With the ignition switch on, voltage is supplied through the HVAC fuse and circuit 41) to the HVAC control assembly. The HVAC control assembly receives ground from G 200 through circuit 1450.

Control of air through the system is regulated by vacuum. At various positions on the heater-A/C control, mode valves mix and direct cooled, heated and outside air through the air ducts.

The vacuum/electric solenoid contains the solenoid valves that control the mode valve actuators. There is one solenoid for each mode valve actuator. When the solenoids are de-energized, the valves vent the mode valve actuators, allowing the vacuum actuator to return to the vent position determined by a spring. Voltage is supplied through the fuse B3-B4 and circuit 41 to the solenoid valves when the ignition switch is in the RUN position .

The air inlet valve closes the outside air inlet so that air from the inside of the vehicle is recirculated through the A/C system for maximum cooling. The outside air inlet is closed when vacuum is applied to the actuator. When the air inlet valve actuator is vented, the spring returns the recirculation valve to position A, which allows outside air to be drawn into the vehicle. The air inlet valve is controlled by the inside air and outside air buttons on the HVAC control assembly.

The air temperature valve determines how much air flows through the heater core, which determines the temperature of the air to the outlets. the air temperature valve is moved by the temperature actuator. The position of the air temperature valve is determined by the voltage level of the temperature control output from the HVAC control assembly. The temperature actuator is powered with the ignition switch is turned to the RUN position. Voltage is supplied through the fuse B3-B4 and circuit 41. Ground is supplied through circuit 1450 from G200.

The defroster valve and the heater outlet valve are connected together by a mechanical linkage allowing the two valves to move together. The defroster valve and heater outlet valve can be moved to one of three positions. The heater/defroster mode valve vacuum actuator consists of two chambers operated by two vacuum actuators. If no vacuum is applied to either vacuum actuator, the valve is in mid position. Airflow with the valve in the mid position B will be delivered to both the defrost outlets at the windshield and the heater outlets at floor level. When the heater solenoid is activated, vacuum to the defroster valve actuator is supplied by the red vacuum hose and the actuator moves the valve to position C, forcing all airflow to the heater outlets. When the defrost solenoid is activated, vacuum to the defroster valve actuator is supplied by the blue vacuum hose and the actuator moves the valve to position A, forcing all airflow to the defrost outlets.

The A/C bi-level mode valve vacuum actuator is a bi-directional vacuum actuator which can move the I/P vent valve to either of two positions. Airflow with the valve in the mid position B will allow partial airflow to the I/P vents. Airflow with the valve in the position A will not allow air to the I/P vents. Airflow with the valve in the position C will allow full airflow to the I/P vents.