what would make the A.C compresser frost over when not being used on 1993 GMC Sonoma

it is on a 93 gmc sonoma i am in the the proses of replacing the heater core and was wondering why the A.C compresser was frosted over after driveing but not useing the ac

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1 answer
If you are using the defrost, then the A/C compressor is still working. Are you using defrost at all?