Troubleshooting W126 Climate Control

When I purchased my 300SE, the A/C worked well.   I assume the overall Climate Control system did too.  I never did test anything more than max A/C as the weather was very hot.  Everything seemed fine, except for the little aspirator fan which was noisy.  Unfortunately 3 weeks after purchase, the A/C stopped working.    I was sitting at red traffic lights and adjusted the settings and it went warm.    This indicated that it was probably not a leak in the system somewhere.    Just to be sure, I had the system checked and it was not low on refrigerant.

The first thing I tried was changing the Climate Control Unit.  This is the push button unit in the dash and they are known to cause erratic climate control if they start to fail.   I was able to purchase a used (but untested) unit for $25.

This change did not seem to make any difference.  I noticed that the compressor was never engaging even when on max cool.  Therefore, the next thing I tried was removing the Klima relay and bridging pins 5 and 7.   The Klima relay controls the A/C compressor and determines when it should engage.   It uses various sensors to control the engagement of the compressor as well as the cooling fans.    This eliminates any of the sensors.    Bridging these pins should bypass all these sensors and simply apply 12v to the compressor clutch.

Doing this and starting the car did result in the compressor engaging briefly.   I even got a bit of cold out of the system!   However a few minutes later fuse 5 blew and the Climate Control System shut down.      Fuse 5 controls most of the climate control functions (except the blower motor), so the next logical step seemed to try and eliminate what could be causing it to blow.   Next thing I tried was to disconnect the mono valve and the Aux water pump.

The Aux water pump is a small pump that increases the flow of hot coolant through the heater.   These pumps are known to draw too much current as they age and are really not necessary in warm climates.    At -20C, having the extra pump does ensure hot coolant from the engine reaches the heater faster.   The mono-valve is used by the climate control system to control how much coolant comes into the system.   (i.e. none for max cool and full flow for max heat).

Fuse 5 still blew, despite eliminating the aux pump and mono valve.   This time, It happened even before I turned on the engine.   Next step may be to test the compressor clutch with an external power source.   This would eliminate some problem with the car’s wiring.