ok my defroster dosent work but i get good heat through my vents when i switch from vents to defrost nothing happens does anybody know how the vacuum runs on this system any help would be greatly appretiacated

1980 model f-150 4x4
stock 351m

 

Every system I have ever seen (which is just a few, but consistent across several brands) uses lower vacuum to switch from vents to defrost. Try really revving the engine up and see if it switches over. That's what I have to do in the 2000 Blazer that I have to drive for work in order to get it to switch over. I can set the selector to defrost and drive for miles with air coming out the vents until I pop it in neutral and rev the engine to about 5500 RPM's. (higher engine speed = lower vacuum).

 

Do you hear a vacuum? I had a 78 Cougar in which the vacuum selector valve broke. The replacement valve didn't cost much. After replacing the valve I decided that I could have repaired the old one.

 

Dillon
Was curios if got this figured out yet as I have the same problem. Should be an easy fix if it can be gotten to without much problem.
Thank You
Michael