Howdy, I've never had to work on the A/C system, but I've got a '84 190D 2.2 and the MB CD-ROM for it. I'd certainly recommend buying the CD-ROM for a W201 as it has been very helpful in fixing things on my car so far.

The pressure switch itself is located on the receiver/drier It's a cylindrical barrel that is on a tee off the top.

The relay should be inside the pushbutton control unit (not such a good design idea). The output of this realy comes out of the control unit on a bl/gn wire to terminal 'O' on the fuse panel (engine bay) then goes out as a bl/ws wire on what looks like terminal 18. This goes over to the pressure switch at the receiver. The output of the pressure switch is a bl/gn wire that goes over to the compressor clutch.

You should really get the CD, it's good although some of the pics are fuzzy.

Anyway, there should be 12V on the pressure switch at the bl/ws wire if there is a call for cool. I'd disconnect it first and ohm out the pressure switch. Then I'd see if there actually is 12V at the switch. You could momentarily light up the compressor (AT YOUR OWN RISK!!!) using battery voltage. The brown wire coming out of the clutch is chassis ground: you could Ohm out that also: grounds have been known to go bad.

Sholin

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What else, '73 MB 280 SEL (Lt Blue)
Daily driver: '84 190D 2.2 5 spd.