A/C Compressor Clutch Not Engaging after A/C control Module theft

Tiny
SCOTCH740
  • MEMBER
  • 2007 GMC SIERRA
  • 6.6L
  • V8
  • TURBO
  • 4WD
  • AUTOMATIC
  • 145,000 MILES
My 2007.5 Sierra 2500 (new body style) was broken into and my A/C control module and Alpine ILX-650 stereo were stolen.

I ordered replacement parts including two different /AC control modules but my A/C clutch will not engage and therefore only have warm air coming out of my vents. All other functions of the A/C control module appear to work. Prior to the theft the A/C worked.

Superfluous Information, possibly:
-Truck has been used several times without the A/C control module or stereos installed.
-The dash trim was destroyed and bin in front of the center console where the cup holders was pulled out during the theft.
-Plugs for modules were likely roughly removed but appear to be in good shape.

Replacements electronic parts I have tried include:
-GM AC control module PN 25936132 (Used off Ebay).
-GM AC control module PN 25936131 (Used off Ebay).
-Alpine iLX-407 stereo with Maestro RR Control module.
-Original 2007 truck Stereo.

I found where Joe had posted wiring diagrams for a 2008 on this site at: https://www.2carpros.com/questions/a-c-compressor-not-engaging-57604804
and have tried to trouble shoot with no luck.

Trouble shooting:
1) I have tried both A/C modules with the new Alpine ILX-407 stereo, the original 2007 stereo and with no stereo installed with similar results.

2) I jumped out the A/C clutch relay from pins 30 to 87 and the clutch engages and AC begins to blow cold. I also swapped the fuel pump and fog light relays with the AC clutch relay, without success.
2.1) I have 14V on pins 30 & 85 of the AC clutch relay
2.2) I have continuity from pin 87 to ground
2.3) I do not have continuity from from pin 86 to ground with the A/C running. (problem, right?)

3) I checked the low pressure switch. It is closed (continuity through terminals) I also attempted to ground the dark blue wire directly to the battery with a jumper with no change.

4) I stupidly touched my jumper from the dark blue wire on the low pressure switch to the positive terminal and got a small spark. (Don't ask why I did that, I don't know SMH) Everything still works as it did/didn't before that, and I did not find any blown fuses.

5) With the low pressure switch connected, I confirmed I had continuity to ground at pin #6 on the A/C control module plug.

6) Checked for codes with my cheap Amazon BT code reader. Only had one that was a high voltage on the A/C compressor relay. I attributed this to my probing around with a jumper wire or #4 stupidity listed above. I reset it and haven't had it pop back up.

Questions:
1) Is the low pressure switch supposed to be closed when you have sufficient pressure?(See trouble shooting item #3)
2) What trouble shooting can I do on the high pressure switch?
3) I saw on the link I posted above that for actuators repairs on other model years a reset is required. Any reason I would need to do that?
4)Is the only communication between the A/C control module and ECM through GMLAN L (Pin 5, dark green wire)? If so is it safe to assume this is working correctly since my fan controls and actuators are working correctly?
5) Are Part numbers 25936132 and 25936131 interchangeable? Will they both work on my 2007.5? I have seen conflicting information.
6) Is is likely I got (1 or 2) bad used A/C control module that don't activate the A/C compressor?

I appreciate any help and have been impressed with previous support I have seen on this site.

Thanks,
Scott
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Friday, September 17th, 2021 AT 5:50 AM

5 Replies

Tiny
KASEKENNY1
  • EXPERT
That is all great information and I think you touched your issue.

One test that we need to do that will get us on the path we need is to see if the PCM is applying the ground with the A/C is on.

The test you did is not exactly definitive of checking for ground through the PCM because the PCM applies the ground but it is not a direct path so there are times that the PCM does not display continuity to ground but is grounding the relay.

So the way to test this is we need to put your red meter lead on battery positive and the black lead of your meter on pin 86 of the relay with it removed and then turn the engine and A/C on. Your meter should read 12 volts.

https://www.2carpros.com/articles/how-to-check-wiring

Theoretically this should show the same as the continuity test but I have been burned by trusting just continuity on other PCMs but this way is definitive.

