you have a problem with the vent door flap.
answer if your heater and a/c are intergrated, you may have a leaking heater core
answer
I thought the 2002 Saturn SL2 computer responsible for air conditioning and heating had failed, because the air conditioning comes on when I put the vent position to defrost (I'd never noticed this before and had experienced other problems with that computer in the past when it had been programmed incorrectly.)
I took it to the Saturn dealer, and the service receptionist had never heard of it. The service technician informed us that the air conditioner comes on by design, to remove humidity from the air. The only problem with that, of course, is that in very cold weather, the humidity is frozen out of the air, and you'd like as much heat as possible on the windshield.
Anyway, maybe someone else will appreciate the strange answer. I personally would like to choose whether to use the air conditioner in the dead of winter!
Add on, Also I was told from a dealer once that the reason the airconditioning pump comes on when the defrost is selected is that the compressor for the airconditioner needs to run every so often. During Summer it is expected people will use the airconditioner and so it is used frequently, however in winter the airconditioner could sit inactive for some time. We regularly use our defrosters with heat in winter and that is a good time to run the airconditioner as well.
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