MB put an rpm sensor on the compressor to keep a slipping or locked compressor from overheating or breaking the drive belt. One belt driving many things...if the belt breaks, you have no water pump, alternator or power steering. The rpm signal from the compressor goes to the Klima relay and is compared to an engine rpm signal. If the difference between them is too great,the compressor clutch disengages.Really a simple system when you think about it.

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W.R.Durrance
ASE Master Tech
Mercedes Tech 19 Years