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Old 09-13-2021, 07:25 PM   #1
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Atwood 8940 aclc won't stay lit

Hiya , after replacing everything but the gas valve , that is next. It lites for a few seconds and goes off does this a few times until I turn off the thermo. Tried to find a solution here in the forum but haven't found any. Anyone else have this problem? What was the solution?

Thanks a bunch
Steve
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Old 09-13-2021, 10:27 PM   #2
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Atwood 8940 aclc
Is this in a Park Model?
'aclc' denotes it is 120VAC gas furnace

Curious what all you have replaced??

Lights for few seconds and goes off........THAT is Flame Sensing/Proving issue
Spark Electrode has Dual functions:
*High voltage spark for ignition if gas
*return path for Flame Proving Signal to circuit board
(Flame generates an AC current that is rectrified into a milliamp current that circuit board must 'see' to keep power on gas valve and keep it open...no Flame Proving/AC gets dropped off gas valve solenoid)

***AC Powered furnace has a transformer with 120VAC Input and 24VAC output to controls

Most common failure to prove flame:
Spark Electrode Gap (S/B 1/8")
Spark Electrode wire to circuit board bad---check continuity of wire
Spark Electrode Bad....ceramic insulator crack allowing Flame Signal to go to ground (replace electrode)
Circuit Board Bad

If issue was gas valve it would NOT light off to begin with
**Both solenoids have to function to open valve
**You can check each solenoid resistance...S/B 30-50 ohms each
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Old 09-14-2021, 01:59 PM   #3
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replaced circuit board , and the following
tem #53 Burner Assembly 35491
Item #6 Burner Orifice 31077
Item #51 Electrode (Single Sense) 34570
Item #52 Limit Switch 37021
Item #36 Sail Switch 36680 (31377)
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Old 09-14-2021, 10:24 PM   #4
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Quote:
Originally Posted by Sdcookie View Post
replaced circuit board , and the following
tem #53 Burner Assembly 35491
Item #6 Burner Orifice 31077
Item #51 Electrode (Single Sense) 34570
Item #52 Limit Switch 37021
Item #36 Sail Switch 36680 (31377)
So basically dang near rebuilt it......
Throwing parts will EVENTUALLY cure the problem

Ohms check the gas valve soelnoid BEFORE you buy/replace that

The 'other' thing you should check before buying anymore parts is the LP System Pressure

LP System S/B 11"WC with at LEAST 50% of appliances in service
***Need manometer to check Regulator output
**S/B test port on Furnace gas valve where you can connect the test tubing/nipple

DIY Manometer
https://www.loveyourrv.com/building-...ube-manometer/
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Old 09-20-2021, 05:28 PM   #5
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So don't I feel like an idiot but at least I have a brand new rebuilt furnace.

It was the regulator. As soon as I replaced it the furnace fired right up.

Now I am an expert at an expense....lol
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Old 09-20-2021, 09:42 PM   #6
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Don't sweat it.......

LP System Pressure is easily over looked
Proper system pressure is 0.4 psi (11"WC)
At 0.36 psi (10"WC) or 0.5psi (14"WC) appliances do not function properly
*difference of 0.04 - 0.1 psi

Very narrow range 2% to 9.5% air/fuel ratio


Got a new LP Reg AND a rebuilt furnace....woo hoo!
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