robw126

I have a 99 expedition 5.4 liter. The problem is when I put the a/c on max it blows cold until you accelerate.It then blows only in yhe top vents for about 2 or 3 min. then it goes back to front vents. ON panel setting it doesnt do this. Only on max setting Please help me

RickMN

There are two door in the heater box---a mode door and a blend door. The mode door controls WHERE the air goes--defrost, dash vents, floor. The blend door controls how much heat gets added to the air. They're operated by vacuum motors. When you accelerate, intake manifold drops, so if one of the vacuum lines or motors has a leak, they'll default to their spring operated position. Engine vacuum is what pulls against the spring.

So check for a broken vacuum line, a bad vacuum switch, or bad vacuum motor. I use a hand-held vacuum pump to pull a vacuum and then see if the line holds it.

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jim56

My 03 navigator has the same problem. I cant find the vacuum leak, so, I will take it to the dealer. Ill post here when i find out where the problem is, since several people are having the same issue. Maybe I can help save you a trip to the dealer