I'v got a '98 Expedition. A few years ago the ac started blowing out the defroster vents part time. You could be driving down the highway and it would go from ac vents to defroster vents at will. It would come out the defroster vents for acouple miles and switch back. Once it started coming out the defroster vents you could even let your foot off the gas and that would change it back to the ac vents faster. Now it only comes out the defroster vents. Any ideas guys?

G.

 

Normal scenario for loss of vacuum to the vent system. Check for loose/broken vacuum connections under the dash and at the control head. Sometimes passengers get their toes up under the dash and dislodge a line or two.

 

No vacuum hoses exposed to little feet. Everything looks like it's high and tight. Do I need to pull the dash out for this inspection?

 

Shouldn't have to. I would check the vacuum connections on your dash controls and also check the vacuum line under the hood for the A/C along with the vacuum booster canister supply line. Not sure wjere Ford mounted the vacuum booster cannister for the Expeditions. These are normally mounted on the inner fender or between the inner and outer fender. Some are plastic and some look like coffee cans painted black. Might also be a bad vacuum control valve which is attatched to the evaporator housing under the hood. Might also try to fire up the engine and listen for a vacuum hiss under the dash. Hope some of this helps.