When the mode rotary knob is in the VENT position, the defroster valve and the heater valve remain closed. As a result, the air coming from the instrument panel vents should be approximately the same temperature as the outside air.

When the mode rotary knob is in the HEATER position, the defroster valve remains closed. As a result, the air from the blower fan receives heat. This air goes to the defroster valve. The defroster valve sends most of the airflow to the heater duct, with some of the air going to the defroster nozzle.

when the mode rotary knob is in the DEFROST position, the defroster control cable moves the defroster valve in order to fully uncover the opening to the defroster nozzle. This restricts the opening to the heater duct. This delivers the maximum airflow to the defroster, with only a small amount of the airflow exiting out of the heater duct.