So I'm driving down the freeway a/c blowing out the vent (like normal) until it hit about 55 mph. Then it begins blowing out the defrost right....well then I begin to slow down , and it comes out the vent again. Now I've noticed it does this Everytime I accelerate for an extended period it switches to the defrost output and when my speed goes back down it goes back to normal.
Im thinking it may be a vacuum line ...anyone had this happen????

 

Welcome aboard..

The default position is Defrost. With system vacuum applied the other vent selections are done. If a vacuum leak, anytime the available vacuum falls off,, Defrost..

Check the lines, canister/ check valves, any place it can bleed off vacuum..