I have a Dometic brand AC unit with a analog thermostat. When I turn the thermostat to off there is a humming noise coming from the AC unit on the roof. The only way it stops is to flip the breaker. The AC still works and blows cold. From what I gathered online it seems that the thermostat is not working properly to shut of the condenser and I should replace it with a digital thermostat. I was thinking that would be a cheap fix. I asked a RV repair shop in the area and they said there is something I would have to change on the AC unit itself to make the digital thermostat compatible. The kept going on about a circuit board and a thermostat (Same thing?) and both would need to be replaced for the tune of $250. Is there any truth to this? Shouldn't I just be able to get a cheap thermostat from Walmart that runs on batteries?

blakeb wrote:...Shouldn't I just be able to get a cheap thermostat from Walmart that runs on batteries?I really do not have an answer regarding the electronics behind the air conditioner. But to respond to his question ^^^^. Why not go ahead and get a "cheap" thermostat and try it. Nothing lost except a small bit of $$$ if it doesn't work. And if it does, ... well then ... you've saved yourself $250, minus the cost of the thermostat.Try it, see what happens.I think, if you were to unwire the thermostat the humming should shut off. If it doesn't, then there's more going on than just the thermostat.

DutchmenSport wrote:I think, if you were to unwire the thermostat the humming should shut off. If it doesn't, then there's more going on than just the thermostat. That is a brilliant and simple point. Thank you. I plan on getting one and trying it but I am not the smartest on these subjects. So I thought I would ask so if anyone gave a hard no it would save me $20

It's not the thermostat on the wall.Sounds to me like a stuck relay for the compressor. Check out this diagram, borrowed from someone here on rv.net.Your fan motor relay is operating properly, turning off the blower, however the compressor relay is stuck on, leaving the poor compressor running (the hum noise) without any cooling fan.

A cheap thermostat with battery power should be able to replace the analog thermostat with no problem.You could easily enough disconnect the existing thermostat and manually connect the relevant wires to see if the AC works properly, just as the thermostat is doing.If it's like mine was (with the slide-rule analog thermostat), there are six wires: +7.5V, ground, cool, heat, low fan, and high fan. The control signals get connected to ground to activate them. Grounding low fan should turn on the fan on low; grounding both low an high fan will turn the fan on high speed. Grounding the cool signal will turn the AC on (and the fan should be turned on at the same time). Grounding the heat wire will turn the furnace on. The +7.5V signal powers the Domestic thermostat, and is unused with a battery thermostat or with manual testing.

GordonThree wrote:It's not the thermostat on the wall.Sounds to me like a stuck relay for the compressor. Check out this diagram, borrowed from someone here on rv.net.Your fan motor relay is operating properly, turning off the blower, however the compressor relay is stuck on, leaving the poor compressor running (the hum noise) without any cooling fan.Is this something I can fix?

AC unit humming when I turn off the system. | Good Sam Community

11 Replies

Dave_H_M
Explorer II
Feb 08, 2018
I had the exact same thing happen some time ago (cam't remember exactly, so it has been a while).

Anyway Gordon would have had the answer then.

At that time I talked to someone at PPL motor homes (think it was) and he said that the whole board in the AC had to be replaced since that relay is permanently integrated into the board. I got him the data he wanted and ordered a new board. Wala for a hundred and a few bucks it was fixed.

That particular board was serviced by removing the inside trim panel and locating it where the romex came in for the power.
GordonThree
Explorer
Feb 08, 2018
Any luck Blake?
GordonThree
Explorer
Feb 05, 2018
blakeb wrote:
Well I unwired the thermostat and turned the power back on. Still had the humming noise so I am assuming the thermostat isn’t the issue like mentioned. I just unwired the cool and the power from the thermostat.

That's probably a sufficient test. If you're feeling up to it, you can open up the ceiling unit (inside the rv), there should be a separate wiring panel to open once inside there, and when open the control board and relays should be visible.

You should also see a similar service diagram like the one previous.

Turn the power on and then use something plastic or wood to bang on each relay. Give it a sharp hard whack. If the relay is stuck that should unstick it and the compressor will shut off. Make note of which one it was, and share a picture with us.
blakeb
Explorer
Feb 05, 2018
Well I unwired the thermostat and turned the power back on. Still had the humming noise so I am assuming the thermostat isn’t the issue like mentioned. I just unwired the cool and the power from the thermostat.
DiskDoctr
Explorer
Feb 05, 2018
Tip: When choosing a thermostat, we decided to go with a digital NON-programmable version. Turn up or down, that is it. It's a pain to change all the programs all the time ;)

Hope you find your solution quickly ;)
GordonThree
Explorer
Feb 05, 2018
The relay should be a replaceable part. BUT

I'd go with DrewE's recommendation first; disconnect the existing thermostat and simulate a new one, touching the control wires to ground. You won't get a shock, it's very low voltage.

Specifically touch high fan wire to ground, and then without disconnecting, add the cool wire also to ground. let it run for a few min, and then disconnect all the wires. listen to see if the compressor stops. If it does, great, just need a new thermo. If not, bummer, probably need a new relay.
blakeb
Explorer
Feb 05, 2018
GordonThree wrote:
It's not the thermostat on the wall.

Sounds to me like a stuck relay for the compressor.

Check out this diagram, borrowed from someone here on rv.net.

Your fan motor relay is operating properly, turning off the blower, however the compressor relay is stuck on, leaving the poor compressor running (the hum noise) without any cooling fan.

Is this something I can fix?
DrewE
Explorer II
Feb 05, 2018
A cheap thermostat with battery power should be able to replace the analog thermostat with no problem.

You could easily enough disconnect the existing thermostat and manually connect the relevant wires to see if the AC works properly, just as the thermostat is doing.

If it's like mine was (with the slide-rule analog thermostat), there are six wires: +7.5V, ground, cool, heat, low fan, and high fan. The control signals get connected to ground to activate them. Grounding low fan should turn on the fan on low; grounding both low an high fan will turn the fan on high speed. Grounding the cool signal will turn the AC on (and the fan should be turned on at the same time). Grounding the heat wire will turn the furnace on. The +7.5V signal powers the Domestic thermostat, and is unused with a battery thermostat or with manual testing.
GordonThree
Explorer
Feb 05, 2018
It's not the thermostat on the wall.

Sounds to me like a stuck relay for the compressor.

Check out this diagram, borrowed from someone here on rv.net.

Your fan motor relay is operating properly, turning off the blower, however the compressor relay is stuck on, leaving the poor compressor running (the hum noise) without any cooling fan.
blakeb
Explorer
Feb 05, 2018
DutchmenSport wrote:

I think, if you were to unwire the thermostat the humming should shut off. If it doesn't, then there's more going on than just the thermostat.

That is a brilliant and simple point. Thank you. I plan on getting one and trying it but I am not the smartest on these subjects. So I thought I would ask so if anyone gave a hard no it would save me $20