Here's a better link to the service manual so you can
see the schematic and troubleshooting flowchart:
http://dms.hvacpartners.com/docs/100...00/SP04-62.pdf

good..

see Fig 12.
Note #5 in the lower right corner.

This wire must be connected to furnace sheetmetal for the flamesensor to work.

THe note is for the return wire of the gas valve.

I'm not sure what they mean exaclty, but something there needs to get to furnace sheetmetal ground for the flame senor to work.

Mark

trader_4 wrote, on Tue, 23 Dec 2014 06:56:45 -0800:

If the ground is affecting the flame sensor, then it't the ground path
from the metal cabinet/frame of the furnace to the control board. That's
where I'd be looking. There must be a ground wire that goes from the control
board to the frame and the frame is also grounded to the incoming AC.

That's a good point in that the *control board* needs to be grounded,
as well, especially, since the *only* DC I can find is in the flame
sensor circuit.

Right now I'm looking up how to test the handful of microamps that
the flame sensor circuit needs.

I'm not sure how it works yet. I suspect the heat lowers the resistance
which then increases the current past a trip point, so, if that's the
case, I will try to measure that increased current when I figure out
how to measure the current.

makolber wrote, on Tue, 23 Dec 2014 07:05:32 -0800:

The flame sensor probe acts as a tiny battery and the
controller needs to sense that current. The flame sensor
current is relative to the furnce ground which needs to
be connectd to the AC ground and to the controller ground.

You're the first person to describe *how* the flame sensor
works, so, I appreciate that input, as the first step of
measuring something is knowing how it works.

I had suspected that the DC current through the flame sensor
changes with heat, such that the controller senses that change.

In the same vein, that's the same as saying the resistance
changes, such that the voltage drop across the flame sensor
and ground changes (which, I think, is your battery analogy).

I guess I could remove the flame sensor and put a bunsen
burner on it and measure the resistance change, but, right
now I'm looking up for how to test the current at such low
microamperes, when my Fluke 75 only has a 300ma scale.

I do have a cheap Radio-Shack emergency open-jaw ammeter in
my car trunk, so, I'll dig that up, but, I doubt it's
sensitive enough to go down to a microampere.

I guess also make sure the burner jets are also grounded
to the chassis. The flame is grounded to the burner
jets actually.

The flame is grounded? (I don't understand that statement.)

makolber wrote, on Tue, 23 Dec 2014 07:05:32 -0800:

it is difficult to measure the 5 uA using a VOM.

This guy tests the flame sensor in situ with an ammeter:
https://www.youtube.com/watch?v=OsIkK3AH1p4

The current for a good flame sensor starts at 0ua with no flame,
and then, with flame the ammeter reads 4ua.
https://www.youtube.com/watch?featur...3AH1p4#t=13 2

He mentions that a dirty flame sensor will read lower
current, so, now I know at least which direction the
thing goes.

a. There is no current when the sensor is cold.
b. There is low current when the sensor is dirty.
c. There is a handful of microamps when it's hot.

This guy removes the flame sensor and checks continuity
all along the stainless steel rod, using a technique
I have never seen before for a metal (conductive) rod:
https://www.youtube.com/watch?featur...mnKXZLzzs#t=18

Here they hook up a Fieldpiece HS36 ammeter into the flame
sensor in series,
https://www.youtube.com/watch?featur...BbZvpg#t=36 5

They had a reading of 0ua when cold, 2ua when dirty, and 3ua
when cleaned. The difference between not working and working was
only 1ua, so, a bad ground would certainly affect microamps!

On Tuesday, December 23, 2014 10:18:15 AM UTC-5, Danny D. wrote:
trader_4 wrote, on Tue, 23 Dec 2014 06:56:45 -0800:

If the ground is affecting the flame sensor, then it't the ground path
from the metal cabinet/frame of the furnace to the control board. That's
where I'd be looking. There must be a ground wire that goes from the control
board to the frame and the frame is also grounded to the incoming AC.

That's a good point in that the *control board* needs to be grounded,
as well, especially, since the *only* DC I can find is in the flame
sensor circuit.

Right now I'm looking up how to test the handful of microamps that
the flame sensor circuit needs.

I'm not sure how it works yet. I suspect the heat lowers the resistance
which then increases the current past a trip point, so, if that's the
case, I will try to measure that increased current when I figure out
how to measure the current.

AFAIK, it's a thermocouple, similar to what you'd have in a water heater.

You can probably buy a new one for not too much, might be easier than
trying to measure what u have.

On Tue, 23 Dec 2014 15:23:20 +0000 (UTC), "Danny D."
wrote:

makolber wrote, on Tue, 23 Dec 2014 07:05:32 -0800:

The flame sensor probe acts as a tiny battery and the
controller needs to sense that current. The flame sensor
current is relative to the furnce ground which needs to
be connectd to the AC ground and to the controller ground.

You're the first person to describe *how* the flame sensor
works, so, I appreciate that input, as the first step of
measuring something is knowing how it works.

I had suspected that the DC current through the flame sensor
changes with heat, such that the controller senses that change.

