A/C compressor relay not working

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

Okay, thanks. I’ll check that and get back to you what I find.

Sounds great. We are here when you get back to it. Thanks for the update.

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.

I also tested continuity from pin 43 on ecu plug to pin on fuse box and it had continuity.

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.

Okay, I’ll recheck like that and I’ll let you know. thanks

Okay, great. Thanks. Please let me know if you have questions when doing it. We can work through any issues. Thanks

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.

Also before it would take a couple of pushes on A/C button before it would stay lit. It’s not staying lit now.

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.

If I jump 87 and 30 compressor clicks on.

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.

I checked green wire with white stripe and nothing coming from ECM it just power coming from fuse box.

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

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.

You are correct.

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

Okay, thanks.

Thanks for all your help. got it working.

That is great. Thanks for the update. Do you mind giving us a couple sentences of what happened to get it working?

That info will surely help others in the future that read this. Either way, thanks for using 2CarPros and come back in the future. Thanks

It was the low pressure switch tested it and it was bad. also the A/C wire harness pigtail was worn out changed that. I now have 34 degree air at the vents.

That is great info. Thanks for doing that. We really appreciate knowing what fixes things because others will benefit from it.

Thanks again for using 2CarPros.

No problem.

Found problem, shorted A/C wire.

Oh wow. That is great and thanks for coming back to update us as I am sure that info will help others in the future.

Thanks

Update: The A/C fuse keeps blowing. I put a test light on the ground wire on the plug going to the compressor and it lights so I have ground from PCM. When I check for power, I have no power. I jump the relay then I get power on A/C plug.

I’m going to ohm out compressor to see if clutch is bad. But could a bad low pressure switch cause fuse to trip or most likely the compressor is bad?

Okay. Just to be sure, you are getting the A/C compressor fuse (shown in the diagram below) blowing?

If this is the fuse that is blowing then yes, it is the compressor. If you look at this circuit, then the compressor/relay is the only thing on it.

So, unless the relay is shorted to ground (unlikely) then the compressor is the only thing on the circuit that can cause this. Basically, what is happening is you have a path to ground prior to the load on this circuit which is the compressor. So that is the clutch as you stated is shorted.

Yes, it’s the A/C fuse 10 amp that keeps blowing.

I’ll update after I ohm out the compressor I I believe 2 to 5 ohms is a good compressor is that correct?

This is going to vary based on temperature. But the coil of a compressor is normally around 3.5 ohms when in a shop that is around 75 degrees F. So, you are in the ballpark.

Hi, sorry to bring up an old thread but I'm having a similar issue on an 06 chevy Express 2500 4.8. Is the diagram you listed at the beginning the same for mine? I'm not getting ground on pin 85 of the relay and jumped the low pressure switch and nothing. I suspect the control head but I see there is a second pressure switch. So 1 where is the 2nd switch and 2 how do I test it. Thank you in advance.

No, the wiring diagrams are for a truck not the van can you please start a new question, and we will be happy to help you. Please post your new question here, you must be logged in.

https://www.2carpros.com/questions/new

Cheers, Ken

Hello! I have the same exact issue HD2424 had at first. I have a 2002 5.3L gmc Sierra 1500 5 Speed. 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. The compressor will kick on, drop RPMs, then immediately kick off. I can watch the GND come in and out from pin 85