The pushbutton unit on the dash sends a +12v signal to the Klima relay telling it to activate the compressor. I don't have a wiring diagram handy, so cannot tell you which pin it is on the relay socket. The +12v signal passes through the low pressure switch mentioned by Larry. That switch is located on the dryer.

There are also some other inputs to the Klima relay which can shut off the compressor. High coolant temperature (>115C) will shut down the a/c system. On diesel cars there is also a full throttle switch which shuts down the compressor - not certain if gasser cars have this function. You need to check these inputs as well.

You really need the climate control shop manual, a wiring diagram, and an inexpensive DVM to troubleshoot this system. It's not really all that complex, but without that basic information you won't get too far.

- JimY