Vacuum System

The auxiliary heater control switch in the instrument panel has four positions: OFF, LO, MED, and HI. In any switch position except OFF, the auxiliary heater control switch routes battery voltage tot he auxiliary heater blower motor when the ignition switch is in RUN, as explaned in AUXILIARY HEATER.

In addition, the switch routes vacuum form the vacuum tank to the vacuum actuator that opens the auxiliary heater hot water valve. When open, the hot water valve admits heated coolant to the auxiliary heater inlet pipe and auxiliary heater core.

When the auxiliary heater control switch is turned OFF, electric voltage to the blower motor is cut off and the hot water valve vacuum actuator is vented to atmospheric air to close the valve. This stops the flow of coolant to prevent unwanted heat.

Conditions most likely to cause vacuum system problems include hoses that leak or become disconnected. With the engine runing, either condition is easily detected from the hiss of air at the opening. Other common vacuum problems are kinked or obstructed hoses, hoses connected incorrectly or binding components.

A vacuum pump can be used as a vacuum source to operate components and test the system. The built-in vacuum gage of the pump provides a means of checking for a vacuum leak as well. Once vacuum is applied to operate the component, the gage should hold steady until the vacuum is purposely released.

On vacuum systems that are controlled by electrical or electronic switches, check for a blown fuse, wiring that is cracked, frayed or burned, and high resistance in connectors of the control circuit (s) before check for a suspected vacuum system problem.