Any suggestions on where to start looking for the cause of the vents to move to the defrost/floor position and then back to the selected AC setting. It seems to happen after driving a mile or two and will last for less than five minutes before it corrects.

 

Sorry I don't have an answer for you, but I have a similar problem except mine won't go into floor/defrost at all. It only does the regular vents. Maybe these problems are caused by the same thing?

 

[font size="1" color="#FF0000"]LAST EDITED ON 10-Jul-01 AT 09:27 PM (EST)[/font][p]Guys, I would start by looking at the vacuum switches on the controls. You might even spray some silicone lube through the hose connections if you can get access to the unit. Sorry I don't know more specifics. Don

 

On my 1990 XLT there is what looks like to be some sort of vacuum control that is located under the hood near where the heater hoses connect to the heater. On my truck, this control is held open by a cable tie. I don't know what this thing does, but maybe it has something to do with the vent controls? Maybe someone had it tied open to keep the vents from switching over? Just a guess on my part. Anybody know if I'm on the right track?

 

I had the same issue (computer I/T term for problem). I fixed it when I was replacing the blower motor. There is a vacuum switch on top of the blower motor housing (90 F150) (have 78 Bronco) the line was rotted and I could hear the vacuum when I moved the lever. The hose goes in to a harness. I covered the hose with Elec. tape and it sealed the leak. I figure there is one hose somewhere on your truck that is leaking. Check the one on top of the blower motor housing first. Hope this helps. Happy AC.

 

I just fixed this on my bronc and it was the vaccum connector on behind the selecor switch in the dash, check all your vaccum lines to find the leak, start at the intake and follow them to the switch in the dash, check the check valve and the vaccum resivor(block box looking thing) first


93 Bronco XLT
3" Rancho suspension lift and 33x12.5 BFG Muds and
rancho 5000 shocks