'96 PA A/C Acting Weird

[Smitty's Buick]

OK, so the popular topic now is obviously Air Conditioning in August! Whodathunkit?

My 96 PA with 207K was parked for about a week when we went on vacation. A/C was working fine before then. I started the car to move it and noticed the A/C blowing hot. I parked it and got into it on Sunday.

The Clutch was not engaging on the compressor, so I checked the fuses, relays, etc. and found no problems. Traced down the low pressure switch and tried to jumper it (has three connections, wasn't sure which two to jump) with no success. Ran wires directly to the battery from the compressor connections, and sure enough, the clutch engaged and ran normally!

I picked up two cans of R134a and added it to the system thinking that the system must have leaked out over that week. The system operated normally, getting down to the low 50's at the vent. Hoped I was done.

Monday I got in the car for work (about an hour drive) and noticed that it was not cooling any more. Figured the charge must have leaked out. The temperature readout was flashing, leading me to believe that yes, this is the problem. The drive home was even more brutal as the outside temp was about 100*F.

Ran today without the air, didn't even try the A/C. Moved the car tonight and decided to check it. To my surprise, it started blowing cold. I checked the compressor, and sure enough, the clutch was engaged. The temp readout was flashing. I tried it several times and it worked fine every time.

Obviously the charge did not leak out (completely anyway), so why is the system functioning intermittently? Is it possible that I didn't need to add refrigerant? One thing I did notice tonight was that the volts (alternator) looked to be a little lower than normal. Not sure if that is at all related. I plan to drive it to work tomorrow, so I guess we'll see if it still works then. In the mean time, what other checks can I do on the system to determine what the problem is?

Thanks guys! Your input is invaluable!
Smitty's Buick

[Smitty's Buick]

Update!
So, I get in this morning, start the car, and try the A/C. Nothing. Tried turning it on and off a couple more times - nothing. The clutch is not engaging. Is it possibly the low pressure switch on the back of the compressor is faulty?

Got the wife's Oldsmobile for today. At least I won't die in the 100 degree heat today!

What do you think????

Smitty's Buick

[HotZ28]

The "switch" on the compressor is actually a transducer which receives 5v from the PCM @ based on line pressure, the output varies from .5v to 4.0v (high cut off). No way to "jump" this system without a variable resistor (potentiometer) to vary voltage input to the PCM. Intermittent problems such as this, are nearly impossible to troubleshoot without an on-site inspection & proper troubleshooting equipment! Even then, it may be difficult to diagnose. Have you verified voltage, or ground @ the relay (first one on the passenger side, under-hood relay center)?

[Smitty's Buick]

Thanks for the reply, HotZ28. I've been researching this more and discovered that what I thought was the low pressure cut out switch was dubbed a high pressure cut out switch. Not sure if that is right either.

So, there is no Low Pressure Switch on the '96?

I swapped out the relay with the horn relay, but didn't check the voltage or ground, other than verify the voltage at the relay. I did find 5V at the switch connection.

I have been contemplating taking it to my "service guy" that I go to when I'm "in over my head". Sounds like this is another one. I wanted to find out if I was missing something with the low pressure cut out switch before I throw in the towel.

Thanks for the help! I will update everyone on what the end result is.
Smitty's Buick

[HotZ28]

The pressure transducer monitors both high & low pressure. The PCM uses the feedback from the transducer (seen as variable voltage) to ground the A/C relay, provided the pressures are within range & the PCM is doing it's job. Several things can cause high/low pressure cut out, i.e. low static system pressure, clogged orifice screen, bad transducer, bad wiring, or bad (stuck) control valve in the back of the V5 compressor. The only way to find out what is going on, is to monitor static system pressure & then high/low side pressure with the clutch engaged (if it will). In addition, with the proper test equipment, the transducer feedback to the PCM could be monitored. The "flashing" temp display is telling you that a fault is present, which could be detected (read) using a Tech II scan tool. Your car is 14-15 yrs old & if the compressor has never been replaced, it may be time.

[Smitty's Buick]

HotZ28 -
Thanks for the explanation. Sounds like I will be making a call in to my local shop.
Smitty's Buick

[Smitty's Buick]

So, the A/C is fixed! The Relay was the problem. I guess I got too hung up on the low pressure/low charge potential, I missed what should have been obvious. Learned that the flashing light is caused (in my case anyway) by a potential problem in the "manifold" where the vacuum lines, valves, and electrical harness all meet behind the glove box. I may have to replace that in the future. Otherwise, the system checked out AOK and is working great!

Thanks for the help HotZ28!

Smitty's Buick

[Smitty's Buick]

Well, so much for the system working....

It worked OK for a couple days. Learned that there is something with the box I described above (don't remember the exact name) that delays the actuator from closing the heater core door and opening the evaporator door for about 45 seconds. I don't think that is my main problem, however.

The clutch is again not engaging. The relay is kicking in (can feel it click when I touch the relay). I suspect that the pressure switch/sensor is maybe bad. Removed it and blew it out. Didn't see anything wrong, didn't expect to. The problem slowly got worse. The first day it worked fine, the next day it took a couple tries to get it to go, more tries the next day, and today, no go.

I'll be calling my service guy tomorrow to get his take on the problem. Any other ideas? Getting tired of fighting this thing.

Smitty's Buick

[HotZ28]

If you can feel the relay clicking in, that means the PCM has a signal from the transducer (pressure switch) and that the PCM is doing it's thing. Do you have power @ the clutch when the relay is engaged? If so, what is the gap @ the clutch disc? Was this a new, or rebuilt compressor?

[Smitty's Buick]

HotZ28 -
The compressor was not touched. The only thing replaced was the relay. I suspect that the clutch has failed. I did not notice much of a gap at the clutch. I will check the voltage at the plug and report back. Thanks again.
Smitty's Buick

[Smitty's Buick]

OK, the voltage at the plug at the compressor is a pretty steady 10V. When the switch is turned on, there is no magnetic field, in other words, I can place a screwdriver by the clutch and it is not pulled towards it. Is 10V causing the problem or should it engage even that low? Sounds like I may have an alternator problem as well now. Am I thinking right???

[HotZ28]

Sorry for the delay responding, storms through the area play havoc with the ISP. Anyway, check the voltage at the battery with engine running to see if the alternator is performing the way it should. With all accessories OFF, you should have 13.2v - 14.5v. Normally, the A/C clutch should have 12v+.

[Smitty's Buick]

OK. The battery is seeing 13.6V when the car is running with no accesories. When I hot wire the A/C compressor, the clutch engages and runs fine. The line to the Accumulator got cold quick. There is now only 9.6V at the compressor connection. I checked the voltage coming out of the relay. It is about 11.6V. Is there anything in between the relay and the compressor that would cause a 2V drop other than wiring?
Smitty's Buick

[HotZ28]

There should be an A/C clutch diode located between the brown/white & black wire, taped inside the wiring just above the terminal that connects to the A/C clutch. The diode acts as a one-way valve for electrons which means, current can flow from the anode to the cathode, but not in the other direction. You could have a voltage leak to ground, a bad diode, bad ground, or simply a bad clutch coil.

[Smitty's Buick]

HotZ28 - Thanks for the quick reply! Is there a way to test the diode?