When entering the freeway the AC switches to defrost position(blowing from top windshield vents) without anything being touched. After about 30 sec. it switches back to front dash vents? This happened twice, both times when entering the freeway. The AC is blowing ice cold. Any ideas??

 

You have a small vacuum leak inn the system somewhere. There is no easy to diagnose it over the internet. You just need to trace the vacuum lines to find a small hole somewhere.

 

Thanks for the tip. Where would be a good place to start? Will I be listening for a sound?