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.