My truck was purchased from the original owner and was mostly original when I got it seven years ago. With 109,000 well cared for miles on it, very little had been changed. One of the first things I noticed that I put towards the top of the "to-do" list was change out the old HVAC controls as it was leaking vacuum in either the VENT or HI-LO positions. Before ordering a new one, I checked all the hoses and vacuum motors to make sure they were not the source of the leak. Then I ordered one from one of the Ford sites... I think it may have been the Bronco Graveyard site. Got the new part, and it was actually a NOS Ford part. But dammit... this replacement hissed just like the original part that was in the truck!

Later, i did more research to make sure I have the correct part number thinking that perhaps I had the wrong one. All avenues I went down pointed toward the part I have installed. Later, I can across another NOS switch that I picked up locally still in the box for a really good price. Put that one in and still got this hiss. Well crap! At this point, I kinda gave up since I really have no use for VENT or HI-LO. Where I live, we use full AC eight months out of the year!

One day recently, I disassembled or the original part that I took out of the truck seven years ago to see how it worked and if there was any obvious problem with it. Everything looked good and the little parts were in nice condition. I examined the little maze of channels in there where the vacuum passes to reach the different hose connections. Pretty cool, actually. So I followed paths and found an opening towards the outer edge of the maze that would allow vacuum to escape. Then I marked the mechanism with a black sharpie in a way that would allow me to know the location of said opening after putting the switch back together. Then I connected the switch in the dash and turned it on. Sure enough, the vacuum hiss I can hear is coming from that very opening.

This makes me wonder... is it supposed to hiss in VENT and HI-LO? I can't imagine that there would be an intended vacuum leak. I tried to think back to when we had these trucks and vans growing up... if those controls hissed or not. But then... my dad always ran AC on full freakin blast... not VENT or HI-LO.

In addition... regardless of which of the three controllers I have installed... ALL functions operate as they are supposed to. All vacuum operated doors in the HVAC system open and close in a timely fashion as they should for each setting. I even verified this with the Ford Shop Manual. It's just that damn hiss that drives me nuts because I feel that it's not supposed to be there. However, it's obvious upon inspection that there is indeed a channel in that switch that allows for a minor vacuum leak.

Since my F100 and F350 are non-AC trucks, i don't have another rig to compare to.