The A/C stopped working over the winter. The compressor does not seem to be running. I removed the control unit compressor cutoff back behind the battery (part number 002 545 12 05).
1. Connected terminals 5 and 7 on the 12-pin connector; started engine, and detected compressor was running, I think. I did detect some change when I removed the jumper. With the engine off but the key on, connecting the jumper caused a click of the compressor's electromagnetic clutch.
2. The RPM sensor checked out OK (0.53vAC between terminals 9 and 11 of the 12-pin connector with the jumper still in place between terminals 5 and 7).
3. Transistorized rpm signal checked out OK (8.4 VDC between terminals 1 and 2 of the 12-pin connector with jumper still inplace between terminals 5 and 7).
I also checked the "refrigerant compressor pressure switch" which is down by the bottle with the little glass window in it. I believe this switch is supposed to make if there is pressure in the lines once the compressor start running. But since the compressor did not seem to be running, this did not seem to be valid test. Shorting out this little switch's terminals did not cause the compressor to run. (For this test I replaced the control unit in its 12-pin connector, connected a jumper across the pressure switch's terminals, started the car, turned on the A/C with temp set to minimum, still the compressor did not run.)
4. The temperature switch was not grounding out pin 12 of the 12-pin connector, I checked.
5. With the A/C turned on and temperature set to minimum, the input side of the "refrigerant compressor pressure switch" was grounded.

My questions follow:
The control unit (part number 002 545 12 05) normally energizes the electromagnetic clutch on the compressor to get it started, but what input to the control unit tells it to energize the electromagnetic clutch? The only input I can see from the push button switch unit in the dashboard goes to the "refrigerant compressor pressure switch." But for this signal to get through the pressure switch, the compressor has to be running. So what gets the compressor running in the first place?

Is there a sensor that detects the level of coolant in the system? (My A/C was converted over to non-R12 stuff last March, and it worked great all summer in Tucson. I don't think this has anything to do with the A/C not working, but thought I'd mention it.)



Now it is the next morning. I don't have a way to test the pressure - will have to take it to the shop (Mark at Benztek in Tucson) probably to get that checked out. Still this seems electrical to me. Shouldn't the compressor run for a little while even if the coolant is all leaked out?