The A/C controls on my 90 XLT with 94k are not working properly. I do not have the problem that so many seem to have, of the vents shifting to defrost by themselves. Rather, mine will not go into defrost or any other mode except the regular a/c vents. I think it is stuck on "norm" a/c. First question: is there a difference in fan speed between "max" and "norm" a/c? The vents are so far away from the driver in the big Bronco that the air hardly seems to reach me on high, which is why I think it is stuck on "norm" instead of "max." Anybody whose a/c is working properly, could you see if there is a fan speed difference between "max" and "norm"? Second question: there was another post here where someone had the problem of the a/c switching to defrost by itself, and they spoke of a valve on the upper left hand corner of the engine compartment near the hood hinge. On mine, there is a valve there, near where the heater hoses go in, and this valve has been kept from moving with a cable tie. I am guessing that my Bronco might once have had the problem of the a/c going to defrost by itself, so they tied this valve open as a cheap fix-it. But I don't know, for all I know this valve has nothing to do with the a/c controls. Can anybody help here? I know this has been a long post, so if you read this far, thanks, and maybe I will be able to help you one day.

 

I tried to look at that for you, but when I turn the a/c on at idle my engine wants to stall...lol. It is hard to notice the change while driving. Once I figure the reason for my problem I will look for you.

 

I had the same problem on my 93 Ford. The climate control reverts to defrost as a fault item. You have vac line problems. The primary differance between MAX and Norm is while on MAX the compressor runs all the time and on NORM it cycles by utilizing the clutch. Have the system checked for leaks too.

 

I always thought that the difference in "max cool" and "normal" was that when in the normal mode, outside air was moved through the evaporator coil and in the max cool mode the air inside the vehicle is continuously recirculated through the coil. All cooling units are designed to provide a specific temperature drop (delta t) which means they will drop the incoming air temp by x degrees. In the "max cool" mode, you are re-cooling air that has already been cooled as opposed to cooling air from the outside that is at ambient temperature.

 

That is what I thought too. On some cars you can feel a difference in fan speed when swithcing between norm and max, which is why I was asking.

 

The recirculation door control vac motor is on the passenger side under the hood, between the blower motor and the cowling. The vac motor metal piece that vacuum pulls into the motor, has a crank on the other end of it. The vac hose to it comes from a harness. The harness comes out from the interior by the blower housing. When in Max, there should be no vacuum on that hose, so the motor's metal piece goes out, turning the crank and opening the recirc door.
If you look inside at the right of the passenger footwell, you can just see a corner of the recirc door opening. When open, intake air is from inside, rather that through the cowling grill in front of the windshield. That vac hose, white on mine, can be seen coming in behind a plastic side panel between passengers feet and the side door. There is a vac connector there, then it goes on fastened up over the passengers feet, across the dash to the middle. There there is a multi-hose vac connector, then on to the dash control. It is easy to see and trace, at least on mine it is.
The fan sounds a lot noisier when the recirc door is open, as air noise from the blower wheel has a straight shot to inside. May speed up a little due to reduced drag on the suction side, but that just happens, not a designed speed change.
Back under the hood, you may see the main vac supply hose coming from the engine to a check valve, then over to a vac reservoir hidden next to the A/C evaporator case. Its under the heat shield foil on mine. Another vac hose goes from a tee at the check valve to inside the truck, to run the dash controls and vac motors.
Hope this helps somehow.

 

Thanks for the advice. I printed it out and went out this morning, started the truck and the a/c, and checked it out. This solved the mystery of the valve under the hood that I was asking it about, it is indeed the max/norm door control. And even though there is a cable tie on it, it is working correctly. I found your advice (no offense here!) to be backwards but still helpful. On max, I had vacuum to the motor and it shut the door. On norm, no vaccuum and the door opened. But you got me on the right track. I looked in with a flashlight where you said, and could see the door opening and closing. Ok, next I want to trace down the motors to open the defrost and floor vents, neither of which work. Does anybody know where these are located, and which color vacuum lines go to them, before I tear stuff apart? I found another vacuum control motor, a larger one on the bottom of the dash, with a blue hose. This one also has a cable tie on it, but it seems to keep it from being able to move.

 

Will have to check the truck again some time, sure looked like the recirc door was open with no vac. Looked it up in the book, it agree's with your observation.
Downstream from the Temperature Blend door is the Panel door. Blue vac hose. Either allows air to dump out the Panel vents, vac on blue, or if no vac on blue, passes air along to the next door. The next door selects Defroster, or Floor, or halfway between these. That vac motor has a red hose and a yellow hose.
Looks like if no vac on red and yellow, is Defrost. Vac on both red and yellow puts it all out the Floor. Vac on Yellow, but not on red is a mix of some defrost, some floor.
The black hose at the control head is from the vacuum source.

Sounds like your truck had a problem, someone who didn't know what they were doing patched it without solving the problem. Maybe they had air coming out the defrosters all the time and didn't figure out the problem, so just forced it. If they didn't break anything real expensive, shouldn't be too hard to fix.

 

I'm going to cut the cable tie off of the second door motor and see what happens. I'll bet I get the famous "a/c switches to defrost at 40 mph" that everybody seems to have. I'll let you know what happens. Thanks for the help.

 

I had the same problem, air coming out of vents only. All vacuum motors were observed to be operating normally, when the control was moved. I found the door that divided the vent duct from the floor/mix/defrost duct had come loose from the hinges and was floating loose. I could reach it with the glove box removed and the access cover(for heater core) removed. Three plies of duct tape seemes to make a workable hinge.

 

I haven't done anything to mine yet. But I am moving out to Colorado and will probably need the defroster. Right now I'm having transfer case problems so I need that fixed first!

 

 

There is a vaccuum line that runs up to a little cylinder lookiong thing in the upper left hand corner of your engine bay, its right by the hood spring. Check to make sure that hose is in tact. Mine was dried and cracked and the AC did the same thing. i traced that hose back and found it leading to a wire loom that runs right by the exhaust manifold and that caused it to dry and become brittle.

 

I have had similar problems with my controls on my '80 Bronco. I replaced vacuum hoses, tracked vac lines etc. to no avail. I even used a piece of 3/8" hose as a stethoscope to listen for vacuum leaks--a handy trick BTW. One day, with the engine running I started pushing on the vauum reservoir and noticed when I pushed near a seam that a slight hissing noise was heard. The vacuum reservoir on my Bronc looks like a recycled Dinty Moore Stew can and when I pushed on the bottom, I could see where the seam had separated slightly.It was only visible wheni pushed on it but it was allowing a vacuum leak which I am sure increased when engine RPM increased while driven. Being a cheapskate, I got out my JB Weld, mixed up a supply and liberally slathered some over the seam and let the Bronc sit for day until I was sure the JB Weld had cured well.

Guees What? Problem solved!