I don't think crank speed is an issue with these cars. I understand that this appeared with the W124 because it has a serpentine belt, and a compressor failure
can break the engine! So those compressors have a built-in speed sensor, and if the
crank speed and the compressor speed are too far apart, the compressor is disconnected.
Unfortunately, the speed sensor makes those particular compressors Mercedes-specific, and therefore pricy.
Update: I have successfully charged the refrigerant circuit with R134a. With a clip lead to the compressor clutch, I get cold air out the ducts. Yay! But the control system still won't turn it on without help from the clip lead
I've half a mind to scan this climate control schematic into a GIF and post it on my website, so I can show people just what I have to work with....
- Jerry Kaidor ( firstname.lastname@example.org )