This is the story of my 2003 F150 HD... About 2 years ago the heat stopped blowing out of the floor vent and started blowing out the defrost vent. This wasn't to bad and I could live with it (when I heard it would cost about $800 to fix). Then late this summer the A/C started to switch back an forth between the dash vent (like normal) and the defrost vent. This happened when the truck was under strain. Then is stopped blowing out the dash vent and only the defrost vent. The question is... would this be a blend door issue or the electronics in the control panel??? Help!!!

 

Simple. Vacuum leak.

SEARCH keywords "+defrost +vacuum"

Steve

 

any simple test to find a leak???