chevy camaro IROC A.C. problem

I have a 1987 chevy iroc camaro. The problem is when the air conditioner is turned off inside the car the A.C. compressor kicks on and off. When the ac is switched to the on position it works fine. What would cause the
compressor to do this. My engine is a 5.0 Tuned port Injection. Any help would be appreciated!!!!!!!
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if the switch is on the defrost position it will kick the compressor on and off to dry the air so the windshield wont fog up

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