A.C. compressor is enguaging when in the "OFF" position

I own a 1987 5.0 iroc camaro.The problem is that the compressor is enguaging and disenguaging when the air conditioner is in the "OFF"
position. Does vaccum control the operation of the compressor or could it be the compressor relay??? The a.c. works fine in the "ON" position. Any one who knows about this or had this happen some help would be appreciated....HELP!!!!! I dont want to get robbed by a repair shop and deal with people who lie.
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