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????

 

 

I believe you are on the right track suspecting a vacuum problem.
The systems are designed such that if normal controls fail, for whatever reason, the system defaults to defrost in case the problem appeared in the dead of winter.
A thorough check of all the vacuum lines and components should be performed including the vacuum reservoir itself.

 

I had the same problem. I put new 007 o rings on all 4 solenoids on back of temp control unit. $5 dollar fix.