Initial Inspection and Diagnostic Overview

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, indicating that vacuum is available at the EATC control assembly. A continuous hiss at the EATC control assembly indicates a major leak somewhere in the system. It does not necessarily indicate that the leak is at the EATC control assembly.

If a momentary hiss cannot be heard as each function button is pressed, check for kinked, pinched or disconnected vacuum supply hose. Inspect A/C vacuum check valves between vacuum intake manifold and A/C vacuum reservoir tank and bracket to ensure it is working properly.

If a momentary hiss can be heard as each function button is pressed from one selection to another, vacuum is available at the control assembly. Cycle function buttons through each selection with blower thumb wheel rotated fully up to the HI position and check location(s) of discharge air. Airflow diagrams show vacuum control motors applied for each function selection and resulting airflow for the settings. Also shown are door positions when vacuum is applied and their no-vacuum position. Airflow for each position of the control assembly is shown. If a vacuum control motor fails to operate, the vacuum control motor can readily be found because airflow will be incorrect.

If a vacuum control motor is inoperative, check operation of vacuum control motor with a vacuum tester. If vacuum control motor operates properly, the vacuum hose is pinched, kinked, disconnected or has a leak.

If the system functions normally at idle but goes to defrost during acceleration, a vacuum leak exists in the system. The leak can best be located by shutting oft the engine and using an external vacuum pump to leak-check each vacuum circuit.