AUTO: GM Trucks: Resolving AC issues

I’ve spent many moons diagnosing nagging air conditioning issues with a 1992 Chevy Silverado K1500 .

Research indicates that many other owners have similar issues.

In my case, a conversion from R12 had already been performed.

My initial problem

Heating worked, control panel worked, but the AC compressor was not engaging.

First step: The compressor clutch running?

I learned that by bypassing the pressure switch on the AC canister I was able to get the clutch running. Low pressure and/or a leak seems like a reasonable guess.

To the repair shop!

Trip to mechanic, who performs leak tests and tops off system. He notes that the conversion from R12 has already been performed. AC works for him, he calls it a day. I pickup the truck and notice that the system isnt very cold, and the ‘AC’ light on the control unit is flashing.

To the internet!

There are known issues with false readings from the pressure switch. Manufacturer recommends disconnecting the pressure switch from the heater control harness. For the K1500, the wire is green #1 (SEE: LOW CHARGE INDICATOR FLASHES DISCONNECT CHARGE DETECTION # 93-1B-129).


BEWARE There are two procedures; one for ‘topkick’ models and one for regular trucks!

I perform the procedure and at this point issue seems resolved. I close up the dash and call it quits.

Not all is well!

Start vehicle the next day, AC is pumping strong, possibly too strong. I smell burning plastic, and the AC control head goes ‘disco lights’ on me. Fsck! Unplug the control unit and assume that its probably fried, judging from the amount of magic smoke escaping from the dash :/

Control unit is dead/questionable, purchase a replacement

Order new unit, NOT CHEAP. $180 US for part #15-72900 via The package really did arrive the next day!

New unit exhibits similar issues. AC clutch is now switching on and off every few seconds. Decide to leave unit in dash but avoid AC functionality. All is well for a while.

More disco lights

Driving down the road one day i smell a familar burning :/ Pull over to a parking lot and remove unit from dash in 5m flat. (I’m becoming an expert in this procedure!) . We’re now approaching winter and i have no heat :\


Purchase vehicle subscription from ALLDATA. Scour through multiple service alerts relating to this issue.

A bulletin i’d never noticed states that there can be shorts

a) where the harness chafes against the ashtray
b) behind the cassette deck.

Shorts, shorts everywhere!

Next workday i pull out the cassette deck, detach the heater control wiring harness from a grey-white 6 pin mini harness which was located about 6 inches back from the AC control head connector.

heater control mini harness

This was a serious pain and scratched my hands and wrists raw. I pull the harness back through to the cassette deck area, peel back several feet of sticky electrical tape to find… 2 adjacent wires with missing insulation! One is attached by exactly 1 corroded strand of copper. Previous electrical tape repairs are present in the same area. The bracket for the cassette deck has sharp edges and is scraping 1mm of insulation from the harness.

Taped up one slightly damaged wire, cut, joined and soldered the terribly burned wire back together. Applied electrical tape to cassette bracket edges and sealed up the entire harness with many layers of tape.


I located yet another short in the wiring harness where it passes below the air filter and above the engine block on the passenger side. Seems that one of the wires had its insulation worn off . Repaired with more tape.

Next i replaced the control unit for the second time, with a brand new part.
It was sending crazy signals to the AC system; opening doors, turning fans on and off.

I can only assume that it was permanently damaged by prior shorts. Upon removal i notice that the unit smells badly like burned plastic. Lucky control unit #3 is installed in its place. The system now works, although the compressor is still being turned on and off in AC mode, albeit in a much quieter manner.

Whats next

Next i intend to disconnect the battery to clear the fault. I will also measure the system pressure to determine that i don’t have too much gas. The low pressure switch has been effectively disabled, and clutch wouldnt cycle at all with the switch connected, so perhaps there is too much pressure.

Vent door actuators

(Coming soon!)


GM bulletin(s) # 93-1B-129, #92-1-118

Document ID# 437695
1991 Chevrolet Chevy K Pickup - 4WD


Some owners of 1991-1992 C/K vehicles may comment that the A/C letters in the middle of the control head face are flashing. The HVAC control system used on 1991 and 1992 C/K vehicles incorporates a low A/C charge warning system. If a low charge is detected, the control head will not allow the compressor to engage and the A/C letters in the middle of the control head face that usually signify A/C operation will blink.

