A/C compressor not kicking in - electrical?

candem

Original poster
Member
Jul 15, 2014
20
NC
2002 Trailblazer LS 4WD, 208,000 miles, Limeswap Tune and 2008 (new) cooling fan clutch.

My A/C compressor is not kicking in, fans are working fine, A/C light comes on when pressed. I have done a lot of research and checked the following:
  • Fuses both front and back
  • Have 4.8 volts on the Low Side switch. Jumpered the switch. Compressor did not kick in
  • Replaced relay 44 with one of the others - no luck
  • Jumpered the relay - compressor kicks in and A/C appears to work fine
  • Bought Ac Pro fill up and checked low side pressure with relay jumpered. Pressure gauge says it does not need a fill-up.
Any other things I should be checking? As it works when jumpered I am thinking it is an electrical issue.
I have a multi-meter and can take basic readings but that's about it. Do not have pressure gauges.

FYI - I was having an issue with the 4WD not switching and traced that back to the ignition switch which I replaced. That fixed the 4WD and it was the first time I tried to use the A/C after that it did not work. As Murphy would have it, that was on a 8 hour trip. I have double checked the ignition by re-installing the old switch before putting back the new one and double checking everything I did - did not solve A/C issue.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
So this leaves just a couple possible things. For some reason, the PCM is not happy with something and not letting the compressor kick on. One possibility is the high side sensor is defective. It's not a switch like the low side but a pressure sensor. You would need a high end scanner like a Tech 2 to see what is stopping the PCM from turning on the compressor.

Attached is the schematics. Did you check the pinouts for the relay to see if the voltages are there? There could be a break between the fuse and the relay or the PCM to the relay.
[pdf]81240[/pdf]
 

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candem

Original poster
Member
Jul 15, 2014
20
NC
So this leaves just a couple possible things. For some reason, the PCM is not happy with something and not letting the compressor kick on. One possibility is the high side sensor is defective. It's not a switch like the low side but a pressure sensor. You would need a high end scanner like a Tech 2 to see what is stopping the PCM from turning on the compressor.

Attached is the schematics. Did you check the pinouts for the relay to see if the voltages are there? There could be a break between the fuse and the relay or the PCM to the relay.
[pdf]81240[/pdf]

Thanks Mooseman.

The problem seems to be fixed although don't ask me to explain it!

On one of the threads I read that the Torque Pro App will give the high side pressure so I figured I might as well see if I could get that. I added the GM extra PIDS to Torque Pro and then setup a gauge to see if it would show. No luck. I jumpered the relay and ran it for a while to see if that made any difference, still no luck in getting it to display. Replaced relay.

I had also read somewhere that you need to scan the PID's before they would show. I started to do a full scan of the PID's which takes quite a while. While waiting for this I noticed the A/C was blowing cold!

Checked and the clutch had engaged. Turn on and off with the A/C switch, all working as expecting. Stop and restart vehicle, all still working.

Obviously I will monitor it but it seems the PID scan triggered something!

FYI - Torque Pro still not showing the high side pressure although I did not complete the full scan.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
Even though Torque may show a possible sensor, they might not work for certain models. I just checked and even though it's there, it's not showing anything.

Maybe the wiring has an intermittent issue. Keep an eye on it and see what happens.
 
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candem

Original poster
Member
Jul 15, 2014
20
NC
Thanks Mooseman. I appreciate your responses and help to both this and the few other threads I have started!

Definitely will be monitoring. I agree that there is probably something else behind this.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON

BanditGTP

Member
Jan 2, 2013
89
Central NJ
Hey guys.

I don't mean to jump on Candem's thread but I wasn't sure if I should start another thread on the same exact subject. I'm having the same issue as Candem and it just started today on my '03 Envoy XL. I haven't had any previous issue this year with the A/C. Just like Candem I checked for:
  • Fuses both front and back
  • Have 4.8 volts on the Low Side switch. Jumpered the switch. Compressor did not kick in
  • Replaced relay 44 with one of the others - no luck
  • Jumpered the relay - compressor kicks in and A/C appears to work fine
  • I have 12 volts at the relay.
The compressor was working fine this morning when I left my house but after stopping for breakfast, the compressor would not engage anymore. I also noticed that as of that time, the ambient sensor was also showing 78F and would not go any higher even though the actual temperature was going up. Even when I got home around noon, the actual temp was around 90F but my Envoy was still showing 78F. I found someone had mentioned that they reset their ambient temp sensor and that helped their truck to start working again. I was able to reset mine and it immediately showed the correct temp but it still didn't help my situation.

