The compressor is controlled by various sensors, like pressure, and on the early cars the speed of the comprssor was matched to the speed of the engine. My 92 drove me nots, it would work I would accelerate or hit a buddle it would stop untill you turned off the ignition and restarted it, slipping belt and then bad tensioner. try diagnostics thru the instrument panel.
http://pages.prodigy.net/jforgione/MB_S500.html