Originally Posted by Jeremy5848

The microswitch is not the main source of failure but rather the Klima relay itself. Klima (AFAIK a German brand name, actually) takes input from engine RPM via the EGR computer, a pressure switch in the a/c line, and the push-button unit (PBU) of the climate control system. It is unique to the 1985 version in the W123 series.

Klima is a "smart relay" and won't allow the compressor's clutch to engage if there isn't enough refrigerant in the line. It cuts off the clutch when you floor the throttle, as previously mentioned. It also won't allow the transmission to kick-down if the engine is within 500 RPM of maximum engine speed.

Failure of any of the inputs or the Klima relay itself will cause the a/c compressor's clutch to not engage. Because the Klima relay is expensive, some people (including the PO of my '85) have replaced it with a simple relay that turns on the clutch whenever the CCU calls for a/c. This bypasses the safety devices and in particular will allow the compressor to run without refrigerant, potentially ruining it.

The first test is to get under the car (engine OFF) and temporarily touch 12 volts and ground to the clutch terminals (doesn't matter which). If the clutch makes a "clack" sound, it is probably OK. Next, short the terminals on the refrigerant pressure switch. This will tell you if you have either a bad switch or low/no refrigerant in the system. Next, test the microswitch on the valve cover by unplugging the connector and testing it with an ohmmeter.

If everything seems OK, the Klima relay itself may be at fault. Some people have reported success in repairing Klima by taking it apart and resoldering the little PC board.

Finally, the climate control vents are indeed operated by vacuum pods. The rubber diaphragms often leak or split with age. They can be rebuilt or replaced. Some are very hard to get to (require pulling the dash). Remember that the center vents are for cooling only, they do not open in heating modes.

Jeremy