In the same vein, that's the same as saying the resistance
changes, such that the voltage drop across the flame sensor
and ground changes (which, I think, is your battery analogy).

I guess I could remove the flame sensor and put a bunsen
burner on it and measure the resistance change, but, right
now I'm looking up for how to test the current at such low
microamperes, when my Fluke 75 only has a 300ma scale.

Danny,
I don't think you fully understood his battery analogy. It really
generates voltage. Google thermocouple. If you put it in a bunsen
burner, use your Fluke on its DC voltage range and measure the voltage
to see if it is good. (It is also possible you are correct and the
device is a variable resistor that changes with heat, but
thermocouples are very common in gas heating appliances).

Pat

trader_4 wrote:
On Tuesday, December 23, 2014 10:18:15 AM UTC-5, Danny D. wrote:
trader_4 wrote, on Tue, 23 Dec 2014 06:56:45 -0800:

If the ground is affecting the flame sensor, then it't the ground path
from the metal cabinet/frame of the furnace to the control board. That's
where I'd be looking. There must be a ground wire that goes from the control
board to the frame and the frame is also grounded to the incoming AC.

That's a good point in that the *control board* needs to be grounded,
as well, especially, since the *only* DC I can find is in the flame
sensor circuit.

Right now I'm looking up how to test the handful of microamps that
the flame sensor circuit needs.

I'm not sure how it works yet. I suspect the heat lowers the resistance
which then increases the current past a trip point, so, if that's the
case, I will try to measure that increased current when I figure out
how to measure the current.

AFAIK, it's a thermocouple, similar to what you'd have in a water heater.

You can probably buy a new one for not too much, might be easier than
trying to measure what u have.

Hi,
I use O'scope to check flame sensor. If that is the problem, I'd just
replace it. If furnace fired and soon after it quits probably sensor
did not sense the flame. Isn't there a test point on the board when
temporarily grounded it will go thru logic steps of the operation.
I believe all Carrier board has it and Bryant is Carrier 2nd tier
product. Control board is quite common amongst brands. My cabin Trane
furnace has Carrier equivalent board in there. Furnace operation is just
a simple one line yes/no logic flow.

"Danny D." writes:
Looking up videos on troubleshooting a Bryant 373LAV flame sensor,
these guys clean it with a brand new five-dollar bill:
https://www.youtube.com/watch?featur...qL4lnk#t=13 0

This looks like a similar furnace to mine, with a similar issue,
and this guy does not recommend cleaning the flame sensor:
https://www.youtube.com/watch?featur...KsZhc0#t=36 6

After replacing the flame sensor, he shows the whole cycle:
https://www.youtube.com/watch?featur...KsZhc0#t=95 0

These guys use #0000 fine steel wool to clean the flame sensor:
https://www.youtube.com/watch?featur...gBB_R4#t=34 4
https://www.youtube.com/watch?featur...qlGfZv8Es#t=59

All say not to touch the sensor with fingers, which I didn't
know when I first had cleaned it.

But, interestingly, *all* (of those) simply clean or replace the flame sensor
without *any* of them testing it for the 5 or 6 uamps of control current.
(I'll google separately for videos *testing* the flame sensor circuit
but this is already long.)

Normally, I test things before replacing them, but, none of the
technicians in the youtube videos I've found bothered to test it
before replacing it.

Even so, I might buy a spare flame sensor and igniter, and keep
it next to the furnace. Any suggestions as to good suppliers?

APED Appliance Parts on W San Carlos street in San Jose has igniters, flame sensors
and control boards for most furnaces. I've been happy with my purchases
from them.

Scott Lurndal wrote, on Tue, 23 Dec 2014 16:49:18 +0000:

APED Appliance Parts on W San Carlos street in San Jose has igniters, flame sensors
and control boards for most furnaces. I've been happy with my purchases
from them.

Hi Scott,

Thanks for that tip, as they're very close.
They're at 2280 Stevens Creek Boulevard, San Jose, CA 95128.

I called them at 408-977-0404, but, maybe they're on vacation
as they didn't pick up the phone (9-6M-F, 9-3 Sat) so I will
call again later.

I think, no matter what, I'll get a price on the flame sensor
and the control board, since they're the two implicated parts
at the moment (other than ground itself).

Tony Hwang wrote, on Tue, 23 Dec 2014 09:09:33 -0700:

I use O'scope to check flame sensor.

I always wanted an oscilloscope; but I don't have one.

If that is the problem, I'd just replace it.

Wouldn't you know it, but none of the service manuals have the
part numbers. I called Bryant and they said that both Bryant
and Payne use the same part numbers, but they didn't have a parts
list at their 800-428-4326x3 phone number.

They gave me four dealers to call, and all had messages, so,
I left a message with all four, hoping they'll get back to me
on the part number for the flame sensor and control board.

If furnace fired and soon after it quits probably sensor
did not sense the flame.

I agree.
The current conditions are classic for when the flame sensor
does not sense the flame (based on all that I've learned yesterday).

