Have someone start the engine while you look at the AC compressor. If it turns on for 1 second or so and than immediately turns off than its most definitely shutting off because the speed sensor in the AC detected a mismatch between the speed of the AC compressor and the speed of the engine. The AC compressor will than not start again, unless you turn your engine off and on. The same cycle will likely than be repeated.

This, as other have said is most likely due to a slipping belt. Fix the belt and you should be OK.

Phil

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