HVAC System - Manual

Air Distribution System Description:

VACUUM SYSTEM

All valves in the HVAC module except the temperature valve are controlled by engine vacuum drawn from the vacuum tank, mounted on the RH hinge pillar. The vacuum is routed by a vacuum selector valve in the HVAC control head to spring-loaded vacuum actuators on the HVAC module. Vacuum stored in the vacuum tank is prevented from bleeding back to its source during conditions of low engine manifold vacuum (acceleration, heavy load, etc.) by a check valve in the vacuum source line.

The vacuum selector valve in the control head is operated by the mode control. For each mode control position, the vacuum selector valve directs vacuum to specific hoses to overcome the force of the spring within the appropriate vacuum actuator(s). The vacuum selector valve also vents the hoses to each remaining vacuum actuator to normal air pressure, allowing the actuator spring to expand. This results in the valves (doors) being properly positioned for the selected operating mode.

The mode, defroster and heater valves use three-position vacuum actuators. A three-position vacuum actuator has a diaphragm with a spring on both sides. At rest (no vacuum applied) the diaphragm will be in the mid-position, allowing for a blend mode. When vacuum is applied to either end of the diaphragm, the diaphragm will move the affected valve to either stop, which will direct airflow to only one of the two possible outlets.

When air is directed to the floor outlets, as in LOWER mode, a small amount of air still bleeds through the defroster outlets to help eliminate re-fogging of the windshield.