I've got a 91 190E and the ac compressor will come on for 3 seconds and shut off. I've replaced the compressor, MAS unit, and checked everything I know to check but it still cuts out after 3 seconds. One thing to mention is when the compressor is on the tachometer acts crazy. Anyone have any ideas?

Steve Brotherton speaks of the compressor speed sensor on the back of the compressor on some models. It's purpose is to let the MAS compare engine speed to compressor speed and disengage the compressor if there is a difference above a certain limit, thus preventing a broken belt. Because of the RPM craziness at compressor disengagement, I'd at least look into this protective circuit. May help, may not. FWIW.

Steve's article:http://www.peachparts.com/Wikka/AcDiags

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

Not enough information. What are the pressures? After it shuts off do you have to turn off the ignition and restart the engine to make it come on again?

Good luck,

Yes if I restart it will come back on for 3 seconds. The pressures are correct. I'm thinking it has something to do with the tach but I've never seen this problem before.

When you restart the car or the air conditioner? I'm assuming the car, in which case the problem is a slipping belt. Either the tensioner has given up or you have an oily belt.

Good luck,

The belt is not slipping, the speed sensor is putting out .3 volts and the pressures are good. If you start the car and run at 3000 rpms the compressor stays on and the air gets down to 36 degrees. What I can't figure out is why the tachometer acts so strange when the compressor is on.