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A/C compressor not engaging

Tiny
WYTSHARK11
  • MEMBER
  • 2005 GMC ENVOY
  • V6
  • 2WD
  • AUTOMATIC
  • 110,000 MILES
A/C quit cooling today. Did some investigation, compressor doesn't engage. Compressor Relay checks good. Fuse checks good. Jumpered compressor relay circuit at fuse block and compressor clutch engages and A/C will cool like normal. Get 5V at low pressure switch, and jumpered that. When jumpered it doesn't make compressor turn on so I'm guessing not the low pressure switch. Probed the relay sense/signal side of the fuse block and with engine running It reads 14V regardless of if A/C button is on/off on the interior HVAC control module. Guessing this means there is something wrong with the control module signal or the ground to the PCM through the relay? Anyway to diagnose those issues? Am I missing something in my diagnosis thus far?
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Saturday, June 27th, 2020 AT 12:29 PM

14 Replies

Tiny
KASEKENNY
  • MECHANIC
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You are not missing anything so far. You have done a good job. However, take a look at the wiring diagram. We need to go to the compressor and see what we have on each wire when the A/C is turned on and the engine is running. Depending on what we have here we can go that direction in the circuit.

The PCM is responsible for grounding the relay and it does that when it receives the input from the A/C button as long as the pressure sensor is closed telling the PCM that it is okay.

So let's start with these voltage readings and go from there. I assume you have 0 volts on the dark green wire and 0 on the light green. If that is the case, we need to go to the relay and see what you have on the relay control wire from the PCM with the A/C on.

Let's start here and go from there. Thanks
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Saturday, June 27th, 2020 AT 4:49 PM
Tiny
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Okay, I pulled the connector off the compressor. Car on, A/C signal on on the control module. Dark green to ground 0V. Light green to ground 0.13V.
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Saturday, June 27th, 2020 AT 5:45 PM
Tiny
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Pulled the relay and measured what I think should be the right one. "85" on the relay/dark green/white although I can't verify the wire going into the fuse block. Just based on finding always hot 30 and hot while on 86 and can deduce 85. 0V when car is off. Then consistent switch to -25milli volts when car is on and is the same A/C on or off. Hopefully that makes sense. Or check out the picture I attached.
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Sunday, June 28th, 2020 AT 12:32 PM
Tiny
KASEKENNY
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Let's test 85 a different way. That should be grounding the relay so you actually need to test the ground for that. To do this, turn the engine and A/C on, put your red lead of your meter on the battery positive terminal and then the black lead on the terminal of the relay. If you have a good ground then the meter will show 12 volts. If not then we will have something else.

Let me know what you find. Thanks
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Monday, June 29th, 2020 AT 7:02 PM
Tiny
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Okay, connected a jumper to 85 post and inserted the relay. Car off 12.5V. Car on drops to 180mV A/C on or off.
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Monday, June 29th, 2020 AT 7:51 PM
Tiny
KASEKENNY
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Okay. That actually doesn't give us the definitive test that we need because it is testing the ground side it is after the load and there should be minimal voltage. I suspect the PCM is okay due to not having an open circuit but I am hoping we can just see that the PCM has the ability to ground 12 volts. So just leave the relay out and put your ground lead on 85 and turn then engine and A/C on. It should be 12 volts. All we are doing is testing the circuit to the PCM and the PCM's ability to ground a 12 volt circuit.
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Tuesday, June 30th, 2020 AT 6:16 PM
Tiny
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Yep. Relay removed, positive battery terminal to 85 slot. A/C on engine on. 14V.
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Tuesday, June 30th, 2020 AT 6:21 PM
Tiny
KASEKENNY
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Okay. Awesome. That checks out the PCM and circuit of the control side. So at this point we need to confirm what you suspected at the beginning because it looks like you proved everything out except for the PCM not getting the signal from the HVAC control module. The fact that you press the button and the light comes on shows the module has power and ground but it sends the signal to the PCM via a bus communication network. So that means we need to monitor the PCM data to see when you press the button if the PCM registered it as being pressed. Due to this being a network communication, you can't just monitor voltage because it is a coded message.

Any chance you have a scan tool that is able to do this?
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Wednesday, July 1st, 2020 AT 7:10 PM
Tiny
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I have a Nexpeak NX501 ODB II scanner but I'm guessing that its probably not fancy enough. It does give me some general monitoring of Temps, pressures, RPM etc, but I don't think I've ever seen a "monitor PCM data" type of option.

I'm guessing without the ability to monitor/check the PCM data stream, we've made it past all the easy stuff that I can do on my own?

I've fixed my ABS control module that suffered from solder fatigue issues by just re-soldering some obviously fatigued joints. Worked like a champ after that, 4 years and counting. I wonder if maybe that could be a culprit on the HVAC module board. Ever come across that problem? Maybe can't hurt to try at this point if we're convinced the issue is isolated to the HVAC module? Or looks like I can get an HVAC module off Rockauto for $130.00. Maybe worth a roll of the dice vs taking it in for a diagnosis.

Thoughts/comments on any of the above? Or any other things to check or isolate?
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Wednesday, July 1st, 2020 AT 7:51 PM
Tiny
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So with that info, I would roll the dice if it were me. If you had the scan tool capable of doing that, then you could confirm the PCM is not "seeing" the command to turn it on. Without it, it is a guess but an educated one.

If it were me, I would go for it and if it doesn't fix it then take it in to a shop. Clearly this is your choice but worst case you will have a back up module if you can't return it. Let me know what you decide. Thanks
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Thursday, July 2nd, 2020 AT 7:19 PM
Tiny
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If only I knew someone else with a compatible model Envoy, Trail Blazer, Rainer to steal from and test. I'm pretty convinced with our back and forth on the isolation to that module. I can easily fix squeaks. Electron problems are a different story :).
I'll probably pull apart the module tomorrow and look for any obvious solder fatigue on any components/boards. If that doesn't turn anything up then I'll prob go buy the Dorman replacement part since it doesn't look like GM still makes/sells the OEM units anymore and see what happens with that. I'll let you know what happens.

Thanks for the help getting this far!
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Thursday, July 2nd, 2020 AT 9:11 PM
Tiny
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You are welcome. That is a good plan. You may be able to find a salvage yard unit. Clearly not desirable but its the next best thing to having a donor part.
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Friday, July 3rd, 2020 AT 6:52 PM
Tiny
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Well, I didn't see anything obvious on the 2 boards in the module but there are hundreds of components. So I gave up on that and bought that aftermarket Dorman replacement, plugged it in. Fired it up and click on went the compressor. Going to call this one a win and wait for the next problem. It was 100F today. No time to waste messing around trying to find a salvage unit with unknown results. ;)

Thanks again for the help!
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Friday, July 3rd, 2020 AT 7:21 PM
Tiny
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You are welcome. I don't use salvage parts either unless I have no other options. I just want to present all options to others. However, this info is great and I am sure someone else will follow through your testing of things and either find their issue along the way or get to the same place you were in and now have the confidence that it will solve their issue as well.

Thanks for sticking with us and then updating with the conclusion. You made this an excellent post. Thanks again.
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Saturday, July 4th, 2020 AT 7:23 PM

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