With the mode knob in the Upper position, the vent valve opens so air can flow to the instrument panel outlet ducts.

When the mode knob is in the Lower position, the vent valve partially closes. The defroster valve remains closed. The blower fan heats the air and directs the heated air to the defroster valve. The defroster valve sends most of the airflow to the heater duct and some to the defroster nozzle.

When the mode knob moves to the Bi-Level position, the defroster valve moves to uncover more of the opening to the defroster. An increasing amount of air flows to the defroster nozzle, and a decreasing amount of air flows to the heater duct.

When the mode knob is in the Defrost position, the defroster valve moves to fully uncover the opening to the defroster nozzle and restrict the opening to the heater duct. The defroster nozzle receives the maximum airflow, with only a little amount of airflow coming from the heater duct.