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Discussion Starter #1
Regardless of selected position, all air from the heater/air conditioning comes out the defrost vents along the front window. What is the most likely problem? (1999 F250 crewcab). Thanks
 

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Vacuum leaks

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Check the vacuum distribution under the glove box.
 

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Mine did that and it was the vacuum pump replaced and all is good do you hear the pump running when you turn the key on it's on the passenger side fender wall hope this helps
 

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Go down to the auto parts store and pick up a dozen or so vacuum lines caps.

Then once you find the vacuum distribution box you can remove hoses and cap off one or two at a time until you get air out of the vents or somewhere besides the defrost. Odds are if your truck is a 4x4 the leak is on the lines to the front hubs.
 

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Discussion Starter #6
Vacuum leak

Thanks, excellent suggestions. I'm crawling under the dash today!
 

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Defrost is the default position when the supply vacuum is lost. Personally, I would start at the vacuum source especially if the truck is a diesel with an electric vacuum pump and electronic shift on the fly 4X4 if equipped. As suggested you can eliminate some components and lines by bypassing the 4X4 solenoid (can stick in the exhaust position) and vacuum reservoir with some test hoses and sections of lines. A vacuum pump with a gauge is your best friend here but not absolutely necessary.
 
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