System Diagnosis

Fig. 66 Air flow schematic & vacuum control chart.:

Vacuum System Diagnosis

1. Start engine and slowly depress function buttons from one position to another.
2. If a continuous hiss is heard, a major leak somewhere in the system is indicated.
3. If no hiss is heard, check for a pinched, kinked or disconnected vacuum supply hose. Also, inspect check valve between vacuum intake manifold and vacuum reservoir to ensure proper operation.
4. If a momentary hiss is heard as each button is depressed, vacuum is available at the control head. Cycle functions through each position with blower on High and check location(s) of discharge air, Fig. 66. If a vacuum motor is inoperative, check motor operation using a suitable vacuum tester. If the motor operates properly, repair or replace vacuum hose as necessary.
5. If function system operates normally at idle, but switches to Defrost during acceleration, a small leak in the system is indicated. Small leaks are best located by shutting off engine and using a gauge to check for vacuum loss while selectively blocking off vacuum hoses.