I have read that the tach "tells" the compressor that there are "revolutions" from the engine. If the tach fails, the compressor thinks the car has been turned off and therefore will not engage. A safety feature so you don't burn out compressor. I don't have this on my vehicle, but have read posts that indicate this scenario. Hope this helps a little.

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Nello Tare

1982 300D (266,001 mi.) looking and running great

Previously owned:
1976 450 SLC
1983 300D
1976 300D
also
1982 Jaguar XJ6 (loved, but gone)