Isn't there a test point on the board when temporarily grounded
it will go thru logic steps of the operation.

Yes. But ...

The test is confusing (to me) as to what the correct procedure is.
Here's the test, on the bottom of page 10, of this manual:
http://dms.hvacpartners.com/docs/100...00/SP04-62.pdf

Here's what that test "says":
"To initiate the component test sequence,shut OFF the room
thermostat or disconnect the "R" thermostat lead. Briefly
short the TEST terminal to the Com 24V terminal. Status
LED will flash code and then turn ON the inducer motor.
The inducer motor will run for the entire component test.
The hot surface ignitor, blower motor-heat speed, and blower
motor-cool speed will be turned ON for 10-15 seconds each."

It's not clear which "R" needs to be disconnected, as there
is are two "R" wires at the thermostat, and one "R" wire at
the control board. I disconnected both.

Then I attached an orange wire to the "COM" terminal and
briefly touched the "TEST" spade on the control board.

Nothing happened.

I then turned on the power, and nothing happened when I did that.
I then turned on the power and held down the door switch, and,
this time "something" happened - but the results were confusing.

First the inducer motor went on (as expected), and then the
igniter lit brightly (as expected) and then nothing was visibly
happening until the inducer went off, about a minute or so
thereafter.

OK. I "think" I ran the test. What did it do?
I don't know.

What did it tell me?
I don't even know what to look for.

I believe all Carrier board has it and Bryant is Carrier 2nd tier
product. Control board is quite common amongst brands. My cabin Trane
furnace has Carrier equivalent board in there. Furnace operation is just
a simple one line yes/no logic flow.

I called Bryant who said that Carrier and Payne and Bryant are all
owned by the same company, and that Payne & Bryant are parts
compatible.

On 12/22/2014 8:40 PM, Danny D. wrote:
Stormin Mormon wrote, on Mon, 22 Dec 2014 20:11:18 -0500:

With a new filter, and open vents, should be much improved air
flow. Lets see who spots the non sequitor.

I'm not sure I understand ...

non sequitor === an inference that does not follow from the premises;
specifically : a fallacy resulting from a simple conversion of a
universal affirmative proposition or from the transposition of a
condition and its consequent.
- a statement (as a response) that does not follow logically from or is
not clearly related to anything previously said.


Sorry, I knew you'd not get it. I ought not wrote
that.

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

On 12/23/2014 10:14 AM, Danny D. wrote:
Normally, I test things before replacing them, but, none of the
technicians in the youtube videos I've found bothered to test it
before replacing it.

Even so, I might buy a spare flame sensor and igniter, and keep
it next to the furnace. Any suggestions as to good suppliers?


I'd mention a couple, but you need to be
in the trade to buy there.

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

Danny D. wrote:
Tony Hwang wrote, on Tue, 23 Dec 2014 09:09:33 -0700:

I use O'scope to check flame sensor.

I always wanted an oscilloscope; but I don't have one.

If that is the problem, I'd just replace it.

Wouldn't you know it, but none of the service manuals have the
part numbers. I called Bryant and they said that both Bryant
and Payne use the same part numbers, but they didn't have a parts
list at their 800-428-4326x3 phone number.

They gave me four dealers to call, and all had messages, so,
I left a message with all four, hoping they'll get back to me
on the part number for the flame sensor and control board.

If furnace fired and soon after it quits probably sensor
did not sense the flame.

I agree.
The current conditions are classic for when the flame sensor
does not sense the flame (based on all that I've learned yesterday).

Isn't there a test point on the board when temporarily grounded
it will go thru logic steps of the operation.

Yes. But ...

The test is confusing (to me) as to what the correct procedure is.
Here's the test, on the bottom of page 10, of this manual:
http://dms.hvacpartners.com/docs/100...00/SP04-62.pdf

Here's what that test "says":
"To initiate the component test sequence,shut OFF the room
thermostat or disconnect the "R" thermostat lead. Briefly
short the TEST terminal to the Com 24V terminal. Status
LED will flash code and then turn ON the inducer motor.
The inducer motor will run for the entire component test.
The hot surface ignitor, blower motor-heat speed, and blower
motor-cool speed will be turned ON for 10-15 seconds each."

It's not clear which "R" needs to be disconnected, as there
is are two "R" wires at the thermostat, and one "R" wire at
the control board. I disconnected both.

Then I attached an orange wire to the "COM" terminal and
briefly touched the "TEST" spade on the control board.

Nothing happened.

I then turned on the power, and nothing happened when I did that.
I then turned on the power and held down the door switch, and,
this time "something" happened - but the results were confusing.

First the inducer motor went on (as expected), and then the
igniter lit brightly (as expected) and then nothing was visibly
happening until the inducer went off, about a minute or so
thereafter.

OK. I "think" I ran the test. What did it do?
I don't know.

What did it tell me?
I don't even know what to look for.

I believe all Carrier board has it and Bryant is Carrier 2nd tier
product. Control board is quite common amongst brands. My cabin Trane
furnace has Carrier equivalent board in there. Furnace operation is just
a simple one line yes/no logic flow.

