NEED HELP AC Stopped Working – pressure = 110psi ???

Dan_G

Well-Known Member
My 06 Envoy Denali AC was working fine. I bought the truck used in 2014. I never touched the AC system. Today it’s 65 degrees and the compressor does not run. I put the AC gauges on it and I read 110psi on the high side and low side. I understand the high side and low side will equalize to the same pressure after the compressor stops. But as I think about the pressure, it makes no sense to me. How on earth can the pressure be 110psi on both sides on a cool day. According to the temperature/pressure charts, that’s way too high for a 65 degree day. R134a should be at about 65psi. If it was 95 degrees out, then sure 110psi make sense.
I initially checked a few things like the low pressure switch (appears to be closed and that is apparently fine) and the relay and fuse. It looks to me like the PCM is not commanding the compressor to engage. I don’t know why yet but I’m wondering if the pressure was too high. I can jump the relay and try to make the compressor run - but those pressures concern me.
 

Mooseman

Moderator
High pressure switch is like in the 400-500psi cut off so that wouldn't be it. It's normal for the pressure be exactly the same on both sides if the compressor is not running. There are a number if variables that would prevent the PCM from commanding the compressor like low pressure (OK) ,high pressure (OK), ambient temperature (below freezing), throttle position (wot), etc.. I'm on my phone so I can't pull up GM-SI but check the manuals in the link in my signature. Check the fuses as well as the relay.
 

littleblazer

Gold Supporter
Check the pressure switch on the low side. It's on the accumulator I believe. (Round metal can thing if it isn't the proper name.) Mine failed on me a couple of years ago.
 

Tiggerr

Well-Known Member
See if your getting power to the compressor/clutch. The clutch could've taken a dump...
Not uncommon for there to be an issue with the connection there, or in the wiring to it...

Also... idk which AC controls you have... I have the auto AC with the dials.. if you have a separate button for your defroster try that as well... maybe your AC on/off switch crapped out.. the defroster on a GM will always kick on the AC ...even when temp is set to hot...
 
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Dan_G

Well-Known Member
Check the pressure switch on the low side. It's on the accumulator I believe. (Round metal can thing if it isn't the proper name.) Mine failed on me a couple of years ago.
Sure, I checked the low pressure switch on the accumulator. It read 0 ohms - so it's closed. This seems correct to me. As long as there is more than like 18psi (and there is) I think this switch should be closed.
 
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Dan_G

Well-Known Member
High pressure switch is like in the 400-500psi cut off so that wouldn't be it. It's normal for the pressure be exactly the same on both sides if the compressor is not running. There are a number if variables that would prevent the PCM from commanding the compressor like low pressure (OK) ,high pressure (OK), ambient temperature (below freezing), throttle position (wot), etc.. I'm on my phone so I can't pull up GM-SI but check the manuals in the link in my signature. Check the fuses as well as the relay.
OK, that helps. It's not in the 400-500psi range. So the system pressure is not too high when the system is off. So it seems to me I have an electrical problem.

I have not tested the high side switch because I don't know where it is. I'll have find it and check it.

It's still strange that there is 100psi in the system on a cool day. That is way higher pressure than I have ever seen in a room temperature R134a system. It should be like 40psi. I tried 2 different pressure gauges on the low side, both are basically in the red zone. That is way to high for the low side. Of course once the compressor runs (assuming there is not an obstruction or some other problem) the low side pressure will drop right away.
 
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Dan_G

Well-Known Member
Does the AC blow hot air when you turn it on? Forgive me if it wasn't posted already as I am not really familiar with the pressures and stuff etc...
Yes it blows warm air, compressor does not run.
 
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Dan_G

Well-Known Member
See if your getting power to the compressor/clutch. The clutch could've taken a dump...
Not uncommon for there to be an issue with the connection there, or in the wiring to it...

Also... idk which AC controls you have... I have the auto AC with the dials.. if you have a separate button for your defroster try that as well... maybe your AC on/off switch crapped out.. the defroster on a GM will always kick on the AC ...even when temp is set to hot...
I can see there is no power going from the relay to the compressor. It looks to me like the PCM is not commanding the compressor to run. I checked a few voltages at the relay. The PCM is not commanding the relay to close. So the relay is open and the compressor is not running.

