Vacuum System

To test the A/C-heater control system, start the engine and rotate the function selector control knob slowly from one position to another. A momentary hiss sound should be heard as the function control knob is rotated 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 the function selector control knob is rotated from one position to another, check for a kinked, pinched or disconnected vacuum supply hose. Also inspect the check valve between the intake manifold and the vacuum reservoir to ensure it is working properly.

A/C System Schematic And Vacuum Control Chart:

If a momentary hiss can be heard when the function selector knob is rotated from one position to another, vacuum is available at the control assembly, then cycle the function selector control knob through each position with the blower on HI and check the location(s) of the discharge air. The airflow schematic and vacuum control chart shows the vacuum motors applied for each position of the function control knob along with a system airflow diagram. The airflow diagram shows the position of each door when vacuum is applied and the no-vacuum position. Using this chart, airflow for each position of the control assembly can be determined. If a vacuum motor fails to operate, the motor can readily be found because the airflow will be incorrect.

If a vacuum motor is inoperative, check the operation of the motor with a Rotunda Vacuum Tester 021-00014 or equivalent. If the vacuum motor operates properly, the vacuum hose is probably pinched, kinked, disconnected, or has a hole in it.

If the vacuum system functions normally at idle but goes to defrost during acceleration, a small leak exists in the system. The leak can be located by turning off the engine and using a gauge to check for vacuum delay while selectively blocking off vacuum hoses.

NOTE: If problem cannot be found go to Pinpoint Test A - Manual A/C-Heater System Vacuum Leak Diagnostics. Manual A/C-Heater System Vacuum Leak Diagnostics