A/C relay not being turned on by PCM

Tiny
CHIEF_CONTROLLERS
  • MEMBER
  • 2001 CHEVROLET SUBURBAN
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 160,000 MILES
First: Thanks for posting the wiring. My Haynes manual is way off base on these. (Just about everything is wrong in their wiring schematic !)

I have a similar but different relay issue.
The PCM is not drawing the ground pin low, so the relay does not turn on. If I draw the relays turn on coil low manually the compressor runs.
I did have to bypass the low-pressure switch to get the compressor to work since the r134 was low, but I am trying to not top it off until I figure out the issue.

My questions are, do you know if these 3-pin high pressure cutoff switches also affect the PCM if the pressure is low since they have a low and high sense wire?
I have good ground at the high cutoff switch and at the low-pressure switch, and 5-volt reference on the gray wire here and continuity on the red wire with black stripe.
I was going to try to bypass the high side switch, but this 3-pin is obviously not as simple to do as a 2 pin and I have no Idea how to bypass that one. Do you?
If this does not also sense a low-pressure cutoff, then I think the issue is between the dash controls and the PCM. The issue is finding this wire since it changes somewhere from white to blue.
FYI: the dash A/C and recirculate buttons light when pushed and the hot cold doors motors do work so power to the control is okay.
On the picture that you marked as fig4 (It is the second row second one in) it shows that wire combo.
Do you know what the heck SP205 in I/P harness is and where it might be?
And why is the daylight sensor shown on some schematics as being wired into this SP205 but not on this fig.4
I reattached that fig 4 schematic.
Thanks
Tuesday, June 7th, 2022 AT 6:02 AM

3 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,513 POSTS
What is your low side pressure? The system has a safeguard to not have the A/C run when the refrigerant is low. Please bring the pressure up to see if the system turns on. This video/guide will show you how:

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

Please run down this guide and report back.
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Wednesday, June 8th, 2022 AT 11:21 AM
Tiny
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Did you read what I wrote? Or just skim over it?

Also, does the 2001 Suburban use a different wiring schematic than the Tahoe?
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Wednesday, June 8th, 2022 AT 11:35 AM
Tiny
KEN L
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You wrote" I did have to bypass the low-pressure switch to get the compressor to work since the R134a was low, but I am trying to not top it off until I figure out the issue."

So, if the bypass worked and the 134a is low, then fill it up so it triggers the sensor. Sorry if I got it wrong. I believe the three wire sensor is used to cut off the compressor when the high side pressure is over spec. I am not sure how it interacts with the PCM specifically.
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Thursday, June 9th, 2022 AT 6:30 PM

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