If I have the fan selector on Max the A/C and blower will kick on for a split second when I start the engine then shut off. The compressor is engaging.. at least it sounds like it is disengaging when the system shuts down.

I've swapped the ACC unit with the one in my wifes '83 300D to no avail. FYI my ACC unit works just fine in her car.

Where should I start to look for a problem?

Beyond my limited knowledge...

Re-post on TECH forum and ask "stevebfl" (resident a.c. guru)