I finally got around to messing with my a/c controls. It was always stuck on "vent" and would not go to defrost or floor. After posting a few months ago, when I was in Florida, I determined that the vacuum motor that controls the vent/defrost door had been zip tied in position so that the vents would be on all the time. I figured that this was because of a vacuum problem that would cause the vents to go to defrost. In Florida, you really want the vents on for the a/c, so they take precedence. Here in Colorado, at least in winter, it is not so important, so I cut the zip tie off. Now I have vents, defrost, and floor. The vents will cut to defrost under hard acceleration, like climbing a hill, so I must have a small vacuum leak. It is not that bad, the vents stay on at cruising speed. But I would like to fix it properly. I started tracing the blue line that goes to the vent/defrost door but lost it. Here are my questions: Where is the vent vacuum manifold located? I am assuming that the blue line is going there. Do I need to take the a/c switch cover off to get to it? And, where might a vacuum leak be located that would cause the blue line to lose vacuum? Specifically, is it necessary for the problem to be on the blue line, or would a vacuum leak on any part of the vent system, or even worse, anywhere on the engine cause this? Finally, do the vacuum motors themselves go bad? Sorry for the book here, but this is a confusing subject. If I fix it I will post a tech article about this.

 

The part of the Novel you missed was the Year of the Bronco.


Dennis
78 F-150 429CJ C6 ,Silver w/Explorer Pkge
641/2 Mustang,Pre-World's Fair Car #8092
64 Fairlane S/C waiting for a 390-4spd.
68-Mustang.Sunlit Gold 80,000 miles

 

I had the same problem on my '80 Bronco. I replaced the engine-side vacuum lines and fiddled about on and off for three months. One day, with the engine running, I started pushing and prying on the vacuum reservoir and sudenly I heard a slight, high-pitched hissing sound.
There was a crack in the seam on the bottom. The reservoir looks like a coffee can--same, cheap metal. I mixed my old standby, J.B. Weld, and slathered on the seam after cleaning the rust away as much as I could.after it cured, there was a major improvement in the performance of my AC/heater/vent controls.

Another thinkg that may be a problem is the little check valve that tees off the main vacuum line.

I would have never found the leak unless I had pressed and pried on the reservoir. I suppose I should replace the reservoir, because I'm sure the JB WEld won't hold forever, but I am sure that would be major $$. and I dont know if i could even locate one. Any suggestions? comments/