Originally Posted by Diseasel300

I know nothing about the W201 system, but I'll share my experience troubleshooting the W126 Gen II system which probably has some similar control circuitry to it.

1: The voltage source to engage the clutch originates in the Klima relay on the diesels (apparently the MAS relay on the gas cars). Not only does it contain the relay contacts for the clutch itself, but it monitors the call for the compressor from the pushbutton unit, monitors RPM from the sensor on the compressor, and monitors the Tach signal from the engine.

2: The control circuit runs from the Klima relay (MAS relay apparently on the gas cars) through the pressure switch and back to the pushbutton unit.

3: The control circuit is GROUNDED by the pushbutton unit which then allows the Klima relay (MAS) to close the compressor relay and engage the clutch.

If the RPM sensor on the compressor is bad, the clutch will not engage. If the tach signal is not present, the clutch will not engage. If the pressure switch is open, the clutch will not engage.

You have ruled out the pressure switch by showing it has continuity.

You have tentatively ruled out the wiring since you show continuity from the MAS back up to the pushbutton unit.

You have proven that the Klima (MAS) relay functions, the RPM sensor is good, the Tach signal is present, and the control signal is present by grounding the wire at the pressure switch.

That leaves a poor connection from the pressure switch back up to the pushbutton unit.

Or:

That leaves a pushbutton unit that has some sort of problem and is not grounding the circuit when the AC is called for. Look for broken grounds, broken traces, poor solder joints (especially around the pushbuttons), or burnt components.