Got rid of my '94 F-150 last year. Right before I did, the A/C started acting weird. Going up a hill or accelerating, the blower would change from vent to defrost. No biggie, I was selling the truck. Picked up an '01 Explorer Sport Trac, and it is doing the same thing. I know that the vent system works off of manifold vacuum. Where should I look for a leak. As this is 2 trucks with the same problem, I am guessing this is a common problem. I guess it is not a BIG deal, just an annoyance.

Thanks, C-Man

Most common place for a vacuum leak on the A/C system is the line that goes to the vacuum canister. These are normally located under one of the front fenders. Look for a black vacuum line going thru the fender. They tend to rub and make a small hole causing the vacuum leak. At least, that is where I would start. Had the same issue on a LTD and Aerostar I owned. Both were holes where the line fed thru the fender.