Funny I should read this today. My belt of my 400E started squeeling lately. And ever since i owned the car, when i get on the gas and after it shifts, the ac shuts off due to slippage. Today, I tightened the belt and it took care of all my problems. The ac stays on all the time, and the belt doesnt squeel anymore. My belt was so loose i could take it off and put it back on with my hands without loosening anything. Also, these motors are notorious for power steering leaks. Check to see if the belt is doused in p/s fluid. If not, its most likely a bad push button control unit like Mike said. Try jumpering the two a/c wires on the reciver drier and see if the ac comp. kicks on and if the pipes get cold.

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93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K