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I am trying to diagnose the A/C system on a 2002 G20. I ran the climate control system diagnostic and get code 20 showing that the sensors are OK, but when I turn on the A/C the compressor clutch will not engage. I removed the compressor relay and measured with my meter to see if it was being commanded on and I see that the relay coil is never told to turn on.

Could this just be that the refrigerant pressure is too low?

Is there no diagnostic report for the pressure sensor?

If the easiest way to test this is to add refrigerant can I safely attempt to charge it myself? If so would I need to bypass the relay with a jumper to force the compressor to run?

Thank you!
 

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I have a similar issue, the AC relay is grounded by the ECM. I was told it could be the ECM but I have not replaced it yet. So I do not know yet.
 

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Put a pressure gauge on the low pressure line A/C port. If it shows no pressure or really low pressure, get a Freon kit with a gauge on it and start adding. If it’s just low on refrigerant, the clutch will engage once it gets up to pressure. If it doesn’t engage even when under sufficient pressure, check your A/C fuse and relay(s). Also, check under the car and around the engine bay at intervals while adding refrigerant. If you have a bad leak, stop filling and get it fixed, otherwise you end up draining the whole can on your driveway/garage floor. Like an idiot. I’ve been that idiot.
 

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Nelson, I pushed the relay coil to allow more freon in since it didnt activate itself automatically. My guess is that the thermo amp control is open and will not allow AC to activate. But I am not an expert in this model, so I am not sure.
 

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Nelson, I pushed the relay coil to allow more freon in since it didnt activate itself automatically. My guess is that the thermo amp control is open and will not allow AC to activate. But I am not an expert in this model, so I am not sure.
I’m certainly no expert, either. Did you check the pressure to ensure it has enough to activate the compressor clutch? In my experience, the most common problem is overwhelmingly a leak somewhere and loss of pressure. Best to rule that out first and then move on to electrical issues. P11s have auto air temp units, so I’m not sure if they have other components in them that can fail to function. In a P10, it’s pretty much just check for pressure/leaks, fuse, relay, and compressor itself.
 

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19853

Correct me if I am wrong, but according to the P11 ecu. . the only input signal is from pressure switch? Btw I do not have a leak in my AC system.
 

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Ok, so I checked my voltage on Pink/Blue wire on ac pressure switch and got 1.6v with freon in the system, 5v on Red/Blue wire. I also checked continuity for Pink/Blue wire to ECM and it is ok and also checked Red/Blue wire to the ECM and is ok. I thought my power would be coming from climate control but it was not. So my conclusion is that if my ECM input is ok and ECM is not outputting ground, then my ECM is bad. Also it could not be thermister because according to the diagram if thermister is open it would cut my 12v for the AC Relay, but both 12v is present in AC Relay 12v from battery and 12v from thermister wire. What do yall guys think?
 
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