1987 300SDL

A/C worked fine on the way to the store, but failed on the way home.
Compressor will not engage. I closed the circuit at the pressure switch next to the dryer and still no action at the compressor clutch. This seems to rule out low freon pressure.
I checked the fuses and they were good.
The belt it good and I don't think it's a clutch problem because I would think the clutch would hold for a while, then slip and shut down the system after a slip is detected.
I'm not sure what the next step in trouble shooting should be.
One thing that I wouldn't think would be related is my LOW, AUTO, HIGH selector does not seem to do a thing. It hasn't for a year and the climate control works comfortably in both winter and summer conditions as if it were in AUTO. I just let the computer control the fan speed and it's comfortable in no time (prior to the A/C failure).

One last comment, I did a hose down the engine compartment and use a bit of degreaser around my #1 & #2 injectors as well as my steering pump located directly above the A/C compressor. I checked the connections at the compressor for water/contamination and didn't see any problems. The system did function properly on the way to the store following the wash down.

Thanks in advance