The most likely failure is the belt tentioning device. If the belt flops the MAS turns off the circuit.

If the system won't reboot with an ignition cycle then this isn't the problem although close attention should be made because a loose belt will drop the compressor almost instanly.

The MAS would be my next choice although most that I've seen bad were caused by shorted clutch coils.

BTW if you want to scope the compressor speed signal; it is a very low voltage AC wave with as low as .75v peak to peak. The engine speed signal is a square wave, having been converted by the ignition comntroller.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician