A/C Down. Replace PCM ??? Help!

[BobcatAZ]

I recently bought a '98 Jimmy that was sitting in a dealer's "Bone Yard". It was headed for the auction. Several things were wrong with it but I like doing my own work. I've got a 2000 Silverado and the complete (4 volumes) service manual. I haven't had a chance to get a set for the Jimmy but the logic seems about the same.
One of the problems was the A/C. There was no pressure in the system so I started looking for the leak. I vacuumed the system down and let it set overnight. It held vacuum so I charged the system and it started working. It has been working fine for over a week and then just quit while driving down the road.
I checked the following on the system:

1. Refrigerant change. = OK
2. AC Request circuit from the HVAC Control Module through the High Pressure Cutout Switch to the PCM. = OK
3. AC Low Pressure Cycling Switch circuit to the PCM. = OK
4. AC Compressor Clutch Circuit including relay, fuses for AC, ING E and Clutch magnet back to the PCM. = OK.

The only circuit that didn't work was the "459" circuit from the Relay Control in the PCM to the Compressor Clutch Relay. The HVAC Control Module will send a request to the PCM for A/C but the Relay Control of the PCM will not ground the circuit.
Do I need to replace the Powertrain Control Module (PCM/VCM) or am I missing something else?
Is it common for everything else controlled by the PCM/VCM to work and the A/C not?
Thank You for your help.
Steve (aka BobcatAZ)

[GMCTech]

Did you try closing the low pressure cut-out switch?

Should be right on the receiver/dryer/accumulator. Just unplug it with the engine running and the A/c on, and jump the two terminals with a test wire or paper clip.

If the compressor clutch engages, then the low pressure switch is bad.

What knid of static pressures are you getting at the low and high sides of the system?

[BobcatAZ]

Did you try closing the low pressure cut-out switch?

Should be right on the receiver/dryer/accumulator. Just unplug it with the engine running and the A/c on, and jump the two terminals with a test wire or paper clip.

If the compressor clutch engages, then the low pressure switch is bad.

Yes. I tried a jump wire at the Low Pressure Cutout Switch. I also checked it and the High Pressure Switch (on the back of the compressor) with an Ohms meter. Both switches are closed, circuit complete.

What knid of static pressures are you getting at the low and high sides of the system?

I can only check the low pressure side as I'm using an old R-12 manifold with an adapter for the R-134. However, the system has been static for a couple of days so the high and low sides have balanced out. I'm reading about 90 lbs on the low side (static).

I have not forced the system to run. (don't know why, just haven't). I have checked the grounds in the wiring harness on the engine side of the firewall. Since my last post I checked the ohms readings of the ground circuits. There was some small resistance but that improved when I cleaned the grounds with a wire brush. No change. The compressor clutch will only engage when I ground the compressor clutch relay circuit at the connection to the PCM/VCM.

I'm thinking of looking at the grounds under the dash. Maybe the signal from the "A/C Request" circuit doesn't have enough amperage to set the PCM/VCM? Or could there be a problem with the temperature control? Does that have anything to do with the compressor clutch circuit?

[BobcatAZ]

Just an update. I now have the entire dash removed from the vehicle.
I know, I was only going to check the "Request for A/C circuit". However, I did find that the air door in the heater box for floor to dash vents didn't work. It was broken so . . .
Anyway, I will put things back together over the next couple of days. I am still stuck with thinking the PCM/VCM needs to be replaced. Can anyone talk me out of it?
Thanks, Steve

[BobcatAZ]

Could the Computer be suffering from Heat Exhaustion? The A/C system works fine when everything is cool, around 80º F. However, after running the vehicle for about 10 minutes and letting the computer warm up to about 100º F. the A/C system starts acting up. (I live in Southeast Arizona).
Up until the last 3 minutes of A/C operation the system cuts off at 21 psi and back on at 39 psi. But, just before quitting all together the "window" of cutting off and on moves up. Example: the system will cut off at 40 psi and back on at 60 psi. The "window" keeps moving up until the cut off pressure reads 50 psi and then the cycling stops.
Now, if I take the computer (PCM/VCM) out of the vehicle and put it in the refrigerator for about 30 minutes and re-install it, things start working again.
Could the computer have such a problem? Nothing else in the vehicle's operation is affected but the A/C.
Could the heat build up of the computer be causing a faulty resistance reading in the A/C circuit?
Thanks.