Originally Posted by S-Class Guru

I had the same issue on my '91 6-cylinder a couple weeks ago. Turned out to indeed be the pressure switch wire connection; my was located on the accumulator, just below the sight glass. I cleaned up the contacts, and all is good. You can just short the contacts and see if the AC engages, that will tell you if the pressure switch is bad also. I think the fuse is #5, and it runs a lot of other things also, so probably not the culprit.
I think the OVP relay is also in the circuit, but don't quote me on this.

DG