Dometic DFMD35 Furnace Issues

[Sherwolfe]

So i de-winterized my 2019 29RLDWE trailer. Fired up the furnace and it worked for a number of cycles but then it would do the "start blower then shut down" So figured the sail switch was the issue. Took it out tested it with a meter and it had good continuity. Blew some air on it and replaced it. Furnace worked great for about one day. Now, same issue with it shutting down without ignition.

I did the "Reset" switch and it worked for a number of heat cycles, now it's doing the blower with no ignition.

The code indicates a "Ignition Lockout Fault" However if I reset the switch it will work for a number of cycles just fine. So any suggestions?

Tanks are full, no air in lines as indicated by the number of full heat cycles it will do. Just makes no sense that it will work just fine for about a day then just stop heating unless i do a reset.

[bsmith0404]

When you turn it on and the blower starts, can you hear it trying to ignite? Typically if you stand outside by the exhaust it’s easier to hear. If it’s trying to ignite, then the sail switch is good and most likely a bad board. If it’s not trying to ignite, could be the sail switch. Since you checked the sail switch with the meter, it’s probably good. You can pull the board and have it tested. Basically, once the blower starts the sail switch will activate and tell the board it’s ok to ignite. Usually, if you have a bad board, it’ll still try to ignite, just doesn’t open the gas valve.

The last time I had a board go bad, it acted very similar to what you’re describing. Would work a few times after resetting, then finally quit working completely.

[Sherwolfe]

Originally Posted by bsmith0404

When you turn it on and the blower starts, can you hear it trying to ignite? Typically if you stand outside by the exhaust it’s easier to hear. If it’s trying to ignite, then the sail switch is good and most likely a bad board. If it’s not trying to ignite, could be the sail switch. Since you checked the sail switch with the meter, it’s probably good. You can pull the board and have it tested. Basically, once the blower starts the sail switch will activate and tell the board it’s ok to ignite. Usually, if you have a bad board, it’ll still try to ignite, just doesn’t open the gas valve.

The last time I had a board go bad, it acted very similar to what you’re describing. Would work a few times after resetting, then finally quit working completely.

I have never really ever heard the clicking of the ignitor on this furnace. Again, I'm am perplexed by the fact that I can do a reset of the furnace manually and it will work just fine for a day or more then just stop. Is it possible the board is bad and maybe the heat and cooling just happens to cause a break in connections? However, why would the reset fix it? Still scratching my head.

[bsmith0404]

Originally Posted by Sherwolfe

I have never really ever heard the clicking of the ignitor on this furnace. Again, I'm am perplexed by the fact that I can do a reset of the furnace manually and it will work just fine for a day or more then just stop. Is it possible the board is bad and maybe the heat and cooling just happens to cause a break in connections? However, why would the reset fix it? Still scratching my head.

Mine did the same thing for a couple weeks before it went out completely and the reset wouldn’t fix it.

[ChuckS]

All that “Reset” switch is doing is turning off the 12 volts DC to the furnace control board..

By doing so you are clearing the “Lockout conditions” the board detected thus shutting down the ignition and LP flow..

I’d be looking at that control board but first I would remove the junk sail switch and install another and evaluate

That sail switch is junk and has been junk for over two years now on that Dometic Atwood furnace ..

[Sherwolfe]

Originally Posted by ChuckS

All that “Reset” switch is doing is turning off the 12 volts DC to the furnace control board..

By doing so you are clearing the “Lockout conditions” the board detected thus shutting down the ignition and LP flow..

I’d be looking at that control board but first I would remove the junk sail switch and install another and evaluate

That sail switch is junk and has been junk for over two years now on that Dometic Atwood furnace ..

I agree, even though mine tested good I have another one coming and will replace it. While the reset does clear the code, I'm certainly curious as to why the furnace will then work for a day or so then stop. I mean I'm not messing with the sail switch when doing the reset, just seems strange. That said, I will replace the junk sail switch and see if this corrects the issue.

[ChuckS]

If the problem persists after a new sail switch then I would take a hard long look at the connections on the furnace control board.. physically removing, checking for corrosion and reconnecting...

If issue still persists the reset switch ( which is a 15 amp breaker ) on that Dometic Atwood furnace may be the culprit .. or the control board..

You could easily bypass that reset switch ( circuit breaker ) on the outer case of the furnace for a bit and evaluate.. The furnace is still protected via a 15 amp fuse on the inside power center panel

AND.. You did remove it and thow it out the window ( if it was a black looking ATC style fuse ) .. that little devil is an auto. resetting 15 amp fuse that has cost many folks a lot of unnecessary troubleshooting.. replace it with a standard ATC 15 amp fuse and see how things go for you.

[bsmith0404]

Depending on how far you are from a dealer/repair shop, you can take the board and have it tested. It takes them less than 5 mins. They didn’t charge me to test mine. A free test is a lot easier than a bunch of troubleshooting and bypassing. If the board tests good, then you can start all the other troubleshooting.

[chuckster57]

Originally Posted by bsmith0404

Depending on how far you are from a dealer/repair shop, you can take the board and have it tested. It takes them less than 5 mins. They didn’t charge me to test mine. A free test is a lot easier than a bunch of troubleshooting and bypassing. If the board tests good, then you can start all the other troubleshooting.

I would call first and make sure they have the test equipment. Of the 3 dealerships I worked at only 1 had the tester and it quit working while I worked there. It isn’t cheap, or I would have bought one by now.

[Sherwolfe]

UPDATE: New Sail Switch and furnace works fine.

Want to thank those who provided input, the sail switch was the issue. As I originally posted I did check, clean and test the old switch for continuity, all good. Even modified the mounting bracket to allow the blade to travel a little farther. Still perplexed by why it would work intermittently. I will just chock that up to maybe some sort of buildup on the contact that I couldn't get too. Regardless, new switch and it hasn't missed a fire up on time since.

[chuckster57]

Good deal.