Do you have any other suggestions I can check before I have to suck it up and take it to a shop?
Thanks
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
Basically the same steps outlined above. The PCM is not happy with something.
 
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BanditGTP

Member
Jan 2, 2013
89
Central NJ
Reporting back...

Thinking that this might be a computer issue, I figured I wouldn't lose anything by trying to disconnect the battery for a little while in hopes to reset the computer. I left it disconnected for approx 20-25 minutes. When I reconnected the battery, I was very happy to find my A/C working again.

Thanks Mooseman for getting me to think about how to fix an issue with the computer.
 

budwich

Member
Jun 16, 2013
2,027
kanata
on my 2008, torque works fine showing the high pressure / pressure that the pcm is monitoring. Based on the schematics that I have looked at, it would seem that this is common among years but maybe not. Anyways, even I am currently having issues as the AC seems to work fine in "spurts". All day yesterday was fine. Today not so ... the hottest for this trip... :-( Not totally sure how the "auto" system works, but when the system dies, didn't notice anything unusual on the high pressure monitor, you can't seem to "restart" it (ie. turn it off and then on)...BUT if you turn off the vehicle, start the vehicle again, select ac or auto, the AC starts again. So either a "hit" on "low pressure" or "high pressure" possibly sets the system in "lock out" until a "restart" as I don't think a "stop/start" (key action) is enough to change "thermal dynamics" of the system.

Didn't mean to highjack the thread... just thought I would add some information plus maybe add an additional "mine's broke in a similar manner".
 

budwich

Member
Jun 16, 2013
2,027
kanata
Was watching the achspressure during trip today. The pressure fluctuates (decided to change the display icon to a sweep plus min max). It was running fine for most of the day but quit around 33C outside on the highway. The pressure range was from 88-399 just before quitting... then dropped to 86 which appears to have caused the "stop working". :-(
Anyways, stopping and starting engine gets things working again. I figure that the system might be on the low side as I suspect it has never been serviced and with normal "non-use" during the winter months (near non-use if no defrost is hit) that perhaps some seals leak / dry out. Anyways, hopefully when we get back to home the GM SI will provide more information although in the AC area, I think it might be thin.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
399 psi is extremely high. IIRC, 400 is the cutoff. GM-SI should give some specifics.
 

budwich

Member
Jun 16, 2013
2,027
kanata
I have seen numbers above 400 when it is operating (ie. blow cold). If I recall from my reads around, the system (pcm) looks at 450 as one limit (foggy memory)... maybe higher.
Thanks for this. It is likely a trip to the AC guy is in order to prevent any bad things from happening if they haven't already. Not that I understand how ac works but I was expecting a bit more constant pressure readings on the high side which seems to move with or without pedal changes (gas off, gas on, acceleration, etc)... without any rhyme or reason... ie. faster engine rpm, slower engine rpm, constant engine rpm etc.
 

budwich

Member
Jun 16, 2013
2,027
kanata
just running down the highway ... stopping at mcd's... :smile:
anyways, saw a 420 high side max value. AC continued to work normally. I am figuring is the low side or the system pcm seeing low side that is triggering a shutdown. The range on the achspres was 109 to 420 and still working. Previous day with shutdown, saw the min value hit 86 which I think is near the limit... possibly 85. Anyways, hope the temp stay in the upper 20's C today as it seems to be able to hold at this temp... assuming no major grades which might also contribute to the issue as running in 2nd with high rpm (3400) might be an added factor.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
High side still seems high to me. I'd get it evacuated and refilled with proper amount to rule out overfilling. The machine will say how much was sucked out.
 

budwich

Member
Jun 16, 2013
2,027
kanata
Finally got around to do some reading of the GM SI. Interesting area. Anyways, to answer some of my questions and to agree with Mooseman, the pressure monitored by the pcm has to be below 350 psi or the pcm commands the unit off til it falls below 229 psi. I haven't done much on this, since I was monitoring the pressure via torque while running done the road so I don't really know if the compressor was cycled off. The strange thing for me was that it never appeared to cycled back on unless I "key off key off / start" when the system stopped working. This was even though from the torque app, the pressure had dropped into the 100's area... so something else stopped the system from cycling. There is also a temperature switch in the compressor which might fire if the compressor gets too hot. This could be the case since during these times (ac "dead"), car reading was in the mid 30C... which likely means the engine compartment was likely a lot higher.