As for why the PCM is not commanding the compressor to run ... I don't know. A reasonable next test seems to be to jump the relay and see if the compressor runs. If the compressor runs and the system cools, then I can be pretty sure that it's an electrical problem (the PCM is not getting the proper signals to engage the compressor).
 

budwich

Well-Known Member
is there power at the relay? it needs to be there first to get anywhere else. Further, the pcm normally switches "grounds" not power.... hence the question.
 
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Dan_G

Well-Known Member
is there power at the relay? it needs to be there first to get anywhere else. Further, the pcm normally switches "grounds" not power.... hence the question.
Yep, there is power at the relay on two of the pins (the low current side and the high current side). The PCM is not supplying a ground to the low current side so the relay is open.

I just found the AC diagnostic procedure on ALLDATAdiy. It uses a scan tool to read the various system values. I have a Tech II scan tool. I'm going to run through the diagnostic procedures and see if that will tell me why the PCM will not engage the compressor.
 

littleblazer

Gold Supporter
Sure, I checked the low pressure switch on the accumulator. It read 0 ohms - so it's closed. This seems correct to me. As long as there is more than like 18psi (and there is) I think this switch should be closed.
You could unscrew it to Check, there is a Schrader valve behind it so it won't lose any refrigerant. But I believe you are correct in it being closed for operation. What others suggested is good directions to go also. Explore all avenues and wiring and such. Good luck
 
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Dan_G

Well-Known Member
You could unscrew it to Check, there is a Schrader valve behind it so it won't lose any refrigerant. But I believe you are correct in it being closed for operation. What others suggested is good directions to go also. Explore all avenues and wiring and such. Good luck
Yes, I think the low refrigerant pressure switch is OK. It's time to get the Tech II out and go through that stuff and figure out what signal is missing. I'm sure it can confirm what the PCM reads for the state of the high pressure and low pressure switches. I have not used the tool much so it will take some effort to find the right screens and locate all the relevant AC system values.
 
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Dan_G

Well-Known Member
I read through the diagnostic procedure. It does not look too bad. First it checks the "A/C Switch" parameter. So if you ask for cooling at the controls, this parameter changes.

If the switch is working, it the procedure checks the "A/C Permissions" parameter. I am assuming this will not be "granted" since the compressor is not running.

If that's the case you check the "Pressure Cycle Switch" parameter. This is the low pressure switch and if it's not "Normal" you can install a jumper and force it to read Normal (and then replace the low pressure switch).

If the low pressure switch is OK you check the "A/C High Side Pressure" parameter and it has to be less than 350psi. If this is reading in range, the procedure says check the connections at the "HVAC Control Module". If the connections are OK, then you replace the HVAC Control Module ... and that's about it. I have no idea of where the HVAC Control Module is but we will cross that bridge when we come to it.

So it's either the A/C switch (or the head unit for automatic HVAC controls) or the low pressure switch or the high pressure switch or the HVAC control module (or some sort of wiring issue). I ought to be able to narrow this down if I can get the Tech II connected and figure out the menus.
 
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Dan_G

Well-Known Member
I read through the diagnostic procedure. It does not look too bad. First it checks the "A/C Switch" parameter. So if you ask for cooling at the controls, this parameter changes.

If the switch is working, it the procedure checks the "A/C Permissions" parameter. I am assuming this will not be "granted" since the compressor is not running.

If that's the case you check the "Pressure Cycle Switch" parameter. This is the low pressure switch and if it's not "Normal" you can install a jumper and force it to read Normal (and then replace the low pressure switch).

If the low pressure switch is OK you check the "A/C High Side Pressure" parameter and it has to be less than 350psi. If this is reading in range, the procedure says check the connections at the "HVAC Control Module". If the connections are OK, then you replace the HVAC Control Module ... and that's about it. I have no idea of where the HVAC Control Module is but we will cross that bridge when we come to it.

So it's either the A/C switch (or the head unit for automatic HVAC controls) or the low pressure switch or the high pressure switch or the HVAC control module (or some sort of wiring issue). I ought to be able to narrow this down if I can get the Tech II connected and figure out the menus.
It turns out the HVAC Control Module is the head unit for the Automatic AC controls. There is also a blower control module so the names are confusing. In any event, there seem to be a few options for getting a replacement unit. If this is the culprit, it's an easy fix.
 

Mooseman

Moderator
Tech 2 is your friend, and a very powerful one at that. Should make diagnostics a lot easier. It should be able to force command the compressor and if not, it will say why. I haven't played with it with the A/C but that's usually how it does things.
 

