurements.

Missed th 1/4 amp - yes, I'd go to 1/2, or possibly 3/4.

Thetransformer is 40va, from previous posts. That's 1/3 amp at full
load. It SHOULD hold on 1/4 amp with no load, but he SHOULD fuse it at
1/2 amp.- Hide quoted text -

And obviously it actually won't hold at 1/4 amp at even
ZERO load. That caused most of the confusion near the
end of the story, where the transformer blew the fuse repeatedly
at the furnace without any load at all attached to it. I thought
from the beginning it was a bad idea to use fast-acting fuses
close to the limit, at least for testing purposes, to be
able to get it running for current measurements.

Ends up it was not "without a load" - just with the BOARD
disconnected. He still had wires connected - that went to the AC unit,
where the wires were shorted.- Hide quoted text -

- Show quoted text -

Wrong again. He made it clear he had the transformer installed in the
furnace with only the primary side connected and the secondary side
open and it still was blowing fuses. That apparently was because he
was using the 1/4 amp fast acting fuses. Your statement makes no
sense because the transformer supplies 24V to the logic board, which
in turn supplies 24V to the shorted AC contactor circuit when it's
time
to turn the AC condenser unit on.

He clearly stated:

"One thing I forgot to do was verify that I was
getting 24V on the output side of the transformer when 115V was
applied to the
input side. I just came back from testing for that, and now I'm
confused
again. I removed both wires from the output side and connected my
meter to
check for voltage, closed the interlock switch, and "piff", the 1/4-
amp fuse
(my last one) blew! "

You really need to follow the thread more carefully.

OK. Why did it work on the bench?????

Probably for the same reason the 1/4 amp fuse held sometimes
on the furnace with just the transformer connected. It was
marginal, so sometimes the inital current caused it to blow and
other times it just barely held.


He was so vague with what he was doing/checking that NOTHING really
made sense.

I didn't have any problems following what he was saying and
telling us he was doing. For example, from the very start
he posted the wiring schematic. Taking a look at that it's
obvious, for example, that with the logic board removed
as he described there was no connection to the shorted
AC contactor circuit. Yet there you were, long into the
discusssion, claiming that the shorted AC contactor
circuit was still involved with the logic board removed.

trader4, look me up if you ever come to Austin. I'll buy you a beer. :-)

Just DON'T ask him to DIAGNOSE your heat pump. He DOESN'T
show much skill there

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


"Steve Turner"
wrote in message ...

trader4, look me up if you ever come to Austin. I'll buy
you a beer. :-)

On Apr 22, 10:19*pm, "Stormin Mormon"
wrote:
Just DON'T ask him to DIAGNOSE your heat pump. He DOESN'T
show much skill there

I found and provided the circuit schematic for the poster
with the heat pump problem in the other thread.
What did you provide? And a little premature for bashing
about the heat pump thread, isn't it? Last time I checked
no one yet has an answer to the fan issue.

Let's look at a sample of your posts in THIS thread:


"It does sound like there is unidentified problem. As the
fellows on alt hvac are often heard to say "replace the
thermostat".

"Glad you noticed. I hadn't looked at the pictures that
closely. I hope your trip charge isn't too much, in this
case? "

"As a heating and AC installer and repair tech, I assure you
that David's writings sound fancy, but he's not competent to
comment. "

"You should contact the manufacturers of furnaces, and tell
them that they are doing it wrong. They will thank you.
Don't take no for an answer. "


There is plenty more bloviating like that, where you did
everything but try to help Steve. Sounds like you're
just ****ed off because someone other than you
HVAC "pros" helped Steve. So get lost, ****wit.





--
Christopher A. Young
Learn more about Jesus
*www.lds.org
.

"Steve Turner"

*wrote in ...

trader4, look me up if you ever come to Austin. *I'll buy
you a beer. *:-)