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2004 GMC Sierra AC Clutch won't engage

abfunk on Sat March 05, 2016 4:38 PM User is offline

Year: 2004
Make: GMC
Model: Sierra
Engine Size: 5.3
Refrigerant Type: 134a
Country of Origin: United States

Hi all,

I'm new and hoping someone here with more experience can offer some guidance. I'm a DIY'er with some electrical knowledge trying to learn as I go to fix this issue.

The Issue:
My AC compressor clutch won't engage when I hit the button on the HVAC module in the cab. This happened suddenly and I did not notice any other signs or issues previously. The AC blew nice and cold as it always has. From inside the truck, everything appears normal. The AC button on the HVAC controls lights when I press it. However, the clutch does not engage.

What I've found so far:
I pulled the AC compressor relay from under the hood and bench tested it with my multimeter and some batteries. It seems okay. I can get it to close when I apply 5V using some batteries. However, when installed, it does not engage (I don't hear it click) when I turn on the compressor from inside the cab with the ignition on. I've checked fuses and have not found any to be dead. After getting a diagram of all the components involved, I started to look at the high and low pressure sensors. With both sensors still installed, I tested for continuity across their contacts and the switches seem to be in the closed position, which makes me feel it's not a refrigerant level issue. For the low pressure switch, I can measure 5V on the wiring harness with my multimeter coming from the HVAC module in the car. However, I don't get 5V on the high pressure sensor wiring harness that comes from the PCM. I tried measuring this multiple ways to try and make sure it was not my methods. This worries me that the PCM may be at fault.

My question is what are the best next steps? Is there a way to send a signal to the PCM from that harness to see if I can trick the PCM into trying to throw the compressor clutch relay? Is there another, more advisable path to take?


GM Tech on Sat March 05, 2016 6:33 PM User is offline

I always start in the middle of the system- at the a/c relay- Is it commanded on by PCM? or not? First test (does relay click on and off in my hand as I gently plug and unplug it with a/c commanded and engine running? If so then problem is most likely compressor clutch (paragraph 2 below) .To further test this, I first jumper the relay socket from corner to corner (relays are reversible). Jumper the hot terminal with the opposite corner that is the lead to the compressor- not the corner that goes to the relay coil. To find the compressor lead- put your DVM on resistance and ground the black lead and test the a/c compressor coil resistance through the relay terminal- find the terminal that is near 3.5 Ohms that drives the compressor coil. This means a/c coil circuit is good and coil is most likely good- go ahead and jumper 12v to this same compressor terminal. Does clutch engage?

If engaged- then your problem is an a/c input- such as pressure switch/or switch connector on accumulater, or the high side 3-wire pressure transducer right near the compressor on the high side line- do the wiggle test with a/c commanded- transducer wires are known to break real close to transducer.

If not engaged above, then leave 12v jumper on relay socket pin for compressor- then go underneath and tap on front of compressor. You have a high probability that the clutch will now engage- because these trucks are known for compressors to get "wide air gap syndrome" where the clutch air gap is too wide to magnetically pull the coil in. Solution is to remove 10mm nut on front of compressor, carefully wiggle clutch driver off, watch for shim(s) to fall out from behind driver, remove the thickest shim and discard- replace clutch driver and 10mm nut and retest- it should pull in now- with jumper and with relay installed.

Have fun with it- since it was a sudden loss of a/c, I can assume you have adequate refrigerant in the system-- the relay clicking in hand (with engine running and a/c commanded) will verify everything on input side (like low pressure switch is) is okay. so like you say, it must be electrical. I have removed shims in probably 50 GM pick-ups, suburban, tahoes, trailblazers from 2002-2007 in my career.and probably 4 or 5 transducer wire broke- so play the odds. A stealership will fix it for you by replacing compressor ($900)- it's up to you...

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......

abfunk on Sun March 06, 2016 4:20 PM User is offline

Okay thank you for the suggestions. I'll give the tests a shot and see what I can find.

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