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Help me fix my furnace!

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I have a 2018 Terry Classic 21 trailer with a Dometic/Atwwod AFMD 30121 forced air furnace. Recently, I turned it on and cold air blew but the burner never came on. I checked the propane and it was turned on. I tried several times, but nada. So I took out the furnace (not easy!) to pull and clean the sail switch, which appears to be functioning. With the furnace out of its cubby, and with the hoses disconnected, but the exhaust pipe inserted,  I am getting 12V at the thermostat and to the furnace itself. But now the blower doesn't come on. The circuit board has an LED, but it is not lit, even though I have confirmed that the board itself is getting 12 volts. Is there a sequence for troublshooting these things?  Do I need a new circuit board? Is there an interlock switch that keeps it from functioning when it is removed from its housing? This is my first attempt at repairing a furnace, but I have some electronics skills. Any and all suggestions are appreciated.

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Welcome to the Escapee forums! We will do all that we are able to help you. I believe that you have the newer model furnace and if I'm right this should be the Atwood service manual for it. It this isn't for the model that you have. let us know and we will look farter. Here is a diagnostic guide that also may be helpful.

Do you have the thermostat leads shorted together as that must be done in order to get power to the circuit board?

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    There should be two blue wires from the furnace. They need to be put together to turn the furnace on.

once the furnace turns on then listen for a dull thud, which will be the gas valve opening. A

so listen for a clicking sound.


Also make sure you have the 12vdc wires connected properly.

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I don't know your furnace operation but FWIW on some when the T stat calls for heat the blower starts running,,,,,,,Then IF there's adequate air flow and its functioning properly (Ive seen many switch failures or even them being blocked) the sail switch closes and IF the high temp limit switch is closed (as should be typically but they can fail) the circuit control board will cause the gas valve to open (can hear distinct clunk) and the HV ignitor tip will fire spark (snap snap snap) and if there's adequate gas present if will light..

 I suspect/check the T stat (closure and 12 VDC to furnace) then for sail switch closure then the gas valve opening and the HV Ignitor Tip sparking and the gas burner lighting. Sure Control Boards go bad HOWEVER  Ive more often seen the sail switch or the HV Ignitor Tip (hairline crack or carbon trace or not gapped correct) cause problems. Ifffffffff the sail switch never closes nothing else happens but if it does then the board has to cause the gas valve to open and Ignitor Tip to fire. As Wrnkrvr said, listen (or feel) to see if the gas valve clunks open ??  If there's air in the gas lines it can take some time to bleed out. Is there an On Off toggle switch on the furnace ?? Is there plenty of gas ?? Check all fuses !!!! maybe thats it !!!! SIMPLE THINGS FIRST  No 12 VDC or no Ground, no working  

PS I have successfully performed troubleshooting with them OUT of the enclosure, but Im puzzled why when the T stat calls for heat the motor doesn't start turning??????? If the T stat isn't sending  12 volts down it wont spin so the sail switch cant make up followed by the boards opening the gas valve and sending HV to the ignitor tip. There has to be motor spin BEFORE anything else happens which takes me back to no 12 VDC (switch or fuse or bad wiring connection or missing ground) ... I once had a t stat with the hard left FULL OFF position that opened the circuit failed so EVEN when I turned up the temp no 12 volts was present to start the blower. Check the T stat also along with fuses and any onboard On Off switches and wiring connections and grounds..

 If all else fails read the instructions lol and the manuals troubleshooting tips NOT me. Unfortunately circuit boards do fail but try the cheap easy simple things FIRST like connections and for 12 VDC and insure a good ground connection.........

    John T


Edited by oldjohnt
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