Section 12-00: Climate Control System—Service | 1993 Mustang Workshop Manual |
Tools Required:
To test the A/C-heater control system, start the engine and move the function selector knob slowly from one position to another. A momentary hiss should be heard as the function selector knob is moved 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 when the function selector knob is moved 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 be sure it is working properly and is not leaking.
If a momentary hiss can be heard when the function selector knob is moved from one position to another, vacuum is available at the control assembly. Then, cycle the function selector knob through each position with the blower on HI and check the location(s) of the discharge air. Refer to the airflow schematic and vacuum control charts in Section 12-02 and Section 12-03A which show the vacuum motors applied for each position of the function and temperature selector knobs along with an airflow diagram of the system. The airflow diagram shows the position of each door when vacuum is applied in the no vacuum position. 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 hole in it.
If the system functions normally at idle but goes to defrost during acceleration, a leak exists in the system. The leak can best be located by shutting off the engine and using a gauge to check for vacuum loss while selectively blocking off vacuum hoses.