Hi folks, new member. I have a new to me 2000 GMC Jimmy SLT w/ 145K whose AC can't be turned off. System is the Auto Climate version. I repaired the common vacuum leaks in the engine compartment thanks to the forum and search. The vents now function and expect some improvement in engine perf now that those are fixed.

The AC on/off button remains illuminated at all times, does not respond to being pressed and the AC compressor only shuts off if you turn the fans off (or pull the plug at the compressor). Is there a possibility that there is a still a vacuum related issue? At first I thought the system may think its stuck in a defrost mode d/t inoperable vacuum related vent control but that's fixed now. The light does turn off when system is set to defrost settings but AC stays on as expected. Not sure if light off in defrost is normal or not.

Other info: the AC system was give a little freon about a year ago and does blow cold. Pretty sure the compressor cycles but I thought that was normal (?). Also seems like the recirc buttons and possibly the rear defrost buttons that are all on the same control module are acting up but not certain since I don't know what normal is (illuminated/not) and only had truck for a couple days. Is there a common fuse or does this point to a failed controller?

Truck was registered yesterday, is having a couple repairs done that I found and is being inspected. Should have it back today to fiddle with.

 

The control circuit for a call for compressor is as follows:

Call for cooling from the HVAC control module on the light blue wire to the high pressure switch on the back of the compressor

Dark green/white wire to the pcm

Dark green/white wire to the Ac compressor relay coil

dark green wire from the relay contacts to the compressor clutch

So the failure to control can be the HVAC module, high pressure switch, pcm, relay or a short to power. You can test the HVAC module by monitoring the light blue wire to ground with a voltmeter while you turn the HVAC module on and off in the cabin. Then move to the next step to each additional component.

Welcome to the forum.


George

 

Thanks for the reply! I am only moderately skilled in auto repair so appreciate the help. I have a multi meter but no diagrams so could need some help when I get the truck home finding components and/or best way to connect for testing.

In that vane, what is the pcm?

 

Yes, relay. The PCM, sometimes referred to as ECM, or VCM is the engine computer. I’ll help in any way that I can.

reading the 12v primer and wiring faults links in my signature may be of help.


George

 

Sorry for the long delay but the truck was in for inspection and a few repairs I didn't want to do. Mechanic seems to assume its the control module but I don't think he diagnosed it. The truck has auto-climate and I'd rather determine if it's bad before tracking down a new/used part. I don't see how monitoring the blue wire on the HP switch can help since the controller light cannot be turned on and off (remains illuminated at all times unless fan is turned off). What am I not getting? Also would vehicle need to be running or could be done in "on" position?

 

I don't know what happened here but my responses to another AC problem (compressor wont turn on) ended up in this thread so yes, my comments here make no sense. Lets start over.

Yes the compressor cycling is normal under certain conditions but not all cycling is normal. The system function on that auto unit changes modes at the extremes of the temp setting. Do you still have the problem with the temp control pointing straight up to say 74 degrees?

George

 

Wanted to confirm but no, adjusting the temp to a position off the extreme left or any call for heat position does not allow AC shutoff.

 

That may be the intended operation, maybe someone with auto AC can confirm:





George

 

My ac light turns off when it's pushed. Also turns off automatically when I is on foot/dash or dash only...

I would take a look at the control unit on the dash, as it tends to get faulty. I'll be going on my 3rd one...

 

Reading the quote from the manual and DonL's experience tells me the AC should be able to be turned off when you want fan only for heat or venting etc.

If it is the controller what do you think about adding a toggle to control the AC? I really don't want to throw $$ at this thing and that wouldn't require much work if I can figure out the wiring.