Vent flow, or lack of

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AWD EXPY

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At least a few dozen times, at the same place, after getting on the highway, my heater moves itself from panel output to another position, without my action. Not the actual dial on the panel, but where the air comes out changes. It's happened in other places seldom, but mostly at one same spot on the highway. Not every morning, but 20% of mornings. It lasts for about 10-15 seconds, then returns to panel output, as per dial setting.

All I can figure is that it is vacuum regulated, and maybe I have a leak of sorts. I could see if acceleration on the uphill highway entrance ramp would be such a load as to reduce available vacuum, but it mostly happens after I've reached a consistent highway speed. Can anyone speculate on this weird issue.
 

Cacique

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I'm having the same problem with my 1998 Ford Expedition 5.4L Eddie Bauer. I had a mechanic work on it over a year ago at a cost of ~$500. Was told it was something with the housing assembly behind the dashboard. It fixed it for about 4 months and then problem returned. This time the mechanic said he had to replace two vacuum hoses. The problem came back after a few weeks. There is a forum about this same issue with other Ford models but the fix seems time consuming and difficult. The link is: http://www.ford-trucks.com/forums/1077168-a-c-blower-defaulting-to-the-defrosters.html
 
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