2015 Bighorn 3750FL - Dometic Rear Air Conditioner

I have a 2015 Bighorn 3750FL and the rear air conditioner in the bedroom is not working. Is there a separate circuit breaker or fuse for this unit? It is a Dometic. The thermostat does not light up and it appears there is no power to the unit. I swapped the thermostat with the one in the front and it works fine. I went on the roof and took the thermostat with me and hooked it up to bypass the phone cord and it still did not work. I took the electronic control board out of the roof unit and moved it to the front air conditioner and it works fine. I took the one from the front and moved it to the back and still nothing. I checked all the circuit breakers and fuses and then I found another set of fuses behind the kitchen drawers that have no labels on them. I checked all those too and they are good. The only thing it can figure out is there must be a separate fuse or circuit breaker somewhere else that I am not aware of.

The air conditioner has its own 120V AC circuit breaker; probably a 15 amp breaker. It'll be in the main circuit breaker panel.

Perhaps someone knows how the thermostat is getting DC power. I would think it's from a fuse in the main fuse box, but maybe someone knows for sure.

How are you checking the fuses?

danemayer said:

The air conditioner has its own 120V AC circuit breaker; probably a 15 amp breaker. It'll be in the main circuit breaker panel.

Perhaps someone knows how the thermostat is getting DC power. I would think it's from a fuse in the main fuse box, but maybe someone knows for sure.

How are you checking the fuses?

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There is a 15 Amp breaker in the main circuit breaker panel that is labeled A/C. There is also a fuse panel with fuse that is labeled for A/C. If you pull the fuse out a Red LED light lights up to tell you it has been removed and it is good or bad. This other hidden panel behind the kitchen drawers that is not labeled, I pulled the fuses out and checked to see if the wires were separated. You can tell by looking at them if they are burnt. The thermostat has a cable that looks like a phone cord that routes to the air conditioner on the roof. I took the thermostat up to the roof unit to bypass the cable and still nothing. That's what led me to think there is no power coming to the unit as I assume the thermostat gets its DC power through the control board inside the A/C unit on the roof. I would think there would be 2 separate circuit breakers, 1 for each A/C unit but there is only 1 and only 1 fuse. The manuals I found online don't show anything about this panel behind the drawers, so I thought maybe there was another hidden breaker panel somewhere too.

Thanks,

I've never heard of a hidden panel with a second set of fuses in a Bighorn. Are you the first owner?

heathcote said:

I have a 2015 Bighorn 3750FL and the rear air conditioner in the bedroom is not working. Is there a separate circuit breaker or fuse for this unit? It is a Dometic. The thermostat does not light up and it appears there is no power to the unit. I swapped the thermostat with the one in the front and it works fine. I went on the roof and took the thermostat with me and hooked it up to bypass the phone cord and it still did not work. I took the electronic control board out of the roof unit and moved it to the front air conditioner and it works fine. I took the one from the front and moved it to the back and still nothing. I checked all the circuit breakers and fuses and then I found another set of fuses behind the kitchen drawers that have no labels on them. I checked all those too and they are good. The only thing it can figure out is there must be a separate fuse or circuit breaker somewhere else that I am not aware of.

Click to expand...

Well, it appears you’ve eliminated all potential problem areas except for the problem itself. Time to check for a mechanical issue, say a non voltage at the unit caused by perhaps a bad wire or something other than a fuse or breaker, or perhaps a capacitor. I say this often, but one more time won’t hurt, it’s probably something simple that will be easy to fix once you find it. Good luck.

Here’s what a “failed” capacitor can look like.




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Hi, I’m having the same problem with the a/c unit and did the exact same things as you. Please help me resolve my problem.