System Diagnosis

Vacuum Schematic And Selector Switch Function Chart:

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.

Airflow/Function Chart - Max A/C:

Airflow/Function Chart - A/C:

Airflow/Function Chart - Panel/Floor:

Airflow/Function Chart - Floor (Full Heat):

Airflow/Function Chart - Floor/DEF (Full Heat):

Airflow/Function Chart - DEF (Full Heat):

Airflow/Function Chart - Off:

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/Function Charts 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 the 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.

If the system functions normally at idle, but goes to To check electrical system operation between the defrost during acceleration, a small leak exists in the control assembly connector and the A/C electronic system. The leak can be located by shutting off the door actuator motor. Refer to Diagnostic engine and using a gauge to check for vacuum loss procedures while selectively blocking off vacuum hoses.