engatwork,

Did you get the a/c working? Here are some things to keep in mind and/or look at.
Compressor won't come on for 10 seconds after the prm is at or above 600rpm.
WOT switch open. (I don't know exactly where it is but I think it is on the valve cover.
2 function pressure switch closed.
Over boost switch open. could be stuck closed.
If compressor comes on, the RPM and compressor speed sensor have to match with in a few seconds.
Could just be a bad clutch.

Does the car run good?
Does the tach work?

The 90-93 300D 2.5's are very much different from the older 300D's.

Paul

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84 500 SEL (307,xxx miles)