Badbart

Well-Known Member
Just had ours fixed last month. It would work intermittently but always worked when we took it to the repair shop. Finally acted up and my wife drove to the shop. Something about the clutch magnets wearing too thin and wouldn't always engage? I watched the guy take a 3/4 PVC inch pipe about 5 foot long and he bumped the clutch while the engine was running. It kicked the compressor on as soon as he moved the clutch. Went ahead and replaced the complete compressor and clutch since it's original equipment.
 

Mooseman

Moderator
Just had ours fixed last month. It would work intermittently but always worked when we took it to the repair shop. Finally acted up and my wife drove to the shop. Something about the clutch magnets wearing too thin and wouldn't always engage? I watched the guy take a 3/4 PVC inch pipe about 5 foot long and he bumped the clutch while the engine was running. It kicked the compressor on as soon as he moved the clutch. Went ahead and replaced the complete compressor and clutch since it's original equipment.
I hate to tell you this but you might have been able to fix it just by removing the spacer in the clutch. Mine did that and as soon as I removed the spacer, was working fine for 6 years.
 

Badbart

Well-Known Member
I figured it served us well but was 13 years old, might as well start fresh.
 
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Dan_G

Well-Known Member
Thanks for all the input. I’m mostly happy to report the AC is working. I’m not sure why it stopped working in the first place. All I did was remove and replace the fuse and the relay and also unplugged the low pressure sensor and plugged it back in. It was not working over the weekend I could see the clutch was not engaging – but obviously did not know why (was it the clutch, was it the relay, was it the ECU failing to command the relay to close? – I don’t know). So as soon as I connected the Tech II, it started working.

The Tech II is not very user friendly to get the vehicle selected. You can just keep trying each menu entry and backing out until you find something that makes sense. You don’t just select the year make and model. It’s got one or more sub menus with letter codes that make little sense to me (since the VIN has a bunch of letters). Eventually you can find the Envoy in a menu and then the 5.3 LH6 engine, at this point you know you are in the right spot. So once you get the vehicle selected, the menu for the HVAC settings is there and the parameters are easy enough to figure out.

I could see the call for cooling come in as I turned the temp down on the automatic AC. Turn the temp up and the call for cooling goes away. I could then see that the compressor was commanded to engage and I heard the click of the relay. Sure enough, it’s working.

One good thing is that I know how to get the Tech II connected and it sure makes it easy to see all the cooling parameters. That should make diagnosing electrical AC problems very simple in the future.

So I guess it was some sort of connection issue with the few things that I removed and replaced. Does not give me a really warm and fuzzy feeling that it will stay working. Perhaps it was just the relay flaking out. I could just pop a new one in there. It’s very inexpensive and the original one is 10 years old. Perhaps it was the clutch as suggested above, not sure yet. I’ll have to see how well it works during the summer.

So chalk one up for the Tech II, it's only the second time I used it. I used it to install a new distributor in my 1999 GMC Suburban Vortec 5.7 (the dreaded "CMP Retard Offset" parameter). If you can afford it and you plan on running older GM vehicles, this is a very good tool to have. After a few repairs, it pretty much pays for itself.
 
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Mooseman

Moderator
I know what happened. When the truck saw that you had a Tech 2, it just said "Damn! I ain't goin' to the dealer so I might as well cooperate. He has THE tool!".

Seriously, it probably was just a bad connection or flaky relay. I'm pretty sure you could swap one from another spot like the horn.
 
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Dan_G

Well-Known Member
I know what happened. When the truck saw that you had a Tech 2, it just said "Damn! I ain't goin' to the dealer so I might as well cooperate. He has THE tool!".

Seriously, it probably was just a bad connection or flaky relay. I'm pretty sure you could swap one from another spot like the horn.
I hope you're right. I'm glad it's working but I would have preferred having a more definitive reason for why it stopped working. I ordered a new relay today. I'll pop the new one in there and hope for the best. The old one is 10 years old, it does not owe me anything.

I never did figure out why the pressure was so high, but it was certainly not a reason for why the compressor did not engage at least for a minute or so until the high pressure switch cut out. I'll put the gauges on there on a warm day just to check. My best guess is that the refrigerant charge is fine.
 

Mooseman

Moderator
If you can, check the pressures with a real manifold set while it's running. It may still be a bit overcharged.
 

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