The inoperative Tachometer may be a clue. See this thread: Particularly post #4 regarding a Klima Relay and Tachometer keeping the A/C from operating...

AC Doest work - where do I start ?

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)