Dometic Duo Therm AC Compressor won’t shut off

[DiverBob57]

We have a 2009 coach with two Dometic Duo Therm A/C 3308120 units installed.

Issue I am experiencing: When I turn on the breaker for the rear unit, I can hear the compressor start humming even though the control panel is still set to “Off”. The fan does not start running until I move the thermostat to a lower temp OR set to “On” position. Once desired temp is reached, the fan stops but I can still here the compressor humming. After an hour or so the coil freezes up and airflow is cut down to almost nothing.

Does this sound like a problem with the control panel allowing power to the compressor while in the “off” position?

[Bsipe01]

Originally Posted by DiverBob57

We have a 2009 coach with two Dometic Duo Therm A/C 3308120 units installed.



Issue I am experiencing: When I turn on the breaker for the rear unit, I can hear the compressor start humming even though the control panel is still set to “Off”. The fan does not start running until I move the thermostat to a lower temp OR set to “On” position. Once desired temp is reached, the fan stops but I can still here the compressor humming. After an hour or so the coil freezes up and airflow is cut down to almost nothing.



Does this sound like a problem with the control panel allowing power to the compressor while in the “off” position?



Attachment 319823



Attachment 319824

It's not likely the thermostat, but more likely a sticking relay in the unit's control box. Sometimes the contacts weld together, that would explain the running with stat set to "OFF". Refer to the schematic to see how it's controlled.