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murphybad 06-30-2012 06:39 PM

New thermostat, compressor stuck on
I just replaced the thermostats in my new house. Within an hour or two of that, I noticed that one of my air conditioner compressors was on even when the thermostat was not calling for cooling. I even cut power to the thermostat, and the compressor stays on. The only way to shut it off is by flipping the breaker for the compressor. From my initial research, it sounds like this is probably a failed contactor. But I am just wondering, the timing seems odd. Is there anything else that could be going on? Could my new thermostat have made the contactor go bad? Is it just a weird coincidence? The air conditioner is pretty old, but until today as far as I know it was working fine.

murphybad 06-30-2012 06:40 PM

Also something I forgot to mention.

One time when I turned the breaker back on, I noticed that for maybe a minute or so, just the big fan on top of the compressor came on, then after about a minute the full thing kicked in. I don't know if that helps diagnose.

beenthere 06-30-2012 06:52 PM

What wires are connected where on the thermostat.

biggles 06-30-2012 08:11 PM

shut the stat OFF....shut the disconnect outside OFF.remove the condenser panel and check that the contactor if free when you push it in and out... remove stat off subbase...jump R to Y go out and see if the contactor is pulled in....nothing will be running just the contactor action i'm looking for...with the condenser disconnect off.unless the compressor is going off on thermal heat over load the condenser fan running has to come on when that contactor closes out there.. lets here back

murphybad 06-30-2012 08:29 PM


Originally Posted by beenthere (Post 954995)
What wires are connected where on the thermostat.

Red (R) -> R
White (W) -> W
Yellow (Y) -> Y
Green (G) -> G

The colors to terminals are the same as what the previous thermostat had. I copied it over directly.

murphybad 06-30-2012 08:29 PM

Thanks Biggles, I will try that and report back.

biggles 06-30-2012 10:34 PM

when you shut off the furnace /air handler that kills the control transformer within,and should drop the condenser fan/compressor right off...contactor might be locking up check the action for tomorrow got to be something easy

murphybad 07-04-2012 10:58 PM

Sorry for the delayed reply, been busy lately. It definitely was a stuck contactor. Replacing it fixed the problem. Thanks for the tips.

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