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Old 03-20-2017, 02:33 PM   #1
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Rooftop AC Compressor coming on when system is OFF???

Hi All,
I've got three Dometic Duo Therm (630515.321) rooftop heap pump units (Comfort Control Center thermostats).
One of them was acting flakey, and I found the breaker tripped a couple-times. Now it's worse, and I know the source of the tripped breakers...
The compressor is turning itself on, and since the condenser fan isn't running, it loads up and trips the breaker.
It's gone from sporadic to constant; now with the thermostat off, the compressor starts running when I turn on the circuit breaker.
Any idea where to start looking for the issue?
Is this as simple as a failed relay in the rooftop unit?
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Old 03-20-2017, 04:54 PM   #2
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It could be a failed relay, a bad fan or starting condenser, or bad control board. You'll have to check each one, eliminate it and move to the next. I'd start by checking the starting condenser to see if it has the secondary wiring to start the fan motor. Look for bulges in the condenser top, a sure sign it's bad. See if you can disconnect the compressor and start the fan alone.
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Old 03-21-2017, 11:34 AM   #3
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Quote:
Originally Posted by BFlinn181 View Post
It could be a failed relay, a bad fan or starting condenser, or bad control board. You'll have to check each one, eliminate it and move to the next. I'd start by checking the starting condenser to see if it has the secondary wiring to start the fan motor. Look for bulges in the condenser top, a sure sign it's bad. See if you can disconnect the compressor and start the fan alone.
Thanks.
Hmmm. I found a wiring diagram (attached). Maybe this will be a little simpler than I thought.
For the compressor to run, the black compressor wire would need to be energized, right? It looks like that's a straight run from power-in through the K6 relay.
So either the relay is bad, or something on the board is bad and energizing the relay.
Could any of those other condition cause only the compressor to run as soon as the breaker is turned on?
Attached Files
File Type: pdf Wiring_Diagram.pdf (74.7 KB, 93 views)
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Old 03-21-2017, 11:52 AM   #4
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I agree with you, now you'll have to determine if just the relay can be replaced. Often they are soldered onto the control board and the board will have to be changed out.
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Old 03-21-2017, 12:12 PM   #5
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Thanks. I'll poke around on the roof and see what I can figure out.
Ha! Just saw your signature. Reminds me of a Kharmann Ghia I got in high school. The thing sat in a field for years and had been through two floods. Gas pedal was stuck to the floor, and got bumped into 4th gear before we took off towing it home. The dang thing started up and was pushing us down the road!
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Old 03-21-2017, 05:55 PM   #6
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So I have Direct TV and a wireless thermostat in my unit.... my furnace/AC used to start on its own and run and run... A phone call to the tech line found that there is a interference from the two remotes.... and the TV remote can trigger a timer circuit in the furnace/AC circuit that you can't do with the wireless thermostat....

In my case there is NO updated circuit board... but I found a work around... I hide the TV Remote when I shut down the TV and I use the thermostat to start/stop the furnace/AC (2) times before I leave the unit....

We looked to see if there was a mechanical thermostat that we could wire into the furnace/AC and throw away the existing wireless thermostat.... but tech support said there is no way to disable the receiver in the ceiling for the wireless thermostat....

OH YEA the other option is to replace the AC.... We also, added a mechanical 12+ switch which supplies the controller to the furnace/AC.... we make sure that switch of off when we don't want it to cycle.... its uncanny when it might start...
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Old 03-21-2017, 06:02 PM   #7
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Thanks.
I'm getting more confident this is a relay issue. I disconnected the 120V leads to the K6 relay, removed the fuse, and unplugged the 12v plug, and I still get continuity across the relay.
So I think I'll go ahead and order the control board. Then if I get a wild hair, I might try obtaining a new relay and soldering it on the board for a spare.
Who am I kidding... I won't get around to that...
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Old 03-22-2017, 05:20 AM   #8
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Bad K6

Spent my life in AC business. K6 is shorted closed.
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Old 03-22-2017, 07:20 AM   #9
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Thanks everybody. I ordered a new control board.
Then I need to figure out why the middle AC blower only runs on HI regardless of the fan setting on the Comfort Control.
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