System Diagnosis

To test vacuum system, start engine and depress function buttons slowly. A momentary hiss should be heard as each button is depressed from one position to the next. A continuous hiss at the control assembly indicates a major leak some where in the system.
If no hiss can be heard, check for a kinked, pinched or disconnected vacuum supply hose. Also inspect check valve between vacuum intake manifold and vacuum reservoir to ensure proper operation.
If a vacuum motor is inoperative, check with a vacuum tester. If motor operates properly, the vacuum supply hose is pinched, kinked or disconnected.
If the function system operates properly at idle but switches to defrost during acceleration, a small leak exists. Located leak by shutting off engine and using
a gauge to check for vacuum loss while selectively blocking off vacuum hoses.