Of course the weather up here in the "great white north" is a lot cooler and I haven't had any issues since I have been back. I hope to use the thing less for the next little while until I get more time (and money) to have it looked at (and schedule some time at the shop).
 

candem

Original poster
Member
Jul 15, 2014
20
NC
If you would consider dropping a buck or two in the Tip Jar, it would be appreciated
http://gmtnation.com/forums/donations/the-tip-jar.4/campaign

Or even better, become a member!
Haven't been on here for awhile and noticed your message so became a member as this site has really helped me out on numerous issues - thank you.

Anyhow this issue is back and I am looking for any input on my best next steps. Here is what I can tell you:
  1. Checked all the issues as mentioned in original post - fuses, relay, low side switch, etc.
  2. Android Torque Pro works to kick the AC back on maybe 80% of the time by running a scan. It seems to work better when it is not that hot outside.
  3. Took it in to one of the auto repair franchises as I found a coupon and figured it was a good way to check if their was a mechanical or pressure issue . They checked and said everything was good. When I went to pick it up, AC did not kick-in. They put their scanner back on and got it to kick in. (don't know what scanner but he did say it was a $5,000 scanner as he was muttering about the display on it)
  4. They recommended I take it to the dealer as they could not identify the issue and said the dealer had "more sophisticated scanners".

    I suspect it is either the PCM or the HVAC control module. Any way to narrow this down?
 

budwich

Member
Jun 16, 2013
2,027
kanata
what kind of torque pro "scan" are you referring to??

My problem got worse in terms of the ac shutting down even when it wasn't very hot. Had the system vacuumed and refilled, didn't help. The one thing that I know was the cutch (bearing) was very noisy (running the engine without a belt, was quiet.... spinning other things by hand were smooth, ac compressor was some what noisy). Anyways, I replaced the ac compressor / dryer, and all is quiet, and the system hasn't shut down since. My guess was that the compressor thermal switch was activated. As part of the testing / checking, I pulled the ac relay while the system had shut down (but I kept the engine running... as previously a restart would get the compressor working again). Check the conditions at the socket for voltage and grounds which were there so that seemed to indicate to me that the system (pcm) thought that the AC system was working fine. That only left the fact that perhaps the clutch was no good OR that the temperature switch in the compressor was activated. Either way, it was a compressor replacement as the clutch / bearing isn't really serviceable.
 

NJTB

Member
Aug 27, 2012
612
Flemington, NJ
Same thing happend to me. I don't have the torque app, just a scanner, so I couldn't check the high side pressures, and also didn't know the compressor has a hi temp switch on it. Spent some time chasing wires and such, then got busy and haven't been able to get back to it.
I was riding with my wife, who refuses to be without AC, and she started playing with the controls, and it started working. Gotta screw around with it a bit, but it does work.
 

candem

Original poster
Member
Jul 15, 2014
20
NC
what kind of torque pro "scan" are you referring to??

My problem got worse in terms of the ac shutting down even when it wasn't very hot. Had the system vacuumed and refilled, didn't help. The one thing that I know was the cutch (bearing) was very noisy (running the engine without a belt, was quiet.... spinning other things by hand were smooth, ac compressor was some what noisy). Anyways, I replaced the ac compressor / dryer, and all is quiet, and the system hasn't shut down since. My guess was that the compressor thermal switch was activated. As part of the testing / checking, I pulled the ac relay while the system had shut down (but I kept the engine running... as previously a restart would get the compressor working again). Check the conditions at the socket for voltage and grounds which were there so that seemed to indicate to me that the system (pcm) thought that the AC system was working fine. That only left the fact that perhaps the clutch was no good OR that the temperature switch in the compressor was activated. Either way, it was a compressor replacement as the clutch / bearing isn't really serviceable.

PID scan using Torque Pro on Android.
 

budwich

Member
Jun 16, 2013
2,027
kanata
interesting.... that would seem to point at the PCM as a point of interest. when it is not working, I would suggest pulling the AC relay and checking the socket for grounds and power. That will may tell you what's up electrically in the main path of the compressor. The suggestion about "dealer analysis" may help but that depends on how good the operator is with the system and how readily the problem manifests itself. You could probably buy a "chinese knock off full scanner" cheaper than having the dealer work on the issue... :smile:

NOTE: you check for voltages first. Then once you have established which pins have voltage and which don't, you do a resistance measurement on the ones that DON'T. You do this test when the system is operating correctly and then again when it is not so you can compare the results.
 
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