There is no connection between the AC compressor and the tach per se, but the tach and the AC compressor require an engine rpm signal to function. The engine rpm sensor is near the oil filter and reads off the flywheel The signal goes to the EGR controller then onto the tach and AC compressor relay. There's a white wire and brown wire with white stripe from the rpm sensor to the EGR controller. I believe you can tap into the wires with a voltmeter set to AC and check for a proportionate rise in voltage with rpm. From the EGR controller, a green wire with black stripe splits to the tach and AC compressor relay.

A wiring diagram is available in the CD manual set available from MB and FastLane.

Sixto
95 S420
87 300SDL

Sixto Sir:
Thanks for the response. This does seem to make sense to me. I noticed, however, that on this car the rpm is actually taken from inside the fuel injection pump. There is another rpm takeoff from the harmonic balancer, i.e up front, but the wire goes to a test module sitting on the left front fender, of course under the hood.

KhRunge