Intermittent A/C!

[crazyd]

Hey all, 

'92 GTP

Been fighting this one all summer and I'm throwing up the white flag to see if anyone else has any ideas.

A/C seems to work the first time I drive the car for the day.  Next time, nothing.  Clutch is not engaging. 

When it runs, I have 40psi at 75° on the low side and 45° at the vents.  When it shuts off, 75psi static on the low side at 75°.  Far as I'm concerned the pressure is fine. 

I have replaced the relay and the pressure switch, but neither has resolved it.

Another heat wave here in the PNW so trying to fix it.  Any suggestions?

[Black92GS]

I'm going to venture a guess that you're dealing with a failing clutch. I've seen them where the air gap becomes too great, and the clutch will no longer cycle on as expected. It's likely borderline to begin with, and getting heat soaked from the engine bay is enough to cause it not to kick on until it cools back down. 

The next time it acts up, check for a couple different things.  When you switch the AC on, the cooling fans should kick on and the idle should come up.  If that occurs, then the PCM is commanding the compressor on.  If the compressor isn't running at that point,  lightly tap the compressor clutch plate with a screwdriver, being ready for it to kick on suddenly. You'd have to do it either through the passenger side wheel well, or from under the car. If it kicks on when you tap it....it's the clutch. 

 

[crazyd]

Wow, great idea.  I had a feeling it might possibly be something heat-related because of the symptoms, but couldn't figure out what.  It's a fairly new compressor as I replaced it in 2018 with a reman from Autozone, but hey I guess you can't assume anything with these.

Edited Friday at 08:44 PM by crazyd

[crazyd]

Revs came up, fans did not activate, clutch would not engage with repeated tapping. 

New Denso compressor from Amazon, replaced in 2021.

Edited Friday at 09:17 PM by crazyd

[Black92GS]

57 minutes ago, crazyd said:

Revs came up, fans did not activate, clutch would not engage with repeated tapping. 

New Denso compressor from Amazon, replaced in 2021.

Fans not coming on may be something else…but the revs coming up tells me that everything is seen as good by the PCM and its kicking the compressor on. 
 

The clutch is 12 volts. I’d check with a test light to make sure you have power and ground at the compressor when the ac is being commanded on. If that checks out, you’re still likely looking at the clutch or the coil being the issue. If there’s something missing…then you’ll have to work back from there. 
 

 

[crazyd]

Checked it this morning after an overnight cooldown. No power at the compressor at all in MAX mode. IGN Fuse tests OK.

[Black92GS]

This is for a 92 Regal so the wiring colours may vary, but the idea will be the same. You first perform test A to determine where to go from there.  

From what I can see, all of the pressure switches, the AC request from the HVAC and such all go through the ECM, and the ECM is what controls the relay, and ultimately the Compressor. The revs coming up when you turn the AC on makes me believe the ECM is receiving all of the necessary inputs and is actively trying to command the AC compressor on through the relay….and that your issue is going to lie in that area of the system. 

 

 

[crazyd]

I thought there was only one pressure switch that sits in the low-pressure line next to the accumulator.  There are more?

[Black92GS]

17 minutes ago, crazyd said:

I thought there was only one pressure switch that sits in the low-pressure line next to the accumulator.  There are more?

I believe there is only 1 refrigerant pressure switch used with the variable compressor systems like your car would have. That will shut down the compressor below or above a certain refrigerant pressure level. Looking at my Regal FSM, 3.1 cars also have a pressure switch in the power steering pump. I'm not 100% certain, but I believe 3.4 cars have the same thing.  Under high power steering pressure, it will command the PCM to kill the compressor.  Same thing happens at full throttle, and at higher than normal coolant temperatures.  The PCM receives the signal from the HVAC unit to turn the compressor on. It will ensure that refrigerant pressure checks out, that the power steering pressure checks out, that the coolant temperature checks out, and that the car isn't at full throttle.  If all of that checks out,  then it will activate the compressor relay, increasing the idle and activating the cooling fans. 

That's why I was asking about the revs coming up when you turn the AC on.  If the PCM is actively trying to command the compressor on, the revs will come up and the cooling fan(s) should kick in or ramp up as required. That would mean that all of the various pressure switches have checked out, and if it's not engaging at that point, your issue is going to be with the clutch itself, the relay, or a broken wire somewhere in that chain.