My first post.... Picking up a 1992 ext cab F150 this weekend. The truck has a problem with the Max AC not increasing, possibly a vacume hose? I've ordered an EBAY chilton's manual. In the mean time, Could someone relieve my worries by telling me that you have possibly had this problem and hopefully it's an easy fix?

The AC is nice and cool on the lower/normal setting. Setting it to max does nothing. I would imagine that a vacume hose would make the outside air vent close or change positions (no sound makes me think that's the part is not working). Also I think the fan increases rpm but it seems like the ducting is closed off and the air volume does not increase.

Any Ideas. This truck is cherry and just what I want except for this AC problem.

Thanks in advance.

 

I just had my 97 crewcabs air serviced. Refrigerant needed topping up and what I believe was an inlet tube required replacement because of corrosion and plugging up. Now it's artic cool, even at +32C.

 

The power (voltage) does not change going to the Fan Motor when going from Norm to Max A/C . The Fan speed switch and the load of the fan control the fan speed.
The only difference between Norm and Max is the closing of the outside vent. The outside vent motor is on top of the A/C Box under the hood. It has a white Vacuum Hose on it.

 

I have a 1994 F150 & had the same thing happen, there was no difference between normal AC & Max AC. I am not sure if the following terminology is correct but I did have a vac line broken. If you find where the heater hoses enter the firewall there will be a valve that needs to acutate in order for Max AC to work. On mine there was a small white vac line that was broken. I did not replace the line, I used a larger piece of vac line and slid over both ends of the broken line and used 2 small zip ties to seal the line. Now the max AC works as good as ever. If this does not help you at least locate a possible problem, let me know and I could take a picture of the location and what I did and send to you on Monday....

 

Ok, electrically speaking, the way the NORM/MAX settings work, there