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A/C compressor relay not working

Tiny
HD2424
  • MEMBER
  • 2003 CHEVROLET 1500
  • 5.3L
  • V8
  • 4WD
  • AUTOMATIC
  • 167,000 MILES
I have the truck listed above A/C compressor not coming on. I know that on relay the following:

Pin 86 has power key on
Pin 30 no power
Pin 85 no ground from ECM
Pin 87 no power to compressor

I did get the compressor to come on by jumping pin 30 with power and grounding pin 85

My question is, what is likely the problem? The fuse box underhood could it be a burnt wire under there? Or a burnt wire coming from ECU to relay? If I run a wire to a toggle switch in place of ECU ground will the compressor still cycle on and off? I also noticed no power to high pressure switch and no power to no power to low pressure switch. I have ground on both.
What sends power to high and low switches?
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Wednesday, June 16th, 2021 AT 10:12 AM

33 Replies

Tiny
KASEKENNY
  • MECHANIC
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Let's start with the relay issue because I suspect that will solve all the issues including the switch issue.

Take a look at the wiring diagram below. You will see that you have a fuse that is responsible for sending power to pin 30. If you have to put 12 volts to 30 then that fuse is most likely the issue.

However, the way this works is power is supplied to pin 30 and 86 from the fuses.

Then the PCM grounds the relay on pin 85 which then closes the relay and sends the power from pin 30 to 87 and then the compressor and then to ground.

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

So if we have no power to pin 30 we need to check that fuse. Then we need to address the no ground from the PCM. That could be a switch issue but we need to find out how you tested it.

You need to put your red meter lead or test light clip on battery positive. Then touch the other lead to pin 85 and turn the engine and A/C on. The light should come on or read 12 volts. If it does not then we have another issue.

Let's start with checking the fuse and going from there. Thanks
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Thursday, June 17th, 2021 AT 6:01 PM
Tiny
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Okay, thanks. I’ll check that and get back to you what I find.
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Friday, June 18th, 2021 AT 1:15 PM
Tiny
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Sounds great. We are here when you get back to it. Thanks for the update.
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Saturday, June 19th, 2021 AT 7:14 PM
Tiny
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Greetings. I put test light on pin 85 started truck and pushed A/C button on and test light did not light up. The A/C compressor fuse lights up test light and ing e fuse as well. Also I have a remanufactured ECU that has been tested and updated. How can the fuse show good but not sending power to relay? Thanks a million for the help.
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Monday, June 21st, 2021 AT 6:21 AM
Tiny
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I also tested continuity from pin 43 on ecu plug to pin on fuse box and it had continuity.
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Monday, June 21st, 2021 AT 6:27 AM
Tiny
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Can we take a step back and clarify how you tested pin 85? I assume the relay was removed.

If it was then did you put your test light clip on ground or battery positive?

This terminal is a PCM controlled ground. So the way this works is the IGN E fuse supplies 12 volts to pin 86. However, this is an open circuit until the PCM supplies the ground to pin 85.

So to test this you need to put your test light clip on battery positive and then touch the probe to terminal 85. Then start the engine and turn the AC on.

This will test the PCM's ability to ground the circuit.

Then when the PCM grounds this circuit it energizes the relay and it pulls the switch closed and it sends power from terminal 30 to 87 which turns the compressor on.
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Monday, June 21st, 2021 AT 8:09 PM
Tiny
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Okay, I’ll recheck like that and I’ll let you know. Thanks
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Monday, June 21st, 2021 AT 8:13 PM
Tiny
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Okay, great. Thanks. Please let me know if you have questions when doing it. We can work through any issues. Thanks
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Tuesday, June 22nd, 2021 AT 7:42 PM
Tiny
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Good morning,

I finally got a chance to test relay. Last night I removed relay put clip on positive battery cable other end to pin 85 on relay turned on ac and started truck and test light came on so pcm was providing ground. I come out this morning and recheck to verify and of course the test light does not light. The ac control inside truck are manual controls and the ac button would not stay lit. That’s only thing I can think of why it’s not providing ground.

Have power fuses ing e and ac fuse. Please let me know what you think.

Thanks for the help.
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Tuesday, June 29th, 2021 AT 4:53 AM
Tiny
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Also before it would take a couple of pushes on A/C button before it would stay lit. It’s not staying lit now.
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Tuesday, June 29th, 2021 AT 5:02 AM
Tiny
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This whole problem started when I had evacuated the system got it down to -30. I started adding the Freon and after the second can compressor did not come on. I checked orfice tube and it was not blocked. The system is empty now.
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Tuesday, June 29th, 2021 AT 6:33 AM
Tiny
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If I jump 87 and 30 compressor clicks on.
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Tuesday, June 29th, 2021 AT 6:58 AM
Tiny
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Okay, I did some more testing. I took the black plug off back of fuse box and the green wire with white strip I’m sure is the PCM wire that is supposed to supply ground to relay. I touched it with test light and it had no ground but had power. I have a test light that has a red plug and a black plug to connect to battery. When testing if light is red that’s positive and green if negative. Checked 3 times and had power. The green wire on back of plug I assume is wire to compressor had no power.
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Tuesday, June 29th, 2021 AT 8:16 AM
Tiny
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I checked green wire with white stripe and nothing coming from ECM it just power coming from fuse box.
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Tuesday, June 29th, 2021 AT 11:01 AM
Tiny
KASEKENNY
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Okay. Let's pause for a minute. The system is empty now and this is when you were testing if the PCM was grounding the relay?

If that is the case then that is expected as the pressure sensor will tell the PCM not to ground the relay and turn the compressor on as the pressure is too high or low.

If this is the case, then we need to refill the system with the proper amount of freon. You need to jump the relay to get the compressor to start pulling in freon once the can will not fill it any more.

https://www.2carpros.com/articles/air-conditioner-how-to-add-freon

Please let me know if I have this wrong. Thanks
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Wednesday, June 30th, 2021 AT 6:50 PM
Tiny
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Okay, great. I’ll try that. I also found the switch on A/C drier is bad. I’m things once I change that and jump A/C relay it should work.

Thanks for all your help.
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Thursday, July 1st, 2021 AT 5:58 AM
Tiny
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You are correct.
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Thursday, July 1st, 2021 AT 6:13 AM
Tiny
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Okay, great. Yes, if it is empty and that switch is faulty then that is where we need to start and retest it.

Please let us know what you find with this and we can go from there. Thanks
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Thursday, July 1st, 2021 AT 7:13 PM
Tiny
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Okay, thanks.
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Thursday, July 1st, 2021 AT 7:14 PM
Tiny
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Thanks for all your help. Got it working.
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Tuesday, July 13th, 2021 AT 5:50 AM

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