A/C problem PCM not grounding pin 85?

HD2424

MEMBER
2003 CHEVROLET SILVERADO
5.3L
V8
4WD
AUTOMATIC
2,224,444 MILES

My question is where should I test next?

A/C is charged I evacuated and ac had no leaks. Ign E fuse has power, and the A/C fuse has power. I can jump the relay and ac compressor comes on. The problem I have is that PCM is not grounding pin 85. I can ground pin 85 with jumper wire and compressor comes on. I believe pin 85 is the green and white stripped wire that goes back to PCM. Is it possible there could be a broken wire or connector in fuse box is loose. Or bad PCM. I know low pressure switch is good. I removed it from a working A/C. Could the high-pressure switch cause no ground to 85? I checked it with multimeter, and it seemed okay.
Don’t know what I should try next?

Thanks in advance for any help or advice you can offer.

4 Replies

JACOBANDNICKOLAS

MECHANIC
109,700 POSTS

Hi,

Pin 85 is to the PCM which provides a ground to actuate the clutch relay secondary. And it is dark green and white.

How are you checking for ground from the PCM? If you take a test lamp, connect the alligator clip to the positive side of the battery, and probe pin 85 in the fuse box, does the lamp turn on when the key is on, and the A/C is requested?

Next, have you checked the relay itself? If not, switch it with a different one having the same part number, or here is a link that explains how to test one:

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

Now, if the relay is good and we have no ground to pin 85, either you have a broken wire, poor connection, or the PCM has failed. To confirm, you need to locate connector 2 pin 43 at the PCM and back probe the wire to see if there is a ground being provided. If there is, then we know the wire is damaged between the PCM and relay.

Let me know what you find or if you have other questions.

Take care and Happy Easter,

Joe

See pics below. Pics 3 and 4 show the connector 2 pinout and the location. Pics 1 and 2 are the wiring schematic.

Images (Click to make bigger)

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Saturday, April 8th, 2023 AT 10:04 PM

HD2424

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191 POSTS

Okay, I'll check that next and let you know what I've found.

Thanks

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Sunday, April 9th, 2023 AT 3:56 AM

HD2424

MEMBER
191 POSTS

Update: checked for continuity from PCM plug the number 43 green and white strip wire. I stuck a small wire into the number 43 put one lead of my multi meter on there and the other to # 85 on ac relay. Multi meter beeped I have continuity on wire.

To verify further I put connector back on and I put small wire from back side of plug at number 43 green and white wire. Started truck, turned on A/C, hooked my test light clamp to positive post and touched wire I put in # 43 on PCM plug and light does not come on. Thus, no grounding.

So, it must be PCM. Does the switch in cab have any effect of PCM grounding #85 on relay?

And just wondering does that happens a lot where truck runs great, but PCM not ground pin 85 for A/C?

From what I've done do you agree it’s the PCM?

Thanks

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Sunday, April 9th, 2023 AT 6:05 AM

JACOBANDNICKOLAS

MECHANIC
109,700 POSTS

Hi,

If there is no ground at the PCM, that is the likely problem. No, it isn't a common issue. Keep in mind, I'm assuming you checked right at the PCM or the ground signal. If it isn't present, the PCM is the likely cause.

Let me know.

Joe

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Thursday, August 29th, 2024 AT 6:47 PM