I have a 1991, L98, auto, with the C68, electronic air conditioning control. Here is the problem, the compressor is working intermittently. The first thing I checked was to see that the system is full, which it is, at approximately 2.25 lbs. of R12 freon. The second was to jumper the high pressure switch, and still nothing. I then moved inside to check wiring behind the dash but still found nothing. Where I'm at now is, if I disconnect the battery, the system will work for awhile then stop. If I unplug the electronic control module in the dash, it will work again for awhile, then stop. I also replaced the relay, no luck there. Finally, with my scan tool connected, when I press the button to call for air, the light illuminates, but my scan tool says it isn't calling for air or the cooling fan, which it should be. When I get the system to work, the scan tool shows that it is calling for these two perimeters, and I have air. Also I'm not showing any type of codes for the system on my scan tool. I guess I'm down to the control module, which, by the way, I can get from my GM dealer, but they want the old one back for exchange, as they get rebuilt. My cost is $200.00, list is $270.00. I'm just wondering if anyone else has had a similar problem with a 91, and if this turned out to be the solution. THANKS!

 

Control Panel sends the a/c request over a serial data line to the Programmer. If the Low Pressure Switch is closed and the Pressure switch has a voltage between .8 and 4 volts, a voltage signal is sent to the PCM to ground the compressor relay. Obviously, sometimes that's happening - sometimes it isn't - so the first thing I'd check is the connection at the Low Presssure Switch. Wiggle it around and make sure it isn't intermittent. Sometimes, just holding the connector on the switch really tight reveals the problem. If that isn't it, sounds like the Programmer, but you might also check the voltage at the Pressure Switch. It's on the high side line. One wire is a 5 volt reference - 2nd the return signal to the PCM - 3rd ground. Each volt on the return line equates to about 100 psi and most scanners show the voltage and the PSI, so you can look at that readout. Bottom line, If the connection(s) are intermittent, PCM senses there isn't enough pressure for the compressor to operate safely and if the low side Switch is popping on and off rapidly, it will set the low charge code (something I don't think you've seen yet or maybe you have since you're having to clear memory to get it running?).

 

SUNCR, you got me thinking about what I worked on over the winter. The air was working fine last year before I took the top end of the motor down. I replaced the hose that runs from the surge tank to the water pump as I had replaced many of the other hoses. This hose was not available any more from any aftermarket or GM sources. I had to splice two hoses together, a 1/2" to a 1", and the connection is right where I believe there is a switch. Correct me if I'm wrong, but are there two switches, a high pressure and low pressure? Also what do you think is happening when I reset the control module in the car by disconnecting the battery or unplugging the module. It only seems to go out when I'm driving as possibly the hose is pushing connector due to the car moving in turns, etc., but not when sitting in the driveway. I would rather not $200, and find I still have the same problem, thanks for your response!!

 

I'd have to see a picture to know whether or not the hoses you replaced are next to either switch. Yes there is a high and low pressure switch, though that high side switch is also used to monitor more than a high side pressure that might blow it all up so it's often referred to as just an a/c pressure switch. PCM also expects a voltage rise in the return signal from this sensor whenever the compressor is engaged and if it doesn't see it, can shut things down, as it will if the signal is over 4 volts (about 400 psi). Best to look at the signal with your scanner.

If the compressor won't engage until you clear memory, that means a trouble code has been set. You should be able to access it through the Display or the CCM link if your scanner will communicate with it. Some Years will also spit out a few a/c codes related to the (High) Pressure Switch via the PCM, though I don't think any are applicable to the 91.

 

SUNCR, I checked the switch that the hose runs near, and tried moving the wires when the compressor stopped running, with no luck. I believe I'm back to the control module in the cabin, which I ordered today through my local GM dealer. Also, when the compressor stops running, the light on the control module starts flashing which according to the owners manual, says there is an electrical problem. Initially, it said to check the freon level, and if low, refill, then pull the fuse to reset the system. As I originally stated, I had a local air conditioning repair shop that I used previously on a 1980 vette check the system, and it had the 2.25 lbs. which is what is called for. It is interesting that the GM dealer wants the old module back which apparently gets rebuilt. The parts person remembered ordering these before, so possibly there was a problem. I guess I'm lucky it ever worked, considering it is 15 years old.

 

The EXACT same thing happened to me last year, but it was after I gave the engine compartment an intense high pressure bath. I forget if I posted it here or CAC, but I remember distinctly that somebody suggested that I had gotten the outside temp. sensor wet, and that was causing my problems. I let it dry out for a few hours, then disconnected my battery to reset the system. I haven't had a problem since. I wonder if your temp sensor could be on the fritz.

 

That's a good thought, but I replaced it last year as the original was defective. The temperature was stuck on 50 F, so I replaced it, the present unit is working correctly, and the temperature updates the way it should. I think I'm still headed toward the control module, which should be in Friday. Another $216.00 out of my pocket, but it is too hot in the car not to have air, especially if it is as hot this summer, as it was last year.

 

If there's an LED flashing on the Control Panel, there's a trouble code (and there are others besides Low Gas). Your Service Manual should tell you how to access it and what it means. Otherwise, I'm pretty sure it's one of the Tech Tips. As long as there's a code, nothing will work until you clear the Programmer's Memory. A new or rebuilt Dash Unit probably won't solve it.

 

It always helps to look for the obvious, I have the FSM in front of me, and there is a diagnostic mode on this programmer. At lunch time (12:00 A.M.!) I will go pull the code, there are a couple which I hope it ISN'T, like the Temperature Door Motor Circuit Fault, or UART or "E" and "C" Bus Failure. I will let you guys know what I find, hopefully it is the Sunload Temperature Sensor, or the In-Vehicle Temperature Sensor.
I already replaced the Ambient Temperature Sensor located in the front air dam. Thanks Again!!!

 

Well, I pulled the code off the AC controller, and guess what showed up? Code 09, low freon!! This must be right on the edge because I watched the guy check it. I guess that is why when I pull the fuse, then tried it again, it will run for a short time, then stop. Does anyone know what is the high side, and what is the low side? The tech put a jumper on the switch near the surge tank, but there is another switch lower near the center section of the wheel well, by the shock.

 

I almost forgot, THANKS SUNCR, hopefully my dealership doesn't make me take that controller for $200.00! I don't think they will, because obviously I didn't try it as they didn't get it yet. I guess next time I should just look at the FSM before wasting anyones time on the board!

 

Since your pressures check out, the most likely fault is the low pressure switch. They simply unscrew from a Schrader (though if you change it out, do it really fast so that you don't lose any juice). I'm thinking you've got a bad connection at this switch and it's opening the circuit just like it would for a low charge - really fast. Another member had the same problem and I had it on my '97 truck. I'd wriggle around that connector just to see what happens - otherwise spend 15 Bucks on a new one.