Weird AC Clutch Voltage Drop?

[aaronec91]

Hi all! Recently acquired a 1979 300SD, runs great and been using it for a couple weeks as a commuter, my first diesel Mercedes! A PO had repaired the AC system (new compressor, upgraded/rebuilt vacuum servo, R134). To clarify this is the auto system with the big temp wheel and push buttons. The system worked flawlessly until yesterday...everything still functioned, except no cold air or compressor engagement.

Ran through some tests, jumped the comp clutch to 12v and checked pressures (about 40/150 when running) and when jumped the system works, blows cold. Probed the unplugged clutch electrical connector with engine on and climate on, get full 12v. However, I backprobed the clutch connector, and as soon as I plug it into the AC compressor, voltage drops to 0 (between 0-0.01 on my meter) and clutch does not engage, unplug it and full 12v again immediately. I also checked the AC On/Off dash switch and it gets power and seems to have continuity where it should.

I would expect if I have a shorting AC clutch, my leads would get hot and/or it just wouldn't work, or fried something else, preventing any voltage from appearing, but unplugged there's 12v to the clutch connector. Looked at the wiring diagram from w116.org and it doesn't appear the clutch is controlled by the climate amplifier, just a series of switches. I'd think if any of the switches were faulty it would get no power.  Am I missing something here?? Thanks!