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RClifton · 08-29-2024, 10:56 PM

Hi All,

I have a 2016 Heartland Bighorn 3750FL with 2-Dometic 15k Blizzard NXT A/C Rooftop units. On Monday I woke up to the Front A/C cycling on and off. When I went to the thermostat I could see the backlight was flickering and the dispay was going on and off. I powered the unit down so I could look at it later in the day (2am isn't a good time to be troubleshooting). The next day I powered the unit on, it powered up briefly still flickering with the display going on and off and then it died.

I removed the thermostat from the wall and swapped it with the one that controls the rear unit. The thermostat worked fine but the rear thermostat when plugged into the front cable was also dead so I headed up top. Removed the cover and could see that the drain had backed up and condensation was pooling in the plenum with a bunch of the wires laying in the water. I removed them dried them all, sprayed the rj11 wire with contact cleaner as well as the rj11 coupler but the thermostat was still dead. I though the control board might be dead and I have a spare so I replaced it, still dead. I took the thermostat up to the roof and plugged it directly into the black rj11 cable that plugged into the coupler and it was still dead. I then plugged the thermostat into the other black rj11 cable, which I believe would be for a second zone, still dead.

I can't find any decent info from Heartland or Dometic with regards to this thing, I measured and there is 120VAC at the unit but I'm not sure about 12V which I think is where the problem is. I can't find any fuses, all the documentation I can find says there is one on the control board but neither board has one. The part # for the control board is 3312227.000. I can't measure the rj11 cable as my meters probes are not small enough to reach the contacts and I am unsure where the 12V is supposed to be at on the control board itself as nothing is labeled.

Sorry for the long post but I'm running out of things to try and it's getting hot here in the next few days!

Does anyone have a similar Bighorn unit and could point me to where the fuse might be if there is one? There is a 10A fuse in my fuse box labeled A/C but I don't believe that's it as there is only one and removing it doesn't kill the rear A/C unit. Barring that does anyone know where the 12V is located on that particular control board so I could measure it at the board? My fear is there will be no 12V at the control board and I'll be back to square one looking for a fuse that may or may not exist.

Thanks!!

Oregonpharme · 08-30-2024, 08:34 AM

You should have two 12 VDC panels, one is the fuse panel you discuss, the other is a 12 volt mini-circuit breaker panel. Check the output on the mini-circuit breakers. There should be a page in your owners manual that will identify the location of the mini-circuit breaker panel.

d23haynes57 · 08-30-2024, 08:51 AM

Yes, the controls for the AC units relay on the house batteries. If the battery voltage goes low the controls will begin to fail. Check the house battery voltage and make sure things are getting charged.

RClifton · 08-30-2024, 09:15 AM

If you mean the little round resettable fuses my unit does not have a panel like that. I just have a panel with automotive style fuses directly above my circuit breaker panel. I have one 50A resettable circuit breaker in my generator bay but it cuts all 12V power.