Vacuum System Diagnosis

Inspection and Verification Vacuum System Diagnosis

To test the EATC vacuum system, start the engine and press the function buttons slowly, one at a time. A momentary hiss should be heard as each button is pressed 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 as each function button is pressed, 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.


Vacuum Leak Diagnosis

The air handling system is designed to provide defrost when no vacuum is applied to any of the three air door vacuum motors. This is done to prevent a situation where defrost cannot be obtained due to a system vacuum leak. Instead, a leak in the vacuum control circuit will send all airflow to the defroster outlets. This condition may occur during acceleration (slow leak), may exist at all times (large leak) and may happen only when certain specific functions are selected, indicating a leak in that portion of the circuit.

The vacuum hoses used in the passenger compartment control circuit are constructed from PVC plastic material. The vacuum hoses used in the engine compartment are constructed of Hytrel. Because of the materials used, the vacuum hoses should never be pinched off during diagnosis to locate a leak. Use Rotunda Vacuum Tester Model No.021-00014 or equivalent to locate vacuum leaks. A wood golf tee can be used as a plug when it is necessary to plug one end of a vacuum hose for leak test purposes. If a momentary hiss can be heard as each function button is pressed, vacuum is available at the control assembly. Cycle the function buttons through each position with the blower on HI and check the location(s) of the discharged air The airflow schematic and vacuum control chart show the vacuum control motors applied for each function selection along with an airflow diagram of the system. The airflow diagram shows the position of each door when vacuum is applied, along with the no-vacuum positions. With this chart, airflow for each position of the control assembly can be determined. If a vacuum control motor fails to operate, the vacuum control motor can readily be found because the airflow will be incorrect.

AIRFLOW, MAX A/C

NOTE: V = Vacuum

NV = No Vacuum (Vented to Atmosphere)



AIRFLOW, MAX A/C













AIRFLOW, MAX A/C














AIRFLOW, Panel/Floor












AIRFLOW, Floor (Full Heat)










AIRFLOW, Floor/DEF (Full Heat)













AIRFLOW, DEF (Full Heat)











AIRFLOW, OFF











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

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

To check electrical system operation between the control assembly connector and the A/C electronic door actuator motor, refer to Diagnostic procedures.