I called Bryant who said that Carrier and Payne and Bryant are all
owned by the same company, and that Payne & Bryant are parts
compatible.

Hi,
If you want to run test just set the 'stat to lowest setting so it won't
call for heat or cool interfering with the test. Just with a short piece
of wire jumper the TP and ground momentarily the test will start.
Regarding sensor only thing could be different would be connector. If
you get one with wrong connector cut the leads and use
connector from old one. I know same is the case with HSI(hot surface
ignitor) Never touch HSI with bare hand, finger oil will shorten it's
life, also it is quite brittle. And cover plate removed, don't forget to
cheat the interlock switch. If you want to reset every thing just turn
the power on/off otherwise it'll take hours B4 it comes out of lock down
mode. All my houses I built I used Carrier product so I am quite
familiar with it's design. I always keep a spare HSI. I once got caught
in one cold winter night with burnt out HSI. I have to light the furnace
with BBQ lighter whenever furnace came on all night. Two fire places
in the house were not enough to keep warm, LOL! Good luck.
NO white Christmas this year. Day time it is above freezing.

Tony Hwang wrote:
Danny D. wrote:
Tony Hwang wrote, on Tue, 23 Dec 2014 09:09:33 -0700:

I use O'scope to check flame sensor.

I always wanted an oscilloscope; but I don't have one.

If that is the problem, I'd just replace it.

Wouldn't you know it, but none of the service manuals have the
part numbers. I called Bryant and they said that both Bryant
and Payne use the same part numbers, but they didn't have a parts
list at their 800-428-4326x3 phone number.

They gave me four dealers to call, and all had messages, so,
I left a message with all four, hoping they'll get back to me
on the part number for the flame sensor and control board.

If furnace fired and soon after it quits probably sensor
did not sense the flame.

I agree.
The current conditions are classic for when the flame sensor
does not sense the flame (based on all that I've learned yesterday).

Isn't there a test point on the board when temporarily grounded
it will go thru logic steps of the operation.

Yes. But ...

The test is confusing (to me) as to what the correct procedure is.
Here's the test, on the bottom of page 10, of this manual:
http://dms.hvacpartners.com/docs/100...00/SP04-62.pdf

Here's what that test "says":
"To initiate the component test sequence,shut OFF the room
thermostat or disconnect the "R" thermostat lead. Briefly
short the TEST terminal to the Com 24V terminal. Status
LED will flash code and then turn ON the inducer motor.
The inducer motor will run for the entire component test.
The hot surface ignitor, blower motor-heat speed, and blower
motor-cool speed will be turned ON for 10-15 seconds each."

It's not clear which "R" needs to be disconnected, as there
is are two "R" wires at the thermostat, and one "R" wire at
the control board. I disconnected both.

Then I attached an orange wire to the "COM" terminal and
briefly touched the "TEST" spade on the control board.

Nothing happened.

I then turned on the power, and nothing happened when I did that.
I then turned on the power and held down the door switch, and,
this time "something" happened - but the results were confusing.

First the inducer motor went on (as expected), and then the
igniter lit brightly (as expected) and then nothing was visibly
happening until the inducer went off, about a minute or so
thereafter.

OK. I "think" I ran the test. What did it do?
I don't know.

What did it tell me?
I don't even know what to look for.

I believe all Carrier board has it and Bryant is Carrier 2nd tier
product. Control board is quite common amongst brands. My cabin Trane
furnace has Carrier equivalent board in there. Furnace operation is just
a simple one line yes/no logic flow.

I called Bryant who said that Carrier and Payne and Bryant are all
owned by the same company, and that Payne & Bryant are parts
compatible.

Hi,
If you want to run test just set the 'stat to lowest setting so it won't
call for heat or cool interfering with the test. Just with a short piece
of wire jumper the TP and ground momentarily the test will start.
Regarding sensor only thing could be different would be connector. If
you get one with wrong connector cut the leads and use
connector from old one. I know same is the case with HSI(hot surface
ignitor) Never touch HSI with bare hand, finger oil will shorten it's
life, also it is quite brittle. And cover plate removed, don't forget to
cheat the interlock switch. If you want to reset every thing just turn
the power on/off otherwise it'll take hours B4 it comes out of lock down
mode. All my houses I built I used Carrier product so I am quite
familiar with it's design. I always keep a spare HSI. I once got caught
in one cold winter night with burnt out HSI. I have to light the furnace
with BBQ lighter whenever furnace came on all night. Two fire places
in the house were not enough to keep warm, LOL! Good luck.
NO white Christmas this year. Day time it is above freezing.
Hi,
One more thing.
If you indeed need board, it could be universal type with wiring adapter
harnesses and instruction sheet. Board is quite pricy,
I hope you don't need one.

"Danny D." writes:
Tony Hwang wrote, on Tue, 23 Dec 2014 09:09:33 -0700:

I use O'scope to check flame sensor.