The HVAC control head determines a low charge by monitoring the circuit from the pressure cycling switch. If the cycling switch will not allow the compressor to run more than 1.5 seconds, ten times in a row, the control head assumes a low charge and disables the compressor until reset.

On some vehicles, it has been found that the wire from the pressure cycling switch to the control head is sensitive to EMI (electro magnetic interference) and gives false compressor cycling information to the HVAC control head which then disables the compressor.

If you encounter a vehicle with the control head A/C indicator flashing the low charge signal but can find no reason for the flash using normal diagnostic procedures, including checking the wiring harness for chafing, remove the pressure cycling switch feedback wire from the control head connector. This process disables the A/C Low Charge detection feature, however, the customer will still be alerted to a low charge by excessive compressor cycling and poor A/C performance.


1. Remove the instrument panel trim bezel.

2. Remove the A/C control head.

3. Disconnect the control head electrical connector.

4. Remove the dark green circuit 59 wire located in harness connector socket number one. Do not cut the wire, remove the terminal (Figure 1).

5. Fold the wire back along the harness and tape it so that it cannot short when the control head is installed.

6. Install the control head.

7. Install the instrument panel bezel.

8 thoughts on “AUTO: GM Trucks: Resolving AC issues”

  1. My ac in my 92 Chevy silverado has an ac problem. It blows slightly cold air but when I start the truck and turn it off there is a clicking notice under the passenger side dash.
    My neighbor said I had a bad cylinoid and my high pressure ac line dies not show up on a gauge. Can some one please tell me what’s wrong and how to fix it?

    1. Sorry for the 2 year old reply 🙂

      The clicking (which i’ve also experienced) is the actuator door which is located right in the bottom corner of the front passenger side.

      This part isnt expensive , however, you need to take a lot of stuff apart to reach it. Since my AC is still broken anyways, I managed to pull the power from the actuator to stop the clicking. I believe that mine is currently stuck open .. 🙁

      1. On the 91 and 92 GMC’s there are actually 3 actuators, if you can isolate where the clicking is coming from you should replace that actuator…the one on the far right is the ‘recir’ actuator and is pretty difficult to remove the top 7mm screw, there are only two screws. Until you replace it, even if you disconnect the power, you will get hot air coming into the cab all of the time.
        The part number for this one is: 16149192 and is available from RockAuto

  2. Hi there. My website may be off as I havent paid for a while. I’m a chef/writer. Mechanical skills a bit suspect. I just bought a 2000 GMC Yukon SLE. AC works parttime and does it’s own thing shutting off on it’s own leaving the AC light flashing. sometimes it kicks back on sometimes not…
    You seem like a much more clever man than me! Many thanks in advance.
    Mark Ceranski

  3. Hello, I’m looking for a replacement 6 pin mini harness as you have pictured above. I had a pin short and melt the connector, melt the insulation a few inches back, etc. I’d like to find a new one but I don’t know what its called or where to look for it. I’m about to get one from a junkyard and splice it in. I’d feel better with new product, though.

  4. Thanks for taking the time to do this! I’ve got a 91′ K2500 that has had a persistent A/C issue for two summers. This is doable and will, I hope, at least give me some clues to narrowing down why the compressor is not engaging. It at least explains why shorting the pressure switch on the accumulator doesn’t “trick” the compressor into thinking the pressure is OK and engage. This is definitely a “Thank Goodness for Harbor Freight” moment. I always wanted a A/C pressure gauge set anyway.

  5. Hi there, on a 88 Silverado truck on the a/c clutch is the green wire hot and should it send a hot signal when the a/c is turned on. I am trying to hook up a flex a lite fan but when I hook the green wire to the moduale it works incorrectly. I am not sure if it is sending a signal. Please help thanks, Al

  6. I bought a 1990 Chevy Cheyenne K2500 a week ago. I removed my AC compressor 2 days ago because it locked up. Then the day after I have had no heat nor does my climate control screen light up. Can anyone help?

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