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Ask GM Tech (Cam) Your Own Question
GM Tech (Cam)
GM Tech (Cam), GMC Mechanic
Category: GM
Satisfied Customers: 5789
Experience:  GM Grand Master Technician 2007. 14 years experience.
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GM Sierra: 2007 GMC Sierra the a/c clutch is inop full on freon

Customer Question

2007 GMC Sierra the a/c clutch is inop full on freon and I can jump the relay and make it work. Jumping the low pressure switch don't change anything.
Submitted: 2 years ago.
Category: GM
Expert:  GM Tech (Cam) replied 2 years ago.
Welcome. This sounds like the PCM may be seeing something it does not like. This could be a temperature issue, sensor issue or something like that. Can you tell me though if you have a manual system or the digital display?
Customer: replied 2 years ago.
Manual
Expert:  GM Tech (Cam) replied 2 years ago.
Ok is there any wiring issues at the sensor for the high pressure line behind the compressor?
Expert:  GM Tech (Cam) replied 2 years ago.
Ok is there any wiring issues at the sensor for the high pressure line behind the compressor?
Customer: replied 2 years ago.
All looks good. I can jump the connector At the compressor and the fans come on.
Expert:  GM Tech (Cam) replied 2 years ago.
Yes, but the computer is in control of the AC system at the request of the heater system. so we need to see what the Powertrain Control Module (PCM) is seeing and why IT is not allowing the AC on.
Customer: replied 2 years ago.
I don't have a scanner. How do we do that?
Expert:  GM Tech (Cam) replied 2 years ago.
You need to use a scan tool for this. A scan tool is very different than a code reader. Scan tools are larger, can scan and control certain systems of the whole vehicle, and usually cost about 2000+. A code reader will only read emission related codes and that limits them to reading the engine and transmission modules. Given the cost of the scan tools, it is usually cheaper to get a shop to read the codes. If they are honest, I would imagine that $20 is fair to stick the machine on and read codes.
Customer: replied 2 years ago.
I don't have that option. I am very familiar with cars and trucks. Have several ASE certifications. Let's assume we hooked up the scan tool. What next?
Expert:  GM Tech (Cam) replied 2 years ago.
We would see what the PCM is measuring for the high side pressure, look at the AC request on the network and see if its commanding the AC compressor on or not. Honestly without seeing codes, its like guessing at why the check engine light is on and the regular things are not the issue.
Customer: replied 2 years ago.
The check engine light isn't on. Most common a/c problem with this truck is the head unit. I put a new GM one in it. Next I'm told is the ambient temp. Then the relay and/or fuse. The least likely thing on the whole truck is the high pressure switch. But if we put the scanner on it and it showed to be reading high pressure what would we do next?
Customer: replied 2 years ago.
Or if it reads low. Is there a way to test the high pressure switch or bypass it to see if that's where the problem is?
Expert:  GM Tech (Cam) replied 2 years ago.
Its a 3 wires sensor so simply bypassing it is not possible. The wiring to the sensor is common for issues. And the check engine light only comes on for issues that relate to emissions, Ac issues do not impact emissions.
Customer: replied 2 years ago.
So the 3 wires are reference voltage, signal and ground?
Expert:  GM Tech (Cam) replied 2 years ago.
yes, correct.
Customer: replied 2 years ago.
Should the ref be 5V or 12V in this case? What should a properly working sensor ohm at? With that sensor unplugged the fans will kick on and stay on. The clutch still does not.
Expert:  GM Tech (Cam) replied 2 years ago.
You cannot simply test resistance. Its a 5v reference from the PCM.
Customer: replied 2 years ago.
What? That's electrical 101. Ohms and amp are the measurement for resistance. You should ohm out every sensor that you take out to replace. Should be no different than a speed sensor. Do you know if the sensor is PNP or NPN?
Expert:  GM Tech (Cam) replied 2 years ago.
Simple electrical devices yes we can measure. Solid state devices we cannot. Let me see if I Can find that info.
Customer: replied 2 years ago.
Ok.
Customer: replied 2 years ago.
Actually don't worry about it. I will just do something different. If I had a scanner I wouldn't have needed help from you.
Expert:  GM Tech (Cam) replied 2 years ago.
Do you have a similar truck around?