Loud flapping noise from center air vent

[JohnPal]

1992 Vanden Plas. With the AC on, there is a loud, rapid, flapping noise that seems to be coming from the center AC vent. If I turn the temperature dial all the way to heat, the noise goes away. When the engine is under acceleration, producing torque, the noise goes away. Just revving up the engine in neutral has no effect, however.

I disconnected the vacuum line at the point where it enters the console beneath the battery and the noise disappears.

All this seems to indicate a defective vacuum actuator somewhere under the dash. It's interesting why the noise goes away while the car is accelerating in drive, but not while the engine is being revved up in neutral.

Any tips or suggestions on how to go about getting this fixed greatly appreciated.

Best regards.

John. /ubbthreads/images/graemlins/england.gif

 

[Dave Russell]

John,
Sorry I can't help much. If the flap actuator is powered from engine vacuum, there is much less vacuum available when accelerating than when in neutral. The "flapper" may be spring returned to the heat side so when it is in the cool position, the vacuum is fighting against the spring & oscillating against the spring. I wouldn't know if there is an adjustment problem or if there is insufficient vacuum available to hold the flapper against the spring. Just speculation. You could put a vacuum gage on the disconnected line & see what is happening while driving. Someone who is familiar with the car design quirks may be able to help. I doubt if you are the only one that it has happened to.
D

 

[JohnPal]

Thanks, Dave.

Guess I'll just dig into the dash and check the vacuum lines and servos.