To test the speed sensor you need to connect a volt meter to the two wires in the plug on the compressor that go to the sensor. The plug has three wires in it, one goes to the clutch and the other two go to the speed sensor. You said you applied voltage to the clutch to make it come on so you need to do that and then check the voltage on the other wires.

Applying voltage and grounding wires randomly on this system can cause damage to the ac pushbutton panel so proceed with caution. A wiring diagram is a must have when trying to sort these kind of problems out.

Best of luck, Robert

__________________
Mercedes Benz Master Tech - Retired