I always wanted an oscilloscope; but I don't have one.

http://www.bitscope.com/product/BS05/

On Tue, 23 Dec 2014 14:48:27 -0500, Stormin Mormon
wrote:

Any suggestions as to good suppliers?


I'd mention a couple, but you need to be
in the trade to buy there.

Easy to get around. I bought a "trade" restricted part from Grainger.
They asked and I told them I was from the "government". They had no
account for the agency. Instead, this was a government cash.

They sold me the part UNDER the Forestry Department account. I had
the electrical supervisor at work have it installed by a convict
prisoner

Cash money talks!

Ok,the flame sensor is just a wire. It does not change resistance with heat. Also the flame sensor has only 1 connection and as you shoild know, a complete circuit needs 2. The circuit is completed THROUGH THE FLAME to the metal burner. And the current does not come from the controller. The flame actually generates the current. The controller senses therescense of thos small current. So the flame is a small battery where the neg terminal is the metal burner and the positive terminal is the probe in the flame. The current is very small and i would not try to measure it unless you have the right kind of meter and know what to do.

Also the flame sensor in NOT like the thermocoouples which have two wires and generate current inthe two wires when heated. The flame sensor is different.

The flowing hot gas from the burner tothe probe creates a small current.

I donT know how important it is for the probe itself to be clean, but the insulator that holds the probe needs to be clean or else the small current willleak directly back to the grounded metal ans not make it to the controller.



Mark

Ok, im a little wrong.
The flame senor works by rectification. The controller applies a small AC voltage and the flame lets the current flow in one direction only in effect creating a small DC current. The battery analogy is not exactly right.
Goggle flame rectification sensor.

Mark

Stormin Mormon wrote, on Tue, 23 Dec 2014 14:45:50 -0500:

Sorry, I knew you'd not get it. I ought not wrote that.

No problem. I had guessed that you might have been alluding to
a situation where a conclusion would be reached without the
scientific logic behind that conclusion.

While a *lot* of people just throw parts at a problem, and
even more make inferences that have no basis in fact, I
"try" to make logical inferences, and I try to test the fact.

I'm not always right, and, in fact, I'm often wrong; but in
the end, the item gets fixed, usually by fixing the right part.

In this case, I "may" have to throw parts at the problem,
because every test I "can" do on the flame sensor circuit
shows a good sensor, yet, clearly, the flame sensing circuit
is not working.

I contacted at least 5 distributors today, and only two
would sell a consumer parts. One had the flame sensor for
$20, the other for $18, but the $20 lady called first, so
I ordered over the phone. Since she didn't require an up-front
payment, I presume she has ordered it by now, so, I'll buy it
from her anyway.

She had the control board for $450, while the other guy had it
for $250, so, that's a bigger difference. I spent a few hours
(literally) on my side, in the crawlspace, staring at that
darn furnace, reading the three PDFs, staring at that darn thing
some more, and, like an idiot, turning it on over and over and
over again (even disassembling and reassembling the burners).

In the end, I gave up on debugging further.
a) I can't definitively test the flame sensor (I can't even figure
out yet how it works)
b) It's either the flame sensor or the motherboard, and,
c) If it's a ground, I can't find it.

https://c4.staticflickr.com/8/7540/1...7fc30d88_c.jpg

Stormin Mormon wrote, on Tue, 23 Dec 2014 14:48:27 -0500:

I'd mention a couple, but you need to be in the trade to buy there.

I called a few today.

My hastily written notes are below, but I may have mixed a bit up
as almost all had to call me back so I invariably didn't have my
paperwork with me when they called back, so, it was hard keeping
them straight since they all sounded the same.

Slakey Brothers 1480 Nicora Avenue, San Jose
(sells only to contractors).

Appliance Parts, Bob 408-265-5030, San Jose
(sells to humans, $18 flame sensor, $250 control board)

APED Appliance, San Jose 408-977-0404
(sells only to contractors)

Residential Heating and AC 888-818-6374, Campbell
(sells to humans, $20 flame sensor, $450 control board)

Air Care Heating & Cooling, San Jose 408-513-3089
(doesn't sell parts, $80 for a service call)

Coldcraft 888-918-8662
(doesn't sell parts, $139 for a service call)

makolber wrote, on Tue, 23 Dec 2014 16:08:42 -0800:

the flame sensor has only 1 connection

I don't profess to understand how that flame sensor works,
but, it does, perhaps, seem to have *two* connections.

1. The spade where the white wire goes, and,
https://c4.staticflickr.com/8/7516/1...0760b5ca_c.jpg

2. Ground (where it's bolted to the chassis).
https://c4.staticflickr.com/8/7480/1...7e94f4f8_c.jpg

makolber wrote, on Tue, 23 Dec 2014 16:19:30 -0800:

The flame senor works by rectification.

There are only two ways to test a component:
a) (Perhaps blindly) following a test procedure, or,
b) Understanding how it works and improvising an appropriate test.

I can't blindly follow the stated test procedure because I can't
easily measure from 1 to 6 microamperes.

I can't logically improvise a reasonable test procedure because I
can't figure out how the thing works.

