The purpose of the compressor comming on in defrost mode is to dehumidify the air comming out of the vents. Dehumidified air will clear a windshield faster than high humidity air. The air passes across the AC evaporator, dehumidifying it, then across the heater core to warm it. Remember, if the ac system goes on the fritz, even the compressor won't come on. Its not a perfect system, but better to have warm air on the floor and windshied in the cold climate areas, than no air or even cold air......

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