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Pogo · 07-17-2018, 08:56 AM

Our front Duo-Therm, model 600312.321 cooling only A/C works great.
But, once in a while, the compressor ONLY will start when the thermostat is off. I removed the thermostat wiring from the back of it, and it still did it.
So, I had to shut off the circuit breaker to stop it. I went on the roof and expected to find a sticking compressor contactor, (similar to a house A/C), but there is no contactor.
Other times, the A/C and fan both work together, and run just fine. So what tells the compressor to run? (It''s not the thermostat sometimes) Hoping for more of an answer than to say it's a wiring issue.

ej1144 · 07-17-2018, 09:16 AM

Quote:

Originally Posted by Pogo

Our front Duo-Therm, model 600312.321 cooling only A/C works great.
But, once in a while, the compressor ONLY will start when the thermostat is off. I removed the thermostat wiring from the back of it, and it still did it.
So, I had to shut off the circuit breaker to stop it. I went on the roof and expected to find a sticking compressor contactor, (similar to a house A/C), but there is no contactor.
Other times, the A/C and fan both work together, and run just fine. So what tells the compressor to run? (It''s not the thermostat sometimes) Hoping for more of an answer than to say it's a wiring issue.

What kind of thermostat do you have

Ljwt330 · 07-17-2018, 09:37 AM

Just a guess.

Since you removed the thermostat wiring from the AC unit itself, the problem has to be with the control inside the unit, something with which I am unfamiliar. Wherever the thermostat wiring connected to the unit is the place to begin. If there is a circuit board or a pig tail there, that would be my first suspect.

Someone who is AC experienced will certainly add to this and point you in the right direction. From what I understand from your post, you have eliminated the thermostat and the wiring to the AC as a suspect.

**Lt Dan** · 07-17-2018, 02:15 PM

Depending on the age of your unit, the compressor relay may be permanently attached to a control board somewhere inside the AC.

Pogo · 07-19-2018, 02:51 PM

I will go back up to the front a/c and look for a circuit board. The unit is a 2004 vintage. Apparently, the relay that starts the compressor has sticking contacts, but the fan relay does not stick.

Ljwt330 · 07-19-2018, 06:32 PM

Quote:

Originally Posted by Pogo

I will go back up to the front a/c and look for a circuit board. The unit is a 2004 vintage. Apparently, the relay that starts the compressor has sticking contacts, but the fan relay does not stick.

Be very careful of the capacitors. If not familiar, do a little "Googling" and learn locations and how to discharge.

bruceisla · 07-19-2018, 06:44 PM

Quote:

Originally Posted by Pogo

Our front Duo-Therm, model 600312.321 cooling only A/C works great.
But, once in a while, the compressor ONLY will start when the thermostat is off. I removed the thermostat wiring from the back of it, and it still did it.
So, I had to shut off the circuit breaker to stop it. I went on the roof and expected to find a sticking compressor contactor, (similar to a house A/C), but there is no contactor.
Other times, the A/C and fan both work together, and run just fine. So what tells the compressor to run? (It''s not the thermostat sometimes) Hoping for more of an answer than to say it's a wiring issue.

Depending on whether your configuration is multi-zone (1 thermostat and 2 or more A/C units, or single zone (1 A/C and 1 thermostat), the control board may be located topside (under the shroud and under a cover front/passenger side of A/C or the control board can be separate ... sometimes in a metal box above the air filter in the cabin air box (vents).
The relays/contactors are on the board.

Trace the wiring backwards from compressor or fan ... either/both will go to controller.