89 300E. after several days and numerous tests
my independent mechanic and other a/c guys
have identified my acc to be toast. they have tested
all the relays, freon pressure, switches and the
klima relay.

before i shell out the cash (not cheap), i would like to
know any comments, if in fact a defective acc will
cut out the compressor. it will again engage when
car re-started.

Are you talking about the pushbutton control unit? Most all the brains of the system are in the pushbutton control unit. I have seen this referred to as the Climate Control Unit, or Climate Control Module.

If this is what you are referring to, it is extremely important to test the current draw of the auxilliary water pump. If the current draw is more than 1.3 Amps, replace the Aux. Water Pump at the same time. Excessive current draw on this unit is the most common reason for Climate Control Unit failure.

Good luck,

thanks Larry, will check it. cheers!!

If the a/c compressor always engages when the car is restarted, I will bet money against the pushbutton panel. Your seeing the compressor RPM circuit trigger. It compares the compressor RPM against engine RPM. If they differ too much, the compressor is shutdown - until the engine is restarted.

There are a couple of possible causes.

1) Excessive compressor clutch gap. Should be 0.50mm
2) Grease/oil on compressor clutch surfaces.
3) Loose serpentine belt/failed belt tensioner.
4) Bad electrical connections between compressor RPM sensor and Klima relay.
5) Bad Klima relay.
6) Bad electrical connections between engine flywheel speed sensor and Klima relay.
7) Bad compressor RPM sensor.

These are more or less in order of ease of checking/correcting. If your guys haven't checked the clutch gap don't let them put in the pushbutton unit.

I know of at least one case where a $1 spray can of degreaser fixed the compressor cutout problem.

As a last resort, the pushbutton units can be purchased rebuit for about $150. Dunno if partsshop sells them, but there are several vendors. Don't pay too much.

thanks for your reply. i've sent you a private email.
your response would be appreciated. thanks