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

Ran out of ideas so I drove my the dealer. I told them I bypassed the pressure switch and still no compressor rotation. They wanted to hook up pressure gages to begin diagnostics. This didn't make since because I already determined that the low pressure switch wasn't failing the system. I left the dealer with the feeling that I may know more about the operation of the A/C than the tech I spoke with at the dealer.

It's going in on Wed to a non-dealer shop that at least seems to know more than I do about the system. We think it's electrical problem.

I'll post the results & conclusion.

There is one thing you should be aware of. On these cars there is a sensor which detects a slipping belt condition. This will shut down the compressor. If it started with the wash down or shortly thereafter, the belt or pulley(s) may be greasy, wet, etc.

Check your tensioner to see if the tension is within spec. Also check the belt, pulleys. Well, you get the idea....

Good luck,

Thanks for the reply Larry,
I had it check out at MBI in Portland OR. They determind that it was a bad temp switch. In the event that the engine is over temperature for some reason, the over temp switch shuts down the compressor to reduce load on an over-temp engine.
I was aware of the rpm/belt slip protection, and of course the low freon pressure switch.
Now I'm also aware of the over temp switch.
Here's a link to a photo of the new switch. It's bright green and located between the #1 injector and the fuel filter.
http://albums.photopoint.com/j/ViewPhoto?u=1667772&a=12785403&p=50676474&f=0