No ground signal at pin 85 on air conditioning compressor relay?

2500HD-BMAN

MEMBER
2009 GMC SIERRA
6.0L
V8
4WD
AUTOMATIC
175,000 MILES

The air conditioning in this truck stopped working at the end of last summer.

When I switch on the ac on the truck it only blows hot air.

I have pulled the compressor relay and jumped pins 30 and 87, while jumped, engine running, call for ac in the cab I get cold air.

When checking pin 86 there is +12v. What is not happening is a ground signal at pin 85. I have checked the low-pressure switch and the two pins have continuity between the pins, leads me to believe refrigerate level is good. I have checked the wiring connector at the low-pressure switch for a ground and there is a good ground at this location on the black wire.

I have disconnected the battery and connected the positive and negative cables together for 3min then reconnected the battery. After I did this the ambient air temp being displayed on the info center was 12deg C. Even though it was 25 deg C outside. After a short while the reading corrected to the actual ambient temp and is still seemingly reading correctly.

I also pulled the HVAC battery fuse for 1 minute, reinstalled and then turned on the key, reading that this should initiate a HVAC control recalibration and I did not touch any controls for 60 seconds then turned of the key.

Still no ground signal at pin 85 of the compressor relay.

At this point I need to understand what else controls the ground signal to pin 85. I can then chase further sensors or broken wiring issues.

6 Replies

BRENDON S

MECHANIC
653 POSTS

Good morning 2500HD-BMAN,

Do you have any codes like P0645?

I am adding a few tests for you. Some are for the relay some are and the clutch on the compressor and there are a couple more for the A/C Pressure switch.

This switch should be on the high-side line and have 3 pins.

This sensor gets a 5v reference from the PCM. The amount of pressure in the system determines what the sensor sends back for a signal. If it isn't within a certain range, then you will not get a ground signal to pin 85.

Sounds like you are on the right track. Try out the tests and let me know what you find out. Sounds like you already have a multimeter. You will also need some pins for back probing and a test light.

Now this wiring diagram shows 85 and 30 should be hot at all times. I am working with another person who has this exact issue on a 2011 GMC Yukon. He says pin 86 and 85 are switched as well. He has power at 86 and 30. So maybe these wiring diagrams are wrong it does happen. For now, just switch 86 and 85 when testing. Either these wiring diagrams aren't exactly accurate or there might be an issue in the fuse box.

I have included a picture of the pins I use for back probing. You can get them from Walmart in the crafts section. I like them because I can attach my alligator clips right to them. A sewing needle will work too.

We found that his pressure switch signal is low when the compressor is jumped on. He was getting 1.06v when compressor off and 1.8v with compressor on. Should be around 3v when on.

Try out the tests and let me know what you find. Let me know if you have any questions.

Where it says "Yellow" lead you would just use your red lead on the multimeter.

I have also added the connector end views for you. Looks like there are a few different ones so I have added them all for you.

Thank you,
Brendon

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