I did try measuring resistance from spade to tip, which varied:
0.2 ohms https://c4.staticflickr.com/8/7516/1...0760b5ca_c.jpg
0.6 ohms https://c4.staticflickr.com/8/7523/1...f1054855_c.jpg

And I verified that the flame sensor "body" is attached to ground:
0.2 ohms https://c4.staticflickr.com/8/7480/1...7e94f4f8_c.jpg

I doubled up on the ground from the flame sensor body to the chassis
ground, (it's hard to see the black alligator clip wire in the photo
below, nor the tape on the door switch, but this is that test):
https://c4.staticflickr.com/8/7534/1...93846665_b.jpg

With the flame sensor body ground doubled, I cycled the furnace,
just to make sure that a bad flame sensor ground wasn't the problem:
https://c4.staticflickr.com/8/7512/1...d60a52f2_c.jpg

In the hopes of doubling the control board ground, I looked (in vain)
everywhere I could on the controller board for a named GROUND,
but never did fine a single connection called "ground":
https://c4.staticflickr.com/8/7564/1...8288d901_c.jpg

All these tests were in vain.

I still don't know if it's a bad flame sensor or a bad circuit board.

trader_4 wrote, on Tue, 23 Dec 2014 08:00:29 -0800:

AFAIK, it's a thermocouple, similar to what you'd have in a water
heater.
You can probably buy a new one for not too much, might be easier than
trying to measure what u have.

The flame sensor is $20, and it's on order.

At the moment, it could be any one of three things:
1. Bad flame sensor (let's hope),
2. Bad circuit board (let's hope not as the quotes are $250 & $450)
3. Bad circuit board ground (I wish).

I know you suggested I check the ground on the circuit board, but,
try as I might, I can't find a single connection "labeled" ground!
https://c4.staticflickr.com/8/7517/1...15b36c3e_b.jpg

Tony Hwang wrote, on Tue, 23 Dec 2014 13:57:56 -0700:

Just with a short piece of wire jumper the TP and ground
momentarily the test will start.

I don't understand that test.

It "sounds" like a good idea, and it's *easy* to run:
https://c4.staticflickr.com/8/7559/1...ed919914_b.jpg

But, whatever it does, doesn't tell me anything.
It just runs 'stuff', and that's about it.

I must be missing the magic of this so-called "component test".
I *wish* it gave me useful information.

But, I get nothing out of it, and, I'm not even sure that the
"component test" tests the flame sensor anyway, since the flames
never start up during that so-called "component test".

So, at the moment, I think it's the most over-rated test that
has been suggested, to date!

Tony Hwang wrote, on Tue, 23 Dec 2014 13:57:56 -0700:

I know same is the case with HSI(hot surface ignitor) Never touch HSI
with bare hand, finger oil will shorten it's life, also it is quite
brittle. And cover plate removed, don't forget to cheat the interlock
switch.

I taped over the door switch, so that's not a problem.

Thanks for the warning on the brittle igniter.

Here, where I've removed the three burner tubes, you can see that
the speckly igniter is only at the top burner, while the rod-shaped
flame sensor is only on the bottom burner, with the middle burner
being left alone:
https://c4.staticflickr.com/8/7467/1...609d3825_c.jpg

Tony Hwang wrote, on Tue, 23 Dec 2014 14:04:50 -0700:

If you indeed need board, it could be universal type with wiring adapter
harnesses and instruction sheet. Board is quite pricy,
I hope you don't need one.

I was only able to get two quotes for the board, the first
being $450 and the second being $250, neither of which was
in stock.

I spent at least an hour (or more) on my back, looking the
board over, and I don't see a single burn mark or obviously
bad component:
https://c1.staticflickr.com/9/8669/1...03e811cb_b.jpg

I did figure out the terminal sequence though, which you can see
I drew a simplified diagram of on the chassis in the picture above.
COM === the unswitched neutral out of the 24VAC (nominal) transformer
RED === the power to the thermostat (which is switched in the thermostat)
WHI === the power coming out of the thermostat to call for heating
YEL === the power coming out of the thermostat to call for cooling
GRE === the power coming out of the thermostat to call for the fan

The transformer, nominally 24VAC, tested at 28VAC, by the way.

Scott Lurndal wrote, on Tue, 23 Dec 2014 21:28:09 +0000:

http://www.bitscope.com/product/BS05/

Thanks Scott,

I have a call to a scientist friend, who might have the capability
to measure 1 to 5 microamperes of current.

A funny thing is that one of the service personnel told me to
measure "5 ohms" (I swear I heard him right) of the flame sensor.

When I questioned him, he said he always measured if it was 5 ohms.
When I asked him *how* he measured it, he *clearly* explained that
he hooked one lead to the disconnected wire and the other lead to
the spade on the flame sensor while the flames were heating it up.

He kept telling me that was ohms, and I told him two or three times
that he can't possibly be measuring ohms that way, but he insisted
he tested the ohms that way all the time, and determined whether a
flame sensor was bad that way.

Sometimes these technicians scare me.

