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2002 gmc envoy_slt
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547 Posts
Discussion Starter · #1 ·
Hi All:
My intermittant AC Compressor problem still exists. In my other posts, after going to the dealer and having them re-flash module with new parameter I thought it was fixed. I was wrong. Compressor still shuts down when it gets hot (anything over 84 degress outside temp) Under those temps it will run perfectly. Dealer said it may be a bad thermo switch INSIDE the compressor.

I'm thinking I can bypass it, at least for trouble shooting purposes. I understand that it's there to protect the compressor, but if it's bad, I'm going to need a new compressor anyways. I'm willing to risk the compressor for my sanity. Also, if it's just a bad switch, the compressor should be OK.

So, my question is....what wires do I jumper to bypass the circuit. There are alot of wires coming out of the back of the compressor, just don't know which ones to switch around and/or jumper. Thanks for any input. c good
 

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2007 chevy trailblazer_lt
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678 Posts
I’m not 100% on this, and cant help you with what wires to tag, but a thermostat operates on resistance, so I would think you would also have to figure out resistance values for normal operation and shut off so that you could trick the PCM into thinking the temp is within the limits.
 

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2004 gmc
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26,181 Posts
This is a switch sort of thermostat, not one with a resistance-based sensor. It's deeply built into the clutch, it looks like, and I don't see way to bypass it. But it's inconceivable (not really) that the dealer couldn't tell if it was the culprit since there's a status signal.

The clutch has three wires. Black is ground. Dark Green is clutch power. If the condditions are proper elsewhere in the vehicle for the AC to be engaged, the PCM commands the AC relay #44 to close. Then the AC clutch gets power and should spin. The light green wire from the clutch to the HVAC control module is at 12V if the clutch is getting the power the relay is sending it. It goes to ground if the overtemp switch opens inside the clutch.

Totally separate from all other functions, this is a trivial one to look at in case the clutch stops spinning and there's power on the dark green wire.

Dumb dealer if this confused them. I'm available today if you want to bring it over to Vista to look at it with me. I'll PM you my cell #.
 

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2003 gmc envoy_slt
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331 Posts
Seems like there may be other issues to cause a compressor to cycle off.
Low airflow over the condensor could cause the compressor to cycle because of high head pressure. My original fan clutch moved very little air at idle. The compressor would cycle on/off before it would cool down. I think with the right scanner, head pressure can be measured or at least what's being reported to the ecm. Even the head pressure sensor could be faulty. Been awhile but recalling it outputs 0-5v proportional to head pressure.
Even could be an issue with the low pressure switch on the accumulator. Generally not related to higher temps but would be worth looking at. You can temporarily short the connector pins if the compressor is off.
 

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2004 gmc
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You're right, but the OP has been to the dealer (we've traded PMs) and the dealer is currently stumped. I think it's a generally incompetent dealer where I'd never take my own vehicle, but I'm a hard-nose. But even a goofball dealer should be able to check pressures and sensors with the scan tool before giving up. They currently can't tell if it's a compressor shutting off for a false overtemp condition or a bad HVAC control module (actually half the functions are in the PCM, but they didn't mention they thought it could be a bad PCM, IIRC.)
 

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2002 gmc envoy_slt
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547 Posts
Discussion Starter · #6 ·
You're right Roadie. Dealer only mentioned it "COULD" be the control head at the dash or "COULD" be bad temp/thermal cutout. They said they wouldn't be able to tell without replacing one or the other..Uggggh..at a price of $500 for the control head and $700 for just the compressor!!! Never mentioned anything about a bad PCM. They did re-flash PCM though. c good
 
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