EATC Vacuum System Diagnosis

Testing For Vacuum At The Control Assembly
To test the EATC vacuum system, start the engine and depress the function buttons slowly from one position to another. A momentary hiss should be heard as each button is depressed 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.

Checking Vacuum Supply Lines




If a momentary hiss cannot be heard as each function button is depressed from one position to another, check for a kinked, pinched, or disconnected vacuum supply hose (illustration). Also, inspect the check valve between the vacuum intake manifold and the vacuum reservoir to ensure it is working properly.

Testing Vacuum Motors

Settings - Max A/C, Vent, Panel-Floor:

Settings - Floor, Floor-Defrost, Defrost:

If a momentary hiss can be heard as each function button is depressed from one position to another, vacuum is available at the control assembly. Cycle the function buttons through each position with the blower on HIGH and check the location(s) of the discharge air. The airflow schematic and vacuum control chart shows the vacuum motors applied for each function selection along with an airflow diagram of the system (illustrations). 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 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 leak.

Locating A Leak When Functioning Normally At Idle
If the function 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 head connector and the blend door actuator, refer to Self Diagnostics Test. Displaying and Reading Diagnostic Trouble Codes