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Old 09-01-2021, 05:45 PM   #1
Join Date: Sep 2020
Posts: 38
No thermostat power at Atwood furnace

Had the Motorhome out this weekend, in the mountains, and of course the furnace wouldn't start. Its a Thor West Residence Model with an Atwood 8940-IIICLP furnace. I've done enough diagnosis to tell that I am not getting power on the blue wire to trigger the furnace to run. There is power on the red wire so with some jumper magic I can fire the furnace up with all of the safety still in place.

But at the Duo-Therm (Comfort Control) Thermostat I just have an RJ11 telephone cable. The thermostat seems to be working for everything else so where do I look next for the issue? Is there a fuse somewhere or a main control box?

Or is there a way to test the Duo-Therm?
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Old 09-01-2021, 09:27 PM   #2
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RED wire comes from a FUSED source and is always HOT at furnace

Red Wire goes to/thru On/Off Switch Circuit Breaker...Manual Reset
Blue wire TO thermostat S/B HOT if CB is closed

See pg 12 for wiring
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Old 09-01-2021, 10:28 PM   #3
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Okay so I have verified that the control board in the front A/C unit is not sending power down the blue wire to the furnace. But I don't know how to determine if its a thermostat issue or the control unit?

Playing with the thermostat I can't get the relay to trigger in the control box, another weird thing is that the thermostat only randomly seems to allow selection of FURNACE when selecting mode, the FAN and A/C are always available. I usually need to power the thermostat off and then back on, and then sometimes FURNACE is shown. But as soon as I hit MODE getting back to the furnace is no longer an option.

Since the thermostat uses an RJ11 connector are there some wires I can connect to call for furnace heat? This would help me identify the thermostat or control board as the issue.
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Old 09-07-2021, 09:15 AM   #4
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While after a ton of investigation I wasn't able to find a way to "hot wire" the thermostat to call for heat. But what I did find is that I could jumper the blue wires at the front A/C unit and the furnace would come on. The thermostat wouldn't even show furnace as an option anymore and therefore you couldn't trigger the relay. I tried cleaning the contacts and a new RJ11 cable but nothing worked. I ended up pulling the control unit from the back of the motorhome and swapping it to the front. Everything now works again.

After everything was working properly I tried and replaced the old 4 button thermostat with a new Dometic 3314080 thermostat that also uses the 4 wires, but even though it would fire up it didn't register the zones correctly and could not see the furnace connected to it.

It does appear though that on these 600 series units with the 4 button thermostat that the RJ11 cable is flat. Not crossed over like a lot of posts would suggest. I compared wire for wire the cable in the wall of my motorhome with a spare telephone cable and they were identical.
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ace, atwood, furnace, power, thermostat

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