Compressor does not turn on - clutch never engages. Occurred right after repair of fatigue-broken fasterners in compressor mounting bracket. Don't think its related, but ... Where should I start looking? What's most to least likely cause?

Everyone:
Shaq was right. It was a broken solder joint on the back of the A/C controller ckt board. The broken one was one of the processor pins. I fixed it in a couple minutes once it was apart.
Thanks all.

Possible cause is the compressor clutch wire or the ground. One of them snapped, probably.

But... for the very easy test, disconnect the pressure switch harness on the accumulator/dryer, and put a wire between the terminals. Then turn on the A/C. If the clutch still doesn't run, then look into a wire problem at the compressor.
--
Chris Herbst
1992 745, 68k

And others:
93 944, 150k
90 245, 110k
88 744, 160k
87 245, 185k

Thanks. I did this. Compressor did not come on.

I separated the connector just above and aft of the compressor in the clip to the engine block and measured resistance to ground and control voltage. The lead going to the compressor measured 0Kohms. I assume this is normal because it runs to ground thru the clutch coil. Is that correct?

The voltage on the other end varied curiously. When the A/C button inside the car was pressed, voltage rose slowly to about 6.5VDC, then hung out a while before increasing to ~10.5VDC. I made sure I had good contact with the voltmeter leads. Is that normal?

It could be that there is a ground around the compressor. What is that circular thing attached to the top of the compressor with a little screwed-down clamp; the one with the compressor lead going into it then reemerging to go to the clutch? Can I take it loose and further localize where the lead may be grounded? Since it appears voltage is making it to the clutch coil, what else might cause the thing to not engage?

Is it cost effective to replace a clutch assembly and not the whole compressor (130K miles)?

Thanks in advance for bearing thru all this.

I side with Shaq. There's some control unit problem causing the voltage to vary like it is. You should see a solid reading.

The later model compressors are pretty sturdy. I don't know who makes them, but they seem to hold up to the job better than some others. I'd suggest taking the time to see what's up inside the control unit and then try it again. You can jump the hot lead to the compressor coil to see if it engages. In other words put a B+ wire on it and see if it turns on. That's not a premier way to do it but it will tell you immediately if your compressor is faulty or has a current problem.
--
Chris Herbst
1992 745, 68k

And others:
93 944, 150k
90 245, 110k
88 744, 160k
87 245, 185k

You should check for 12 volts at the low pressure switch on the accumulator located on the passenger side of the firewall. If you are not getting a good, consistent 12 volts there may be a broken or cracked solder joint in the a/c control head. To remove the contoller you will need a phillips screwdriver, a torx25, and a 10mm socket. Remove trim around controller. Remove the fuse panel cover and cigarette lighter panel. Pull out the radio, but do not disconnect unless you know the code. Take out the screws in the top of the radio slot pull it out. Take out the 4 bolts holding the metal bracket with the two tabs that go over the bottom two holes of the controller. Remove 4 screws for controller and unplug vacuum hoses and wires. With the controller out of the car bend back the 4 metal tabs that hold the platic cover on. Flip the unit over and remove the 2 screws that hold the circuit board in place. Flip the circuit board over leaving the wires attatched. Upon close inspection you should be able to see at least one cracked solder joint. Resolder and reinstall. This usually takes 30 to 45 minutes and saves about $500 dollars.

Thanks.

I've got 12V at the accumulator switch, and have verified that the clutch engages by hot-wiring it, so the clutch coild is good. It has to be an issue with the controller. When I get some time this weekend, I'll pop it out and check it over.

Thanks again.

The a/c compressor isn't engaging on my '93 945, either. I turned on the a/c and checked the voltage at the connector on the accumulator and get no voltage at either wire. Do I understand correctly that there needs to be voltage to one of these wires before the compressor can engage? Would system pressure have any bearing on whether there's power to one of the wires?