A/C compressor will not engage without jumping the relay

2006 CHEVROLET SILVERADO
185,000 MILES • 4.8L • V8 • 4WD • AUTOMATIC
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MARK CRAWFORD
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Hey guys, I am having a problem with getting my A/C compressor clutch to engage. I put gauges on the low and high side. Without the compressor running I have 90 on the low and 100 on the high. I jumped the low side switch, and it still did not kick on. I jumped the high side and had the same outcome. I took out the relay and jumped it and it kicked on. I checked the gauges when it kicked on and the low side was around 50 and the high was around 80. I changed out the relay with a relay I know is good to no avail. I checked every fuse in the box under the hood and in the compartment, all good.

I did have to change the wiring for the blower motor this winter because the heat wasn't working. It turned out the resistor was bad, so I replaced it and the plug as it was burnt. I am wondering if I have a bad HVAC module

With the A/C turned on and running I have 4.5v on the plug for the low-pressure switch. I have not checked the voltage at the high side plug. I know I am getting power to the compressor because it will kick on when I jump the relay 87 to 30.

I am kind of at a loss and any help would be appreciated. I could use a wiring diagram for the A/C system if anyone had one that could help. Thanks.
Apr 27, 2022 at 10:12 AM
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JACOBANDNICKOLAS
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Hi,

The pressure readings you have with it running are way off (at least the high side). What was the outside temperature when you were testing it?

If you can jump 87 and 30, we know power is available. Did you check pins 85 and 86 to make sure one supplies power to trigger the relay and the opposite is providing a ground path? See pic 1. Note: Pin 86 supplies 12v from the ignition E fuse in the under-hood fuse box. Pic 86 is the ground side which provides a ground path via the PCM.

Let me know if you checked that. Also, let me know the outside temperature when you checked pressures and confirm the pressures you mentioned. In addition,

Let me know what you find with the relays and if this has automatic climate control or manual.

Take care,

Joe

See pic below.
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Apr 27, 2022 at 7:35 PM
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MARK CRAWFORD
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Thanks, Joe, for the reply. It was approximately 85 degrees outside when I checked the pressures. I didn't leave it running long due to the high pressure rapidly increasing due to the fans not running. It is a manual climate control. I have not checked pins 85 or 86 but I will in the morning before work.
Apr 27, 2022 at 8:04 PM
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JACOBANDNICKOLAS
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Hi,

Check that first. If the coil in the relay isn't powered, the switch won't close completing the circuit.

However, (and this is a theory) I have a feeling the compressor is bad. First, the high side reading shouldn't be where it is at any temp. At 85F you can expect the low pressure to be near 50 psi, which it was and the high side to be around 230 PSI. Either you are very low on Freon, or the compressor isn't working properly.

Let me know what you find with the relay.

Take care,

Joe
Apr 27, 2022 at 9:14 PM
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MARK CRAWFORD
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Hey Joe, I was running a little behind this morning and didn't have time to check the relay. I checked the pressures again this morning. It was around 75 degrees and with the relay jumped my pressure on the low side was around 50 and the high side was at 250. I am sorry about giving you the wrong readings the first time. I had checked them about a week before I asked the question on here and couldn't remember correctly, I guess. I am going to check the relay when I get back to the house and I will get back with you. Thanks for your help.
Apr 28, 2022 at 7:13 AM
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JACOBANDNICKOLAS
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Mark,

No problem whatsoever. Let me know what you find, and if the pressures sound correct. LOL

Take care and let me know what I can do to help.

Joe
Apr 28, 2022 at 5:59 PM
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MARK CRAWFORD
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Hey Joe, Sorry I have not gotten back to you sooner. I have been out of town. I got everything ready to run some voltage checks last night and my meter laid down on me. I had to go and buy a new one last night and by the time I got back it was dark and I called it a day. My plan is to get on it today and I will let you know what is going on. Thank you for your help.
Apr 30, 2022 at 7:48 AM
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MARK CRAWFORD
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Okay, I just tested it. Pin 85 is grounded and pin 86 has 12 volts.
May 2, 2022 at 6:19 PM
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JACOBANDNICKOLAS
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Hi Mark, if pin 26 has 12v and pin 85 is ground, then the relay isn't switching on. Have you tried a new relay? Based on your test, pins 87 and 30 should be completed sending power to the compressor. It runs when you jump those two pins, so the same thing should be happening with the relay being powered.

Let me know.

Joe
May 2, 2022 at 8:38 PM
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MARK CRAWFORD
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Hey Joe, I have changed out relays with other relays on the truck and it still doesn't work. I am going to test the relay pins again and make sure I get what I told you the other day. The weather has been terrible here for the last few days and I have not been able to work on the truck. I am going to get back on it when these storms finally move out and I will let you know.
May 5, 2022 at 12:40 PM
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MARK CRAWFORD
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Okay, I moved the truck to the shop and tested it again. 85 is not grounded.
May 5, 2022 at 5:22 PM
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JACOBANDNICKOLAS
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When you checked the ground, did you do it with the key on and the A/C turned on? That ground is a "signal ground" which means the ground path is provided when needed. In this case, it is by the PCM.

Do me a favor. The signal ground is provided by the PCM via a dark green wire with a white stripe. I need you to check the condition of the wire, and connectors, and then test the wire itself for continuity from end to end.

See pics below for the schematic.

Let me know.

Joe
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May 5, 2022 at 8:31 PM
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MARK CRAWFORD
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Hey Joe, yes, I checked it with the truck running and the ac on. I checked the green and white wire, and it looks good on the back of the fuse box. I will have to check the other end tomorrow.
May 5, 2022 at 8:33 PM
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JACOBANDNICKOLAS
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Hi,

Okay, let me know what you find. There has to be an issue at one point.

I will watch for your reply.

Joe
May 6, 2022 at 7:16 PM
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MARK CRAWFORD
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I know this has been a long time gap between messages. I am still having the same issue with the truck. I was working on this with Joe before and I ended up getting hit by a guy pulling a trailer and he put the truck out of commission until now. I have checked continuity on pin 85 to the connector at the pcm. The wire is good. I checked the high pressure switch wires with truck running, the ac switch on, and the high pressure switch unplugged. I am not getting any voltage on either wire.
Aug 23, 2025 at 8:22 PM
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STRAILER
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So the high pressure switch has three wires, do you mean the low pressure switch? The high pressure switch should have 5 volts at the white wire only, while the low pressure switch should have no voltage to either wire. How much residual pressure in the system do you have?
Aug 24, 2025 at 10:57 AM