JIMMY ELECTRICAL PROBLEMS?

Hi guys just purchased a 1998 GMC JIMMY 115000kms on the clock, and everytime I switch on the windshield defroster there is no heat coming through because the A/c compressor turns on. Then when I turn the knob to
dash vents compressor shuts down and normal heat blows out. The A/C works great when in it's position. So can anyone help me figure out why the A/C compressor turns on when I turn the knob to defrost ?
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It's supposed to do that. The incoming air hits the A/C evaporator first, which removes the moisture from the air, then it hits the heater core which heats it before blowing out your vents. The A/C compressor helps dry the air so your window defogs faster and stays fog-free even in very humid conditions.
Doc

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lol, electrical problems, righttttttttt

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One of the "low fuel economy" trouble shoots in my service manual is excessive use of the defrost option. BUT - my truck blows hot air out the defrost vents.
GMC Gremlin

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Drive 5 mph slower than the posted speed limit and you can save up to 25% more gas that way, but you also piss off 50% more drives on the road, its a win lose situation.

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