I’ve got a situation where my AC compressor will only kick on some of the time. If I press the AC button it will blink 6 times (when it won’t come on). This doesn’t seem to be temperature dependent as I experience the problem in hotter and cooler temps. Once the compressor kicks on it will stay on and there are times in a given key cycle that it will not come on initially, but will eventually kick on.

I’ve went through the FSM and the first thing it says to check is DTCs. I do have communication errors stored in history (however, I have always had these since I owned the car and they never caused issues). They are U1016, U1064, and U1096.

Beyond the DTCs I have went through the troubleshooting procedure. There is about 60 psi of freon in the system when static. The FSM refers me to the “AC does not engage” section which walks me through the electrical checks which turn out OK. That leads me to the “Insufficient cooling” section of the FSM, which then leads me right back to “Compressor Does not Engage”, so I am going in circles.

Notes of interest:
-I have recharged the system twice (putting a vacuum on first). That did not help.
-I replaced the accumulator/dryer when recharging the system
-I replaced the AC pressure switch. (did not help)
-I swapped HVAC modules with a friend (did not help)
-When the AC compressor does not come on with the switch on the console, I can hook HPTuners to the car and manually turn the compressor on. This leads me to believe the electrical harness is just fine and that the communications between the BCM and PCM or HVAC and PCM are causing this issue.

Any thoughts in general or ideas on what I could do to either troubleshoot more to pinpoint this problem or simply to try to remedy the communication DTCs (or figure out if they are even causing the issue)? I can only find one sentence in the FSM that talks about the AC compressor light blinking “when it is not allowed to turn on.”

Any help or ideas are appreciated.

Thanks

 

I really don't like the GM HVAC systems. I have a similar one in my Oldsmobile and they both annoy me.

If you're having trouble with the AC button blinking at you, just hit "AUTO". Sometimes it won't let the AC compressor run depending on which vents you have the air blowing out of. Seems stupid to me.

 

The AC compressor doesn't come on whether I push "Auto" or if I push the compressor button.

 

bad low freon pressure switch?

 

That is unfortunate.
When your compressor is working, is the air cold or just cool?

I'm having a similar issue with my daily driver (Oldsmobile). Except mine will never come on. I think I have a leak somewhere.

 

If your external air temp sensor is not working properly (showing too low an external temp) the compressor will not cut on. Does your external temp reading on the display seem to be correct? Are you experiencing the problem with external temp above 50 degrees? You say hotter and cooler temps but exactly how hot? Don't remember the exact number but somewhere around 40 degrees is where the compressor gets turned off by the computer.

If your internal air temp sensor is not working properly it would tell the system the compressor does not need to cut on. However, I doubt if you put it in defrost mode the internal one would keep the compressor from cutting on.

Hope this helps some.

 

I see a lot of parts changing, but no troubleshooting with a DMM to verify operation, grounds, voltage, etc.

 

I'm not an HVAC expert so with that said, all new cars are running R134 Halogen systems these days and they're not similar to the old R12's. You need a formal system to extract and re-pressurize the system. Its based upon local temperature at the time as well as the volume of the system. Its not like the old days where you could add a can of freon. I would recommend that you take it to an HVAC shop and have them do it for you. Should cost $50-$60.

 

I'm beginning to think this more and more even though I replaced the pressure switch as one of my steps. The FSM troubleshooting procedure basically ignores this switch in my 1998 manuals.

I retested all electrical/relay connections downstream of the PCM last night and everything checks out fine. I can turn the clutch on and off by grounding the harness that plugs in at the PCM.

I believe my problems are communication between the HVAC and PCM or input upstream of the PCM that is preventing the AC compressor from being allowed to turn on (like the low pressure switch).

Does anyone know how to troubleshoot the pressure switch? I can read the voltage with HPTuners, but don't know what the min threshold is.

 

The AC works fine when the compressor does choose to kick on.

 

I have trouble at temps from 50-90+ degrees. The PCM will not allow the compressor to kick on below 36 and may not below 45 deg per the FSM. I have done all my testing at 50 or above.

My external temp sensor is working fine.

I don't know about the internal temp sensor, but do know that on cool days when I don't need the AC it heats the cabin on AUTO just fine, so I assume it is fine. I'd be interested in knowing if I can test this. I think there is a DTC for it, but my car is not throwing that code.

 

I've checked everything downstream of the PCM with a DMM and double checked by turning the compressor on with HPTuners (forcing the PCM output).

Again, I don't see any troubleshooting in the FSM for the pressure switch or the "upstream" items before the PCM. Can you enlighten me?

 

I've put a vacuum on the system (extract) and filled per the FSM specs (freon and oil) based on temp/humidity twice.

 

If you put the control panel in defrost mode does the compressor clutch kick in then?

 

Yes. PM me your email, and I'll send you the procedure for the pressure sensor.

 

Just a stupid question....but....

Have you checked your pressures on a set of gauges when the system is on? I am sure you have. Do they stay in the range they should be in?

I think the flashing light tells you more than anything else (sees something "stuck" in a mode).

I agree with Lucky you need to do some diagnostic work here. I would personally verify the mechanical componentry with a set of gauges before I dug into heavy electrical work. They take more time to get out of the toolbox and hook up than to test with!

good luck, hope you get it solved before summer.
Cassidy

 

Yeah, after all this work, I'm making the assumption that low/high side pressure has been verified. Even if the compressor will not stay on using the AC request from the control panel, you can still command it on to get a quick reading.

 

Are you getting an AC request signal when you think you are getting it? From the 2000 FSM:
HVAC Compressor Clutch Does Not Engage



Has the HVAC Diagnostic System Check been performed?
2

1. Install a scan tool.
2. Start the engine and allow to idle.
3. Display the PCM Eng 2 Data List.
4. Turn the A/C button ON.

Does the scan tool indicate the A/C Relay Command is ON?

 

Nope. I double checked last night. When it doesn't want to come on, it doesn't come on no matter what mode I select or what buttons I push.

 

PM sent...