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the heater controls are run on engine vacuum. When a engine is accelerating it has a drop in vacuum. There is a one way check valve that keep vacuum in the heater circuit that has gone bad. You could have it fixed or you can deal with it, its not that big of a deal

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โˆ™ 2004-10-25 16:57:04
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Q: Why would the blower of a 1990 Lincoln Town Car switch from the heater to the vent to defrost when accelerating or decelerating?
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