Question about 1986 Buick Century

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Ac and heat air flow stuck in defrost postion up top .

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I have seen times when battery acid melts the vaccum line that supplies vaccum to the a/c controls. Sounds like you have no vaccum to the a/c controls and its default to defrost. Check for damaged vaccum line near the battery

Posted on Sep 05, 2010

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The hvac blend doors are vacuum actuated. It takes vacuum from the intake manifold via a line that goes through the firewall just right (passenger side) of center. Open the hood and look for a disconnected or broken line in that area and repair as needed. The default position for the blend doors is the defrost position. When vacuum is lost you'll experience what you've described.

Posted on Sep 05, 2010

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