Pat wrote, on Tue, 23 Dec 2014 11:01:12 -0500:

I don't think you fully understood his battery analogy. It really
generates voltage. Google thermocouple. If you put it in a bunsen
burner, use your Fluke on its DC voltage range and measure the voltage
to see if it is good.

I will fully agree with you!
I do *not* know how the flame sensor works.

It appears to have two *external* electrical connections.
1. The spade end that has the controller wire connected to it, and,
2. The body (which is bolted down and presumably grounded as a result).

What is not clear are the *internal* connections (if any).

When I tested it cold, there was no connection between the spade end
and the body ground:
https://c4.staticflickr.com/8/7470/1...d3a691fb_b.jpg

Oren wrote, on Tue, 23 Dec 2014 14:46:23 -0800:

Easy to get around. I bought a "trade" restricted part from Grainger.
They asked and I told them I was from the "government". They had no
account for the agency. Instead, this was a government cash.

I remember that story, as you've said that before, I forget why, but,
probably in reference to an AC part, when I had to fix my sister's AC
unit.

If it matters, I took all three burners out, but, there was nothing I
could see to do with them once they were out, except snap a photo:
https://c4.staticflickr.com/8/7510/1...f06fc142_c.jpg

That did enable me to confirm that the flame sensor, which is on the
lower burner in the photo below, was mounted squarely in the flame:
https://c4.staticflickr.com/8/7467/1...609d3825_c.jpg

Googling for how the flame sensor works, it seems very few people
actually know, or, if they do, they are all explaining something
different.

How does a flame sensor work and how do you test it:
http://heatingandaircooling.com/2012...-flame-sensor/

How does a furnace flame sensor work:
http://www.ehow.com/how-does_4924695...nsor-work.html

How does the gas furnace flame rod work:
https://www.youtube.com/watch?v=uJU3806j2CE

etc. (I'm still reading up on this...)

makolber wrote, on Tue, 23 Dec 2014 16:08:42 -0800:

The flowing hot gas from the burner tothe probe creates a small current.

I'm trying to make sense out of these four descriptions of
how the typical gas furnace flame sensor works (because if I knew how it
works, I could devise a test that might not require measuring from 1 to
5 microamperes, which I can't adequately measure with my Fluke 75).

How a flame sensor works:
http://thehvacguy.wordpress.com/2012...ensor-problem/

How does a flame sensor work and how do you test it:
http://heatingandaircooling.com/2012...-flame-sensor/

How does a furnace flame sensor work:
http://www.ehow.com/how-does_4924695...nsor-work.html

How does the gas furnace flame rod work:
https://www.youtube.com/watch?v=uJU3806j2CE

Danny D. wrote, on Wed, 24 Dec 2014 04:45:28 +0000:

The transformer, nominally 24VAC, tested at 28VAC, by the way.

It's not clear, to me, whether that's because of RMS versus
average measurements, but the measured 28 VAC is close enough
for our purposes to assume the switched power to the RED terminal
isn't a problem.

What's harder to understand is the unswitched COM terminal.

It seems to be an unswitched neutral, coming out of a center-tapped
transformer, especially since it doesn't seem to go to the thermostat,
but, it does go to the AC unit.

Anyway, for some purposes, I assume it's not a center-tapped
transformer, and that the COM is simply the unswitched other side
of the transformer, where the RED is the switched side of that
transformer.

But, for other purposes, I assume the COM is a "neutral" of sorts,
coming out of the middle of a 12VAC + 12VAC transformer, which makes
it a ground, of sorts.

All this probably just tells you that I don't really understand
what exactly the switched RED and unswitched COM terminals are, but,
in reality, that's not the problem with this furnace, so, that's
just a diversion away from the real problem, which is determining
whether the failed part is, either:
a) A bad flame sensor ($20 part is on order), or,
b) A bad control board ($250 to $450 part can be ordered), or,
c) A bad circuit board ground.

Scott Lurndal wrote, on Tue, 23 Dec 2014 21:28:09 +0000:

http://www.bitscope.com/product/BS05/

I got an idea for a test of the "flame rod" from this video.
https://www.youtube.com/watch?featur...06j2CE#t=15 8

The guy shorts out a good flame rod, and the flames stop.

If I short out mine, during the flames, it should also stop the flames
quicker, and, if it doens't stop the flames right away, that would be
an indication that it's not doing anything.

Of course, either way, I still wouldn't know if the control board is
bad, but, it's worth a test to see what happens.

Also, I noticed his autopsy of a flame rod shows that there is apparently
*no connection* from the flame rod to the steel body of the flame rod.
https://www.youtube.com/watch?featur...06j2CE#t=18 6

So, it *is* a single-wire sensor!

Danny D. wrote, on Wed, 24 Dec 2014 05:11:13 +0000:

a) A bad flame sensor ($20 part is on order), or,
b) A bad control board ($250 to $450 part can be ordered), or,
c) A bad circuit board ground.

Now that I've seen the insides of a flame-sense rod, I can't imagine,
other than getting dirty, how they can *not* work.

