Heating problem


Old 01-19-02, 06:33 AM
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Question Heating problem

OK, I'm working on a 81 Mustang 4 cylinder. This problem has got me baffled so I was hoping someone might give me some ideas. When the heater is on, and set to vent and you apply throttle going up a hill, it quits blowing. The blower motor doesn't quit, but no heat comes out of the vents at all.

Now if it's set to defrost, it continues to blow heat even going up hill. Is there possibly a something broken in the vent side that causes this strange problem??
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Old 01-19-02, 11:17 AM
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The mode door is vacuum operated, and spring loaded to the DEF position for safety.

When you accelerate, there is very little vacuum available, so the mode door defaults to the DEF position. To prevent this from happening, manufacturers install a check valve in the vacuum hose that supplies the HVAC controls. Your check valve is bad or you have a vacuum leak inside the car.
Old 01-19-02, 06:11 PM
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Thanks Knuckles. The whole thing suddenly makes so much sense after reading your reply. I'm gonna check it out tomorrow and I'll let you know how it goes.

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