My 300SD has an intermittent (usually off)flow of air from the center vent. This occurs when the pushbutton is on Auto Hi or Lo.
Flow is fine to the side vents. A/C does need a charge of freon. Compressor switch seems to have no effect on this.
Air flow appears normal with pushbutton in Bi-level position.
Unit worked normally most of last summer, then air to center vent just stopped. Air flow OK for winter heating. Tried A/C this summer and, again no flow to center vents. Last week, got flow to center vents for a day or two, then did not work again.
Would appreciate any advice or suggestions on where to look for problem, how to test, etc.
Thanks for your help.

Steve

There is a vacuum element that controls this flap. It is located behind the dash and will probably need to be replaced.

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Benzmac:
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