Ventilation Air Flow System Tests

To test the ventilation air flow system, start the engine and rotate the mode selector knob slowly from one position to another. A momentary hiss sound should be heard as the mode selector knob is rotated indicating that vacuum is available at the climate control assembly. A continuous hiss at the climate control assembly indicates a leak in the system. It does not necessarily indicate that the leak is at the climate control assembly.

If a momentary hiss cannot be heard when the mode selector knob is rotated 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 A/C vacuum reservoir tank to ensure that it is working properly.

If a momentary hiss can be heard when the mode selector knob is rotated from one position to another, vacuum is available at the climate control assembly. Cycle the mode selector knob through each position with the blower motor on HI and check the location(s) of the discharged air.

Check for correct mode door movement and air distribution:
- FLOOR
- VENT
- DEFROST
- MIX





The system schematic and vacuum control chart shows which vacuum control motors are applied for each position of the mode selector knob, and a vacuum schematic of the system. Air flow through the doors for each position of the mode selector knob can be determined using the chart and schematic. If a vacuum control motor fails to operate, the fault will readily be found because air flow will be incorrect.




If a vacuum control motor is inoperative, check the operation of the actuator with Rotunda Vacuum Tester 014-R1054 or equivalent. If the vacuum control motor does not operate properly with the vacuum tester attached to the hose connector at the climate control assembly, connect the vacuum tester directly to the vacuum control motor. If the vacuum control motor operates properly, the vacuum hose between the vacuum control motor and the climate control assembly is pinched, kinked, disconnected or has a hole in it.

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