Hi,
Thanks for replying. I will post a photo later but it is a 1980 300sd (w116) with the old style climate pushbuttons. There is a separate a/c switch to the right of the pushbuttons which directly controls the compressor. As long as the pushbuttons are set to auto lo or auto hi the switch will control the compressor directly.

The Mercedes shop here did the repair and they are a reputable Benz shoo with a lot of experience. They put in a new compressor and expansion valve and receiver/drier, and said they had to flush the condenser due to seeing metal particles on the old seized compressor.

They told me I needed a new auxiliary fan and temp switch, but I repaired the fan easily, tested the switch and circuit according to the service manual
And it works now. It comes on while the car is idling and it is hot out, but only after idling about 2 minutes when outside temp is 95or so.