Steve, thanks for the replay.
I checked the pressure switch on my '88 260 with a ohm meter and got infinite ohms (switch is open). There is a possibility that there is no gas in the system although there are no signs of leaks anywhere.
The next question is if the system is low, how can I add R12 if the compressor does not run? On other cars I have owned, when the pressure gets low the compressor cycles on and off so it will at least suck in the R12.