I have a 2018 class A gas unit with an Atwood afm series furnace. My problem is that it was sooting up and smelling, even to the point of sounding off the CO alarm. I thought I had a dirt dauber problem maybe so I took the unit completely apart and flushed the heat tubes. I was wrong and that is not the issue. Ran a snake from stem to stern through it and got nothing at all. Flushed it with water also and got nothing but good flow. Here where my plot thickens. Not only have I not found the original problem, but now have another one. After putting it all back together, I now have a problem with nothing at all happening when turning the heater on. I checked the wiring diagram and see not issue, however it is possible that I am overlooking something. When we first put it back together I got a single flash light on the circuit board which pointed to the limit switch. Now though, after swapping out a couple of wires that were put in the wrong place initially, I get nothing. Any ideas? Could the limit switch getting wet during the flush, could it be stuck open not allowing the circuit to be complete upon startup. ALSO, could someone send a picture of their circuit board just to make sure the wiring is correct?

Did you verify with a meter that you have a good 12 V going to the board.Also with the meter you can ohm out the switches to see if they are working properly.Did you take a picture of the board or make a diagram of the board connections before you took it apart?

Here is manual that should help.ManualI would blow the furnace compartment out good with compressed air, compressor or can will work.Have you tested the thermostat? At furnace connect the two blue wires on terminal block. Furnace fan should start immediately. In 15-20 seconds burner should kick in. separating the two blue wires the furnace should go into cool down mode and then shut off.If no operation, Check the circuit breaker to insure it is energized. This is mounted on motor housing.

I sure wish now I had taken pictures but didn't. I do have 12 volts at the board. I didn't know if it would hurt to jump the circuit breaker, sail switch and the limit switch, not at the same time but one at a time? I will test the thermostat like one of you said next. Thanks again.

It would not hurt to jumper circuit breaker for testing only! You could take test with voltmeter. One lead to ground, other lead to each side of breaker terminals. You should have 12 volts one each side.

I,d STOP right now. your not sure of the wiring. you could damage other things by just flipping wires.

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warpz256
Explorer
Jan 13, 2024
sounds like low gas pressure or the air flow is low. check the burner blower wheel for dirt
enblethen
Nomad
Feb 06, 2020
Thanks for the update!
VilleDeal
Explorer II
Feb 06, 2020
PS... thanks for everyone's help!!
VilleDeal
Explorer II
Feb 06, 2020
UPDATE... After replacing the board with a Dinosaur brand, it now is working as designed. I do take responsibility for shorting the board out, but not only did it start and run after it's replacement, but did so without sooting or smelling. So while I am perplexed about what exactly did fix it, I am also happy to move on.
VilleDeal
Explorer II
Jan 28, 2020
It has no diagnostic code flashing at all.
I will recheck my test on the sail switch, I do know it was operating from closed to open after removal, just cannot remember at this point in time whether it was closed before the startup or open.
Thanks, I will update when I have a change or more information.
mchero
Explorer
Jan 26, 2020
VilleDeal wrote:
I have now tested the limit switch and it is closed as it should be.
I have also tested the off/reset switch and have resistance when it is on reset and have no resistance when it is off which should also be correct.
I do have 12 volts to the board, so am I to suppose I need to replaced the circuit board at this point do you reckon?

You stated the sail switch is closed "as it should be"
My Atwood 8531 sail switch is normally open when the furnace is not running. When the thermostat calls for heat one of the pre checks is looking for an open circuit on the sail switch. If it's found closed the board goes into lockout. No fan, nothing.

3rd poster provided a link, open link and scroll down to the wiring schematic. Sail Switch N.O. (normally open).
Any diagnostic codes from the ignitor board?

Even a failing sail switch can look good using a volt meter. My rear furnace would occasionally not run when calling for heat. Dyno board flashing a sail switch closed error. I pulled sail switch and it tested good. Purchased sail switch, installed and now furnace starts when called for heat.
enblethen
Nomad
Jan 24, 2020
I would suggest a Dinosaur electronics version of the circuit board.
Call them for correct unit. They do not sell direct but available on Amazon.
Dinosaur Electronics
VilleDeal
Explorer II
Jan 24, 2020
I have now tested the limit switch and it is closed as it should be.
I have also tested the off/reset switch and have resistance when it is on reset and have no resistance when it is off which should also be correct.
I do have 12 volts to the board, so am I to suppose I need to replaced the circuit board at this point do you reckon?
VilleDeal
Explorer II
Jan 23, 2020
Yes, I'm confident that the wiring is correct now per the wiring diagram.
Tied the two blue thermostat wires together and still got nothing at all at last check.
I took the reset/off switch to check it and the resistance of the limit switch while I am at it.
dougrainer
Nomad
Jan 22, 2020
There are 3 wires and a multiwire quick connect on the Circuit board
1 wire is the spark ignitor wire and is distinctive from the piezo tower to the burner
the other 2 wires are for the Fan Motor relay and the spade terminals are different sizes and CANNOT be attached wrong
the multi quick connect only fits one way.
So, your Circuit board is connected correctly.

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