97 ford f150 you turn on the ac and it works fine until you put it in motion when you start driving you hear a noise in the dash that sounds like a vent closing it then starts blowing hot air why?

Answer 1

AC Switches Vents During AccelerationThe AC/Heater switch on 97 F-150 is controlled by vacuum. I had a similar problem with my F-150 and it turned out to be a vacuum leak. My battery terminals had corroded and battery acid leaked behind the battery and ate holes in several of my lines at the vacuum harness. The vacuum harness is located directly behind the battery. In my case, the AC blew correctly from the panel vents until I accelerated, it would then switch and blow out of the defroster vent and continued to blow from the defroster vent until it decided to do otherwise.

I did a temporary repair by splicing the damaged lines but a permanent fix will require replacement of the vacuum harness assembly. If you are considering buying a used vacuum harness, make sure the specs are the same as your vehicle, i.e.: Engine size, trans type, 2-WD or 4-WD, etc. If you are mechanially inclined this is a fairly easy fix, otherwise I suggest the repairs be made by a reputable mechanic.

The mode doors in the F150 are vacuum ac

actuated and if you have problems switching between floor/vent/defrost, it's probably a vacuum leak. The temperature of the air is controlled by the blend door and this system is electronic with an actuator motor. It's common on the F150 and Expedition for the plastic blend door to break and this sounds like what your problem is. Search "F150 heater" on YouTube for a video showing how to diagnose and repair the problem.