The engine vacuum regulates all of the actuators (3, 10, 12 and 13) (except the temperature valve) in the heater and air conditioning (A/C) evaporator module. The vacuum is drawn from the vacuum tank (1). The tank is mounted to the right side body rail.

A vacuum selector valve routes the vacuum to spring-loaded vacuum actuators (3, 10, 12 and 13) on the heater and A/C evaporator module.

During conditions of low engine manifold vacuum, a check valve in the vacuum source hose prevents vacuum in the vacuum tank from bleeding back to the vacuum's source.

The mode control regulates the vacuum selector valve. At each mode control position, the valve performs the following actions:

The vacuum (bi-level) (10) and vent valves have two-position vacuum actuators. When vacuum is directed to either actuator's vacuum chamber, the vacuum pulls in a flexible diaphragm. The diaphragm forms one wall of the chamber. This action draws a movable shaft attached to the diaphragm. The shaft is linked to the corresponding valve. The shaft operates the valve when the vacuum is applied.

An external spring moves the shaft back to its original position when the vacuum chamber is vented to normal air pressure. The shaft, in turn, returns the valve to its original position.

The heater and defrost valve (13) uses a three-position vacuum actuator. A three-position vacuum actuator has a diaphragm with a spring on both sides. At rest (no vacuum applied) the diaphragm is in a mid-position. This allows for a blend mode. Apply vacuum to either end of the diaphragm in order to move the heater and defrost valve to either stop. This action directs airflow to either the defroster outlets or the floor outlets.

When air is directed to the floor outlets, as in the HEATER setting, a small amount of air still bleeds through the defroster outlets. This action helps eliminate the refogging of the windshield.