31
Mar

Why Does My Heater & A/C Only Blow from the Defroster Vents?

Dear Ken, the heater and A/C on the car I just found on Used Cars for a Grand both work, but they only come out of the defroster vents on top of the dash.

Moving the switch makes no difference.

Could a vacuum hose be off?

I don’t know where to begin.

- Alex R.., OH

Dear Alex, the internal doors that serve as valves to control the cold and hot air flow are not being activated. They are stuck in the defrost-mode position and aren’t able to to move in order to close and open the lower doors leading to the regular A/C and heater ducts.

First find out whether your vehicle has vacuum-activated doors or electrically-activated doors by looking at the control head.

If electric, check fuses and for loose electrical sockets and wires.

If vacuum, check for one or more leaking or disconnected vacuum lines. These sometimes come loose from the control head.

Start checking for these conditions at the control head.

Hope this helps.

- Ken the Car Guy

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This entry was posted on Wednesday, March 31st, 2010 at 12:03 am and is filed under The Questions. You can follow any responses to this entry through the RSS 2.0 feed.

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