I cant imagine why you would have a drive belt squeal only when the ACC is on, whether or not the compressor is engaged. The only ACC-related item that would put added load on the drive system resulting in drive belt squeal is the compressor....nothing else. The only other components that are engaged by the ACC system are mono valve, elecronic control unit, blower, vacuum switches (for flaps), and the aux. water pump. None of these put a load on drive belts. Of these, the only thing that can squeal is the aux. water pump..not a belt squeal, but squeal due to motor bearing or pump housing. I think that the aux. water pump functions at idle rpms, ensuring adequate coolant flow to monovalve/heater core. I suspect that your squeal happens only at idle, and is from failing aux. water pump...this is the thing mounted in the engine compartment just in front of the pass. side wheel hump.

Use the broomstick stethoscope method...take a broomstick handle, dowel or other piece of wood and put one end on your temple bone and use other end to probe for noise sources...i bet when you touch aux. water pump, you will know that this is the problem. (maybe not, but thats my best guess)

If this is the problem, fix it soon, because if it fails in a mode that causes very high current draw, it can fry the ACC pushbutton unit.

If this is not the case, give more info....squeal only at idle? also at highway speeds? are you positive about no relation to compressor?

good luck

mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)