If that does not show 12 volts then the PCM is not commanding the clutch on. This is either due to a PCM issue or other input that is telling the PCM to not turn the clutch on.

Let us know what you find. I attached the wiring diagram below.

Thanks
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Friday, September 17th, 2021 AT 1:38 PM
Tiny
SCOTCH740
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Great suggestion.
Voltage between the 86 terminal and the positive battery terminal was 10.5V with truck running and A/C turned on. I repeated the test with the other A/C control module (turned truck off to swap them) and got 10.7V. During both tests the battery was at 14V

I performed the same test on the fog light relay terminal (known to be working) and as expected got 14V.
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Friday, September 17th, 2021 AT 4:13 PM
Tiny
KASEKENNY1
  • EXPERT
Okay. I can honestly say, I didn't expect that but clearly that is not correct. So what that means is we have resistance in the circuit somewhere that is requiring voltage to be used to get through the resistance.

Could be wiring or PCM.

So the next test is to do what is called a voltage drop on just the wire.

So put the relay back in and we need to back probe the connector on the fuse box (so you will have to take it loose and try to get enough slack in the harness to flip it over or turn it). Or you will need to install a jumper in the relay that will give you access to the pins with the relay installed. They sell these at most parts stores.

Then back probe the connector on this wire at the PCM. Turn the AC on and see what the voltage is on this wire. The normal scenario for any circuit is the load uses all the voltage except for what is needed to complete the circuit back to ground. So on a typical 12.6 volt circuit, the load will take about 12-12.2 volts and the remaining voltage will be used by the wiring and circuit boards to return it to ground.

In this case, if you have 14 volts coming into this relay, I would expect to see less then a half volt on this wire. If the wire is the only issue then you will see all but what the circuit board of the PCM requires which would most likely not be measurable. This means you would have about 4 volts on this wire.

I doubt that is the issue because I don't think a wire with that much resistance would still be intact. Which means I think you have a PCM issue.

Take a look at the wiring diagram below for detail on this.
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Friday, September 17th, 2021 AT 5:58 PM
Tiny
SCOTCH740
  • MEMBER
I too have my doubts it is the wire between the ECM and relay/fuse block.

I tested voltage between Pin 86 and positive battery with the A/C controls off, then stereo unplugged, and finally the A/C control module unplugged. Each time I got a voltage of 10.5V.

Performing the same test on the fog light relay with them off resulted in 0.4V.

I would have expected voltage between pins 86 of the fog light and compressor relay to be similar.

Currently I have the batteries disconnected to "reset" everything.

Can anyone answer my questions above? Specifically #1, 4 and 5?
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Saturday, September 18th, 2021 AT 6:32 AM
Tiny
KASEKENNY1
  • EXPERT
Okay. Sounds good.

1) Yes. The low pressure switch is closed when the pressure is above a minimum threshold.

2) The high pressure switch is a three wire so you need to remove the connector and check for 5 volts on the reference wire from the module. Check the low reference at the same time by putting your meter in pin 1 and 2. You should have 5 volts.

Then the 3rd wire is the signal wire that needs to be monitored by back probing to find out if the voltage is moving steady and not jumping around. This would indicate a faulty sensor. In order to see proper pressure you need to hook a scan tool to the module and see what the sensor is actually telling the module. I don't have a voltage conversion chart on this vehicle but normally higher voltage is lower pressure and it drops as the pressure increases however, GM has used the inverse as well.

Connector view is below

3) Not for this issue.

4) Correct. These modules communicate on the bus network. We would have to monitor the voltage on the bus in order to determine if it were off but if you have some functionality it is a safe assumption it is working fine. Plus you would most likely have bus performance codes indicating one of the modules received a faulty message.

5) I am not seeing any difference in the modules other then the model years. However, just because your vehicle may have been built half way through the year doesn't mean it was equipped any differently then any other 2007. Unfortunately the only way to know if it will work is to plug it in and try it. We don't have any conversion charts for this module.

6) It is very unlikely that you got two faulty modules but it is possible as I see this happen all the time.

I would just continue running through your diag like you are and if it leads you to another module, then that is what it means and I would not question the results just because you replaced.
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Saturday, September 18th, 2021 AT 7:46 AM

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