1991 Bronco Eddie B. with 302 CI. I can be running along at say 40 mph with my AC on max and the cool air blowing out of the dash vents. I hit the interstate and kick it on up to say 60 mph and the air stops blowing out of the dash vents and starts blowing out of the defrost vents! If I run the AC on normal instead of max, I don't have this problem. Any ideas?

 

I just started having the exact same problem. I was thinking either a vacuum or seal problem, but I really do not know for sure. I also have a 302. I can not find any references to this symptom anywhere. Someone please help. Resources are scarce here in Hawaii.

 

You both have a vacuum problem in the A/C-Heater control area. When you open the throttle, intake manifold vac decreases. If this vac was used directly, your controls would go to the no-vac position, which is defrost. Its defrost for safety, if something fails, for snow country. So there is a one way check valve, that traps vac in the controls side of the system. Also a vac reservoir on the controls side of the check valve. Could have bad check valve, leaky reservoir, vac leak on a hose (the most probable), or leaky dash control switches ($$$), or a vac motor with a bad diaphram in it.
You need to check it from the intake manifold pickup, and work back into the passenger compartment.

 

Wow, thanks Torky. Now I have something to do on Memorial Day. Trace vacuum leak in HVAC. WooHoo! I will let you know what I find Fast Freddie.

 

OK, I checked my handy Haynes Manual, and could not find any related material. So I went out and looked. And looked. What am I looking at. A trip to the mechanic I fear. If you do not figure it out, fast freddie, tell me and I will share what I find out from the HVAC Shop. It will be about 2 weeks until I have enough money to visit the Broncobra Doctor, so I will continue digging until then.

 

[font size="1" color="#FF0000"]LAST EDITED ON 28-May-01 AT 08:39 PM (EST)[/font][p]I think Torky is probably right on. I'm in no hurry to fix mine. I'll just drive slow and wait until I hear back from you...lol. Thanks Guys.

 

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.

 

There was a post along these lines on the My Big Bronco site last month. Here is the link: http://forums.mybigbronco.com/cgi-bin/topic.cgi?forum=1&topic=1046

I think they got it figured out.

1991 Ford Bronco, 351W, E4OD

 

I had the same problem with my 93 bronco. I think I solved the problem by fixing a vacuum line. There is a little round cylinder in the upper left hand corner of the engine bay, right near the left hood hinge. The vacuum line was dry rotted and cracked so I traced it back through the truck and replaced it. The line ran into a factory split loom line which made it hard to remove. The problem was that it runs near the exhaust manifold that caused it go bad. I replaced that line, not because of the A/C but because I saw it was broken. I have never noticed the max A/C working funny again.

 

Cool, I'll check There. I have not even messed with the hoses under the dash. Looks like a nightmare. We have a new baby, so playing on the truck has taken backseat. I'm going to look at that canister, though. Thanks for the tip.

 

You spoke of the cylinder on the left side of the engine bay. Is that the left side as you are looking at it? Near where the heater hoses go in? Does this thing have anything to do with the a/c controls? On my 90 the cylinder is tied in one position with a cable tie, and my defrost won't activate, nor will it blow air to the floor etc. Vents only, regardless of speed. Does this cylinder have anything to do with the a/c controls? Thanks for your help. I also started a thread about this called "a/c controls not working properly."

 

Fixed it! Looked under the hood near the hood hinge on the passenger side and there it was as big as as anything. A vacuum motor with a little "stubb" of tubing sticking off of it. The tubing was very brittle and it took me a few minutes to find where the other end of it was. It was hanging down out of a wirng harness (looks like a white wire) right near the blower motor. I un-taped the harness far enough to get to some good tubing and then reconnected to the vacuum motor. Problem solved! Thanks to all for the hints as to where to look!

 

I'm glad you finally found the fix to your problem. I found out, through the thread I started about my ac controls, that the vacuum motor that you located near the hood hinge on the passenger side controls the max/norm ac door.