Vacuum-actuated doors control the airflow through the HVAC module. Outside air is either cooled or heated. The air is then discharged according to the control setting.

You do not have to replace the entire vacuum hose assembly if the vacuum hose leaks or collapses. You can replace the hose by cutting the hose and inserting a plastic connector. If you must replace an entire hose, cut the hose off at the connector. Then attach a new hose directly to the vacuum solenoid control assembly.

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

A system of ducts and outlets directs air to the passenger compartment. Check the following components in cases of poor air output:

Check for the following obstructions: