On your receiever/dryer are mounted two switches:

1. Refrigerant temperature switch: Activates the aux. fan when this switch detects a temperature above a present point (~62C if I recall correctly). This switch is mounted at the top of the receiver/dryer. Shorting the leads to this switch will force the aux. fan to be always on. You do not have to evacuate the refrigerant to replace this switch.

2. Refigerant pressure switch: Disables the AC compressor if there is insufficient refrigerant. This switch is mounted in the side of the receiver/dryer. Shorting the leads to this switch will cause the compressor to engage regardless of the amount of refrigerant in the system. You must evacuate the system when replacing this switch.

How do I know all this? I have the exact same car and my temp switch was broken so I jumped the leads so the fan would stay on. My pressure switch then gave up the ghost and vented all my new freon. I have ordered a new receiver/dryer and both switches and am contemplating a switch to R134A. (I read up on the subject in my W126 Service CDROM).

Chris Blanchard
1984 300SD 140.8k