Vacuum Tests

Fig. 85 A/C & heater system vacuum control:

To test the A/C heater control system, start engine and move function selector lever slowly from one position to another. A momentary hiss should be heard as selector lever is moved from one position to another indicating that vacuum is available at the control assembly. A continuous hiss at the control assembly indicates a major leak somewhere in the system. It does not necessarily indicate that the leak is at the control assembly.
If a momentary hiss cannot be heard when function selector lever is moved from one position to another, check for kinked, pinched or disconnected-vacuum supply hose. Also inspect check valves for proper operation.
If momentary hiss can be heard when function selector lever is moved from one position to another, vacuum is available at the control assembly. Cycle the selector lever through each position with blower on HI, while checking location of discharged air. The air flow vacuum control charts, Fig. 85, indicate the vacuum motors that should be applied for each position of the selector lever along with an air flow diagram of the system.
If a vacuum motor is inoperative, check operation of the motor using a suitable vacuum tester. If motor operates satisfactorily, check for a pinched, kinked or disconnected vacuum hose. Also inspect check valve in supply line for proper operation.