No Air Comes Out My A/C Vent But Does Come Out Defrost and Heater – Why?

Reader Question Hi Austin,
I couldn’t find my answer on your website so here it goes:
I have a ’92 Bonneville. The air vents in the dash board do not open. The floor vents and the defroster vents work fine. It’s not a big deal in the winter but last summer it was uncomfortable. The heater and AC work fine. Any suggestions?
Thanks,
John

Hey John

Thanks for your email. When you say the vents do not open, I am assuming you mean no air comes out the vents, but does come out the defrost and floor vents ok.

I would first look to see if there is a broken vacuum line going to the dash from under the hood. Listen under the dash board for a hissing noise to indicate that there is a vacuum leak in the dash. This system should be using vacuum from the engine to open and close the air diverter door inside the dash.

If there is no vacuum from the engine to the dash, or if the valve in the dash is leaking that vacuum then it will default the diverter door to blow air out the bottom and top vents only.

I would open the hood and look at listen (with the engine running) for a vacuum leak from either a broken vacuum hose or from a plastic connector attached to a vacuum hose. If you don’t see or hear anything under the hood, move inside the car and listen and look under the passenger side of the dashboard for a hissing noise or an obvious hanging rubber hose.

There could also be a chance that the diverter door itself is either broken or restricted. I have seen a pencil fall in the vents on top of the dash, and block the diverter door from moving. Inspection of the diverter door and the vacuum control valve will require dash removal which you will want an experienced mechanic to perform.

Keep me posted as to what you find out.

Blessings,
Austin C. Davis