Atwood 8535 IV fires three times wityh flame and then quits.

[MrVan]

I've done a search on RV.net for this kind of problem and there are a few closed threads but I'd like to try one more time to get some help. RV has not been used for a few months. Tested the furnace about a week ago and it would fire and flame three times and quit. Too outside cover off but didn't see anything obvious and the connections look good. Tried starting it several times an all of a sudden it started working right. Ran it for several minutes and all seemed ok. Then let it set in cool rainy weather and tried it again and today it won't start again even after several tries and even waiting between tries. Turn the furnace on, fan starts, a few seconds later can here the gas valve open, flame starts for a few seconds and shuts off. Does this three times and then finally the fan shuts off. I've repeated this sequence several times today. Get the "ignition lockout fault" three blinks of the led. Gas water heater and three burner gas stove work fine.
I've disconnected all connections and reconnected them several times in hopes to clean off any corrosion on the contacts. They do not look corroded at all. I've run a small but long wire brush in the exhaust port in case there might be any spider webs or anything in the exhaust pipe. Wire brush comes out clean with no noticeable obstructions. I'm stuck. Is the next step getting my control board checked?

[tatest]

Three tries on an Atwood usually means the sparker/flame sensor failed to show the tiny current that indicates the spark gap is inside a flame.

Any discontinuity or excessive resistance is enough to give this condition. The high voltage sparker can spark across a small gap in the wiring or overcome a bad connection, but when control board goes into resistance measuring mode, it sees open circuit, as if the flame failed to ignite.

Most of the sparker failures I've seen have been the HT lead burning inside the ceramic insulator, so the defect is not obvious.

Replace/adjust sparker first (particularly for the light three times failure mode), then suspect the control board. Too many good control boards get replaced before the sparker is suspected. I suspect this is because the control board is a much more expensive (and profitable?) part, and easier to replace than the sparker, in an Atwood.

[DaHose]

I had a similar issue on my water heater. WH and furnaces will have the same symptoms.

Replaced spark module first to no avail. Then installed a Dino board and the problem was fixed.

Of course, I then had a bad solenoid in the gas valve, but that was another can of worms.

Jose

[Chris_Bryant]

I'll say the circuit boards are replaced *way* more than needed- back in the days of the Suburban NT series and Hydroflame DC82 series, the circuit board was next to the fire box. This literally fried them. Dinosaur came out with a much higher quality board- fiberglass with plated through holes. It would last longer, but even Dinosaur finally made a board saver which relocated the board outside the furnace cabinet.
On today's furnaces (except the NT series- still), the board is in the return air flow, which keeps it cool. I used to replace a board a week- now I replace a board a season. They just do not go bad that often.

[SteveB]

Had to do the Dinosaur board in my 94 Dutchmen, Atwood furnace. Same symptoms as OP and it fixed it right up.

[MrVan]

Old control module still working after a month and a 1/2 in AZ

[pugslyyy]

Sounds like a bad control board to me. As others have said, the Dinosaur boards are the way to go for replacements.

I carry a spare water heater control board and a spare furnace control board with me.

[Dutch_12078]

I've changed the board on our last few RV furnaces to a Dinosaur Fan 50, and have never been sorry. I stick the OEM boards in my spares, but so far I've always ended up giving them to someone else having furnace problems.

[dougrainer]

The furnace will do what you have experienced if this happens.
1. The Spark Electrode GAP is not correct
2. The spark plug wire is damaged in anyway (nicks or cuts which degrades the millivolt return signal to the board showing flame)
3. The Spark Electrode has hairline cracks in the white porcelain insulator the metal spark plug wire is imbedded in (which also causes a disruption of the return millivolt flame voltage).
4. Then the Module board. Doug

[MrVan]

Stewed over this problem for awhile and wondered if it had anything to do with the wet cool weather we have had recently. Took a hair dryer to the furnace control module and in about 15 minutes of drying all of a sudden the furnace started working. Now running the RV at a higher temperature to have the furnace run more often which may dry things out. But ordered a Dinosaur replacement control module ... "Fan 50 Plus pins" just in case this problem shows up again.