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Old 09-18-2018, 03:57 PM   #1
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Do You Have An Atwood Furnace That Won't Light?

So did I. Everything seemed to be working fine in my 8535 -lll. Had a propane smell, blower worked, thermostat worked, just didn't want to light reliably or stay lit. Atwood/ Dometic was no help. Told me to look at the sail switch which I KNEW had to be working or the gas solenoid wouldn't actuate.

WARNING: DO NOT TRY THE FOLLOWING UNLESS YOU ARE COMFORTABLE WORKING WITH HIGH VOLTAGE AND LPG. I TAKE NO RESPONSIBILITY FOR ANY INJURY OR DAMAGE THAT MAY OCCUR! Before doing ANYTHING get yourself a furnace schematic and a furnace blow up diagram so you know what goes where.

I took out the igniter. Some fun. First you have to remove the exhaust which just pulls out after some persuasion. Then, shut off the LPG at the tank, disconnect the ignition wire and solenoid wire at the ingiter. Four screws and the gas fitting later you can persuade the burner to come out.

Now for the good part. Measure the space between electrodes. It should be about a quarter of an inch. Mine was an eighth. Gently bend the electrodes apart or together until you get about a quarter of an inch spacing. To test, hook the ignighter and gas solenoid wires back up to the igniter. Hang the igniter assembly from the door with the electrodes facing up. Fire the heater as you normally would. After the appropriate delays you SHOULD get a visable, repetitive, rapidly snapping spark between the electrodes for about five seconds. If you get no or a weak spark the problem could be the board or electrodes. Mine originally tested very weak and did not repeat, but when I opened the electrode gap to one quarter of an inch the difference was dramatic.

Reassemble and test. I highly recommend using SNOOP or some other leak detector at the propane fitting after you have reattached it. A leak could be deadly! Remember that the propane line will be empty so it will take a few tries to ignite. After all of this my furnace now lights and runs flawlessly.

Once again, this procedure is NOT for someone who does not know how to work around flammable gasses and / or high voltage. You could be killed, or even worse! You have been warned!
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Old 09-18-2018, 06:36 PM   #2
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Spark electrode gap too narrow...no spark
Spark electrode gap to wide........no spark


Atwood factory spec is 1/8" -- 3/16"


Here is link to 'Atwood Furnace Service Manual'
http://bryantrv.com/docs2/docs/hflamefurn04.pdf
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Old 09-19-2018, 06:12 AM   #3
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Quote:
Originally Posted by Old-Biscuit View Post
Spark electrode gap too narrow...no spark
Spark electrode gap to wide........no spark


Atwood factory spec is 1/8" -- 3/16"


Here is link to 'Atwood Furnace Service Manual'
http://bryantrv.com/docs2/docs/hflamefurn04.pdf
Appreciate the link I have the manual and, in my case anyway, the 1/8"setting does not produce enough of a spark rapidly enough to ignite the propane. I'm an engineer who makes my living troubleshooting in the field so I like to tinker. After playing with things, including verifying and setting the propane pressure, I found that with the electrode gap set at about 1/4" the burner fires every time. I've also found that everyone with furnace problems seems to have a Sail Switch fixation. If you get propane feed as I did, the sail is good as it is a permissive for propane in the firing sequence. After about 40 minutes of phone work getting through to Atwood / Dometic tech support and explaining the situation to them in detail, to be told that the Sail was the problem, or maybe a blocked exhaust was disappointing to say the least. The way I did things is a great diagnostic to see if the firing mechanism is working.

In any case, should anyone decide to remove their igniter, there are videos on YouTube to help you, but once again, this evolution needs to be done carefully and precisely to be safe.
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Rookies Plus Three Years. 1998 6330 National Tropi-Cal 460 Ford, full Banks kit, Trans-Command, full internal and external makeover and lots of suspension upgrades.
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Old 09-19-2018, 12:08 PM   #4
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Sail Switch only comes into the equation IF you are not getting DC to circuit board.......OR if nothing happens (no fan starts) which could be sail switch stuck closed.


Got DC to circuit board then issue is circuit board, gas valve/solenoid, spark electrode


Too wide of a spark gap MAY result in a strong spark BUT could be an issue for 'flame proving'
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Old 09-20-2018, 06:13 AM   #5
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Quote:
Originally Posted by Old-Biscuit View Post
Sail Switch only comes into the equation IF you are not getting DC to circuit board.......OR if nothing happens (no fan starts) which could be sail switch stuck closed.


Got DC to circuit board then issue is circuit board, gas valve/solenoid, spark electrode


Too wide of a spark gap MAY result in a strong spark BUT could be an issue for 'flame proving'

I'll have to see about that. I was pretty sure it was either the board or the igniter as, per the schematic, all of the permissives were working and I got gas, just no ignition. Actually on the few occasions when it DID light I was having problems with flame-proving with the smaller gap. So far it has been flawless in every test!
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Rookies Plus Three Years. 1998 6330 National Tropi-Cal 460 Ford, full Banks kit, Trans-Command, full internal and external makeover and lots of suspension upgrades.
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