AC Troubleshooting Question

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VTRider001

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Jul 21, 2019
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My Coleman Mach AC compressor runs non-stop right now regardless of what the thermostat is turned to.  I had it set to auto at 72 degrees then woke up at 4am to a 62 degree camper.  Turned off the thermostat to find that the compressor wouldn't shut off. 

I turned it off at the breaker and let it sit for a few hours.  Then the compressor turns on as soon as you flip the breaker back on.  Furthermore, the low setting on the fan doesn't work, only the high setting on the fan.

I did some research and found that sometimes it could be a problem with the thermostat on the wall having a short with the yellow wire that goes to the compressor.  So I replaced the thermostat with the same thing. No change.

Could this be the controller board on the unit or a relay.  Do you know of a good video that will help me check continuity to diagnose this?

My feeling that I have lost control of the compressor and low fan that I lean toward the controller board but I'm not sure.

Our local shop is scheduled a month out and we have a trip planned in a week with a family of five in the middle of summer.  You get my drift. 

Any thoughts would be appreciated. 
Thank you.
 

SpencerPJ

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Smart ones on AC troubleshooting will be along shortly.  Just make sure you have a decent battery, silly stuff happens if not.  I do tend to agree about the control board.  $200 might not be a bad investment.  You could always call a Mobile Mechanic and pay hourly rate and parts.  Hopefully others will be along shortly and help you troubleshoot.
 

Henry J Fate

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Disconnect the thermostat and check to see if the unit continues to run full time. That test could rule out the thermostat and give you some more info to consider.
 

Neal

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I have seen cases where the compressor contactor points were stuck closed (no voltage to contactor coil) which would make the compressor run any time current was on. In such case, install new contactor. If you have 12 or 24 volts to the contactor coil, while the thermostat is off, then something else is telling it to run.
 

xrated

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If you've replaced the thermostat with the correct replacement brand/model...AND hooked up the wires correctly....AND assuming there had been no damage to the wiring between the thermostat and the A/C unit, the finger is pointing at the control board in the A/C unit.
 

xrated

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Neal said:
I have seen cases where the compressor contactor points were stuck closed (no voltage to contactor coil) which would make the compressor run any time current was on. In such case, install new contactor. If you have 12 or 24 volts to the contactor coil, while the thermostat is off, then something else is telling it to run.

Aren't most of the compressor contactors built into the control board these days?.....in other words, not replaceable as an individual part but as a whole control board.
 

Neal

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xrated said:
Aren't most of the compressor contactors built into the control board these days?.....in other words, not replaceable as an individual part but as a whole control board.
We are not familiar with RV rooftop units, but I would bet you are probably right.
If you can determine it is the compressor relay/contactor on the board, and can get a part # off it, you might be able to order and replace the relay on the board. We were able to do this on our home dishwasher and if I remember correctly, saved possibly over $150.00.
 

John From Detroit

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Davison Michigan
xrated said:
Aren't most of the compressor contactors built into the control board these days?.....in other words, not replaceable as an individual part but as a whole control board.

A lot depends on the unit.. But generally if you know how, AND can find a suitable replacement they are replaceable.
 

VTRider001

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Jul 21, 2019
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Looking up top the connector is mounted directly onto the upper control board and it is all one piece.  I found it new on Amazon for $60.  I'll wire it in tomorrow and see if that does it.  Many times running on low power can weld the connectors together.  I'll let you know if that does it.  Thanks for all the input.
 

VTRider001

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The upper control board was the problem.  I replaced it today and it worked just fine.  The low setting on the fan was because I over-tightened the wire nut on the thermostat wire that controlled the low-speed fan.  It tore off the wire. 

Thanks for all the help with thinking through it.  Take care guys.
 
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