Look at yesterday's (Sunday's) Diesel Forum. You will find a discussion called "A/C & Tach problems" The answer to your question is there.

Below is an extract.

The AC speed sensor, and the engine speed sensor both feed their signals to the KLIMA relay. The output of the two speed sensors is then compared and it if differs by more then approximatlery 10% the KLIMA relay shuts off the AC. The presumption being that the belt is slipping or that the AC compressor is seized.

The engine speed sensor also provides the input for your Tach. In your case it sounds like the engine speed sensor is not functioning properly, thus the Tach stops working and the KLIMA relay shuts off the AC as it sees different speeds at the engine and AC Compressor.

Fix your engine speed sensor and you should be OK.

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]