After replacing the A/C compressor and condenser, still not cooling properly?

RB95

MEMBER
2002 GMC SIERRA
5.3L
V8
2WD
MANUAL
257,850 MILES

Hello! I have the truck listed above 1500 with a 5 speed. I recently replaced my compressor and condenser, and I am still having cooling issues. Compressor runs when jumped at the relay. All sensors are bypassed. Freon is adequate.

Probing AC Relay pin 85 shows an Intermittent Ground from the PCM. It works when I drive down the highway. It works when I first crank the engine. But when I put the truck in gear and roll it, the ground immediately goes away. Then the compressor will kick on, drop RPMs, then immediately kick off. Over and over.

With the multimeter on my hood, I can watch the GND come in and out from pin 85

5 Replies

KEN L

MASTER CERTIFIED MECHANIC
48,505 POSTS

It sounds like the PCM, or the HVAC controller is bad, you might what to do a CAN scan to see what codes come up. You can get a CAN scanner (Controller Area Network) which will work on most cars from Amazon.

here is a video to show you how:

https://youtu.be/u-4syLc-ifQ

Here are the AC system and compressor wiring diagrams so you can see how the system works. Check out the images (below). Please let us know what happens.

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Friday, August 11th, 2023 AT 2:51 PM

RB95

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4 POSTS

That's all great information! I'm not seeing any codes with an odb reader. I just bought an ODBLink LX Bluetooth scanner. Do you know if the CAN system can be read using LSDroid or similar app?

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Friday, August 11th, 2023 AT 11:00 PM

KEN L

MASTER CERTIFIED MECHANIC
48,505 POSTS

You need to look at their product information, but I don't think so.

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Saturday, August 12th, 2023 AT 10:09 AM

RB95

MEMBER
4 POSTS

So, I cleaned out the ground on the harness below the alternator - using your diagram. It seemed to help! I reset the PCM by disconnecting the battery and grounding the power cable. I tested resistance from AC relay pin 85 and pin 43 on red connector of PCM. Sat 0 ohms. Cleaned out dust from the back of the PCM connectors.

Is there any chance that an old battery or alternator could cause this? I have an Optima with accessories on it. My headlights are showing weird symptoms. Running aftermarket harness. +12v at each plug. HIDs will not work at all. Halogen comes on one at a time and dim constantly. Any info helps. Thank you

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Sunday, August 13th, 2023 AT 9:08 PM

KEN L

MASTER CERTIFIED MECHANIC
48,505 POSTS

It is possible if the system is low on voltage, it will not pull the A/C clutch in. With the engine running are you getting about 14 volts? Please go over this guide:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Please go over this guide and get back to us.