It's just a steel rod inside a ceramic insulator.

Nothing more than that. It's about as simple a construction as can
be imagined.

In fact, this guy, who says he's an instructor, says he can't get
a flame rod to fail!
https://www.youtube.com/watch?featur...8450mM#t=21 7

He says you can clean it with just about anything (even a file):
https://www.youtube.com/watch?featur...8450mM#t=39 0

Tomorrow I'm going to three more aggressive ways to clean it:
https://www.youtube.com/watch?v=6DsKn1Rw9Yo

But, it's looking less and less like the flame rod went bad.

Scott Lurndal wrote, on Tue, 23 Dec 2014 21:28:09 +0000:

http://www.bitscope.com/product/BS05/

Here, they use a $10 cheap (#69096 Harbor Freight) meter!
https://www.youtube.com/watch?featur...4OJBc4#t=20 1

Danny D. wrote, on Wed, 24 Dec 2014 05:44:44 +0000:

Here, they use a $10 cheap (#69096 Harbor Freight) meter!
https://www.youtube.com/watch?featur...4OJBc4#t=20 1

As much as I hate cheap tools, I might pick one of these up:

http://manuals.harborfreight.com/man...9999/69096.pdf
Harbor Freight 69096, Cen-Tech 7 FUNCTION DIGITAL MULTIMETER

trader_4 wrote, on Tue, 23 Dec 2014 08:00:29 -0800:

AFAIK, it's a thermocouple, similar to what you'd have in a water heater.

This is probably one of the better explanations of the flame sensor:
http://www.robertshawtstats.com/spaw...163_RevB_1.pdf

Pat wrote, on Tue, 23 Dec 2014 11:01:12 -0500:

I don't think you fully understood his battery analogy. It really
generates voltage. Google thermocouple.

Here's a good explanation of how the flame rod works.
https://www.youtube.com/watch?v=be_it7G01gI

Now that I know how it works, I should be able to devise
a test that doesn't require me to measure 5 ua of current.

trader_4 wrote, on Tue, 23 Dec 2014 06:56:45 -0800:

If the ground is affecting the flame sensor, then it't the ground path
from the metal cabinet/frame of the furnace to the control board. That's
where I'd be looking. There must be a ground wire that goes from the control
board to the frame and the frame is also grounded to the incoming AC.

You're definitely on to something here!

This video says reversed polarity will cause teh flame sense to not work!
https://www.youtube.com/watch?featur...WNRqMM#t=31 6

Look at the test here for ground using AC voltage ( 2VAC):
https://www.youtube.com/watch?featur...ApSN7E#t=49 8

From this video, the FS terminal (Flame Sensor) on the board is key:
https://www.youtube.com/watch?v=JE3MzApSN7E

That control board FS terminal sends out a signal of 90VAC to the flame rod.
The flame itself, acts like a diode and a resistor, connecting the flame rod
to ground, which is the burner metal itself. Completing the circuit causes
the control board to hold open a relay, which keeps the gas flowing.

I noticed my burners were rusty on their ends, so, I will try to clean
them up. https://c4.staticflickr.com/8/7554/1...49404dd7_c.jpg

Any ideas how to clean up this burner end?

On Tuesday, December 23, 2014 7:19:34 PM UTC-5, wrote:
Ok, im a little wrong.
The flame senor works by rectification. The controller applies a small AC voltage and the flame lets the current flow in one direction only in effect creating a small DC current. The battery analogy is not exactly right.
Goggle flame rectification sensor.

Mark

Well, you learn something new all the time. I thought the flame sensor
was a thermocouple. But from some googling, I see you're right. I never
even knew this phenomenon existed. It apparently works by using the
flame itself to conduct a small current. And somehow the AC gets half-waved
rectified in the process.

One big difference with testing, would appear to be that the rod type
sensor won't generate any voltage from just heating it. It has to have
an AC current applied, so the current test would have to be done with
it installed, connected to the controller.

Now the advice to clean them makes sense. If it was a thermocouple,
whether it had some dirt on it or not should not affect the operation.

On Tuesday, December 23, 2014 11:32:58 PM UTC-5, Danny D. wrote:
trader_4 wrote, on Tue, 23 Dec 2014 08:00:29 -0800:

AFAIK, it's a thermocouple, similar to what you'd have in a water
heater.
You can probably buy a new one for not too much, might be easier than
trying to measure what u have.

The flame sensor is $20, and it's on order.

At the moment, it could be any one of three things:
1. Bad flame sensor (let's hope),
2. Bad circuit board (let's hope not as the quotes are $250 & $450)
3. Bad circuit board ground (I wish).

I know you suggested I check the ground on the circuit board, but,
try as I might, I can't find a single connection "labeled" ground!
https://c4.staticflickr.com/8/7517/1...15b36c3e_b.jpg

Presumably the AC line is connected to the controller. Follow the
green wire/s. The incoming ground will be connected to the cabinet
frame, typically right where the connection is made. And typically,
you'd have a green wire at the controller, together with the hot
and neutral, that probably goes back to that incoming connection point
too.