Within the heater module assembly are mode valves. These mode valves are hinged parts that act like doors to direct the airflow through various sections of the heater module. They provide the proper airflow for the selected temperature and operating mode.

All mode valves except the temperature valve are controlled by the engine vacuum stored in the vacuum tank in the engine compartment. The vacuum is routed by a vacuum switch in the control assembly to any of the spring-loaded two or three-position vacuum actuators. One vacuum actuator is linked to each air valve.

The vacuum switch in the control assembly is controlled by the mode control knob. For each mode knob position, the vacuum switch directs engine vacuum to specific hoses and vents the remaining hoses to normal air pressure. When a hose vents to normal air pressure, the vacuum actuator's internal spring expands and holds the associated air valve in one position. When the vacuum is routed to the hose, the force of the vacuum overcomes the internal spring pressure and moves the air valve to the opposite extreme of its travel. In this way, the mode control knob positions the mode valves for the selected operating mode.

The temperature of the air discharged from the heater system is regulated by controlling the relative amounts of warm and cool air that are mixed together downstream from the heater core area. The part of the total airflow that is allowed to pass through the heater core is governed by the position of the temperature valve.

Hot coolant from the engine's cooling system is directed through the heater core. The heater core transfers heat from the coolant to the air passing through the fins.

After the heated and unheated air flows are mixed beyond the heater core, the combined airflow comes to the mode valves. These valves work in combination to control airflow through the passage that feeds the instrument panel outlets. For the VENT mode, the vacuum is directed to the vacuum actuators and the mode valve(s) open to allow air to flow to the instrument panel outlet, as shown by the symbol. In the HTR, and DEFROST modes, the vacuum actuators are vented and the mode valve(s) close to prevent airflow to the instrument panel outlets, as shown by the symbol.

When the BI-LEV mode is selected, only the lower mode valve is opened. This divides the airflow so the instrument panel outlets, the floor air outlet(s), the defroster nozzles and the side window defogger outlets all receive a share of the air, as shown by the symbol.

Another vacuum controlled mode valve in the air distribution system is the defroster valve. In the HTR and BI-LEV modes, vacuum is applied to the defroster valve actuator. This moves the defroster valve to divert almost all of the airflow to the floor air outlet(s) to heat the passenger compartment. Only a slight amount passes through an opening in the defroster valve and goes to the windshield defroster nozzle, as shown by the symbol.

In the DEFROST mode, the defroster valve actuator is vented to normal air pressure and the spring in the actuator holds the defroster valve in a position to route most of the airflow to the windshield defroster passage in the air distributor, as shown by the symbol. In the VENT mode, the defroster valve also is held in this position. Only a slight amount of the total airflow passes through the opening in the defroster valve and goes to the air outlet(s), as shown by the symbol.