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Flyitall · 01-16-2022, 05:45 PM

Hoping there is an easy fix!
My Dometic DFMD35 was working fine for several nights then we had warmer weather for 3 nights and it didn’t need to come on. I then had the propane tank filled. The next night it failed to come on. I got the flame signal on my interior control panel, but the blower did not turn on. My first thought was the propane fill may have caused this, but I cannot find anything amiss. I retried the start procedure many times to clear any potential air in the line. My propane water heater works fine, the furnace fuse is good, the furnace off/reset switch behind the furnace outside panel does not fix it.
So…I am beginning to think the failure after the propane fill was merely coincidental.
Troubleshooting further, I see a failure light sequence on the internal control board of a light flash every 3 seconds indicating an air flow problem. I am thinking the most probable failure is the sail switch. I’ll be in a spot where I can start to dismantle it in 2 days. We are in a spot now with power and getting by with a small electric heater until I get this fixed.
I thought I would reach out to the forum first to see if I am missing something obvious before starting the disassembly. Any thoughts?

Old-Biscuit · 01-16-2022, 08:12 PM

Sail Switch Stuck CLOSED will not allow furnace to run...circuit board checks and if it is not open the circuit board will not send DC to Fan Motor

Only One Blue wire in connector on circuit board (2nd one) should have 12VDC Voltage....until Fan runs then when Sail Switch closes the other Blue wire in that connector (top one) will have 12VDC Voltage

See wiring diagram pg 8 (AFM manual---same as DFM)
*AFM---Atwood Furnace Medium/DFM---Dometic Furnace Medium
https://manuals.heartlandowners.org/...wood%20AFM.PDF

Flyitall · 01-18-2022, 07:08 PM

Both the sail switch and the over temp sensor are working properly. I did find the voltage to the board is only 10 volts and should be 14.4 from my house batteries. I am currently trying to run this down but can’t follow the orange lead thru the cable bundles and walls. Will try tomorrow from the other end.

Old-Biscuit · 01-18-2022, 08:27 PM

Low DC Voltage.......

DC to furnace should be from a FUSED source

Good luck IDng the wiring!

Toby Dog · 01-19-2022, 04:42 AM

Would running a good 12 volt lead from a known good battery to the circuit board be worth trying?

Flyitall · 01-19-2022, 07:55 AM

Good idea! I’ll try that if I can’t figure out what’s happening with the existing wire.

Old-Biscuit · 01-19-2022, 08:37 PM

Don't forget about having a good ground!

Flyitall · 01-21-2022, 02:50 PM

I found my issue!
First, my fuse box labels were not correct which led me astray.
Second, the actual fuse in the distribution box was loose and not always making contact. There is a long line of automotive type blade fuses stacked closely together. When I tracked down the correct fuse, and detected looseness, I pulled it and examined it and could see the blade on one side was gouged by the mating connector and spread the metal on the connector. I think when the fuses were installed at the manufacturer, they forced two fuses in that spot and then removed one. This left the metal spread out so that the single fuse never made good contact. It did work fine for the few times I used it, but ultimately failed open. I just squeezed the metal back down and the fuse is snug.