Vacuum regulates air flow through the system. Depending on the mode selected, the mode valves mix and direct either cooled, heated, or outside air through the air ducts. The various modes of operation are as follows:

Outside air is delivered through the instrument panel (IP) outlets to ventilate the vehicle. If preferred, the air may be warmed by setting the temperature control knob to the desired setting.

Electrical power is supplied to the blower motor. Outside air is drawn in and heated (depending on the position of the temperature control knob). Then the airflow is split between the IP outlets and the floor outlet.

Outside air is drawn into the vehicle, heated (depending on the position of the temperature control knob) and directed to the floor air outlet. A small amount of air is routed to the windshield through the defrost outlet.

Outside air is drawn into the vehicle, heated (depending on the position of the temperature control knob) and split between the defrost outlet and the floor air outlet.

Outside air is drawn into the vehicle, heated (depending on the position of the temperature control knob) and directed to the defrost outlet. A small amount of air is routed to the floor air outlet.