Automotive Air Conditioning Information Forum (Archives)

MONSTA on Thu April 19, 2012 7:13 PM

Year: 1999
Make: GMC
Model: YUKON
Engine Size: 5.7L
Refrigerant Type: R134a
Ambient Temp: 82
Country of Origin: United States

My truck's AC compressor will not turn off or cycle even if the controls are turned off.

What I know...

1. The relay has been replaced by a known good relay.
2. The system was recently charged with 2 12oz cans.
3. If I disconnect the HVAC control unit the compressor still runs.

Please help me diagnose and suggest the proper tools to buy. :-)

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Aloha from Maui

GM Tech on Thu April 19, 2012 8:26 PM

Must have been hot-wired by someone-- pull the relay and or fuse in underhead fuse box to see if that disables it and help you diagnose it. If relay is wrong one- or wrong style it maybe crossing up the circuit somehow.

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

mk378 on Thu April 19, 2012 9:55 PM

Does it disengage if you stop the engine but leave the key on? If it does, the PCM has control of the relay OK, but something on the input side of the PCM is telling it to stay on.

chris142 on Thu April 19, 2012 10:13 PM

Is the clutch gap nonexistant?

MONSTA on Fri April 20, 2012 1:40 AM

Quote
Originally posted by: mk378
Does it disengage if you stop the engine but leave the key on? If it does, the PCM has control of the relay OK, but something on the input side of the PCM is telling it to stay on.

Not sure how to check that. It is difficult to turn by hand when it is off...if that helps. But I haven't seen if there is a difference with the key on. I'll go check that now.

What sensor tells the computer when to cycle on and off? The high pressure switch??

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Aloha from Maui

Edited: Fri April 20, 2012 at 1:42 AM by MONSTA

MONSTA on Fri April 20, 2012 1:42 AM

QUOTE Originally posted by: chris142
Is the clutch gap nonexistant?


I do not know how to measure that. I could if you provide instructions, though! :-D

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Aloha from Maui

Edited: Fri April 20, 2012 at 1:45 AM by MONSTA

NickD on Fri April 20, 2012 6:38 AM

Crazy interference fit GM has been using for sometime now that holds on the compressor clutch plate, plate can either creep in or out, sounds like yours crept in. Should be a 20 mil gap between the idler pulley and the clutch plate. Need a special tool to adjust it.

Its a puller/installer tool, if that isn't bad enough, GM keeps on changing the design of this tool, so far have three of them. I hear Autozone loans them out, but never needed to do that.

Your not the only one with this problem, join about a million more.

mk378 on Fri April 20, 2012 11:35 AM

To check for mechanical problems with the clutch, simply unplug clutch wire and see if the compressor disengages. I think this is going to be a basic CCOT which cycles on a low pressure switch on the accumulator, but would need to see a schematic to be sure.

Edited: Fri April 20, 2012 at 11:58 AM by mk378