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usally it is because of a cracked vaccume line(hose) from the intake manifold to the dash controls.usally heat makes the line crack-some vaccume canisters develope a crack in them - this will cause the same problem probably the vacum value connected to the vent controls. If there is a broken vacumn line or if the value is bad, it could cause this to happen

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โˆ™ 2004-06-14 08:16:32
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Q: What makes the AC on a 1989 Ford F-150 not blow from the dash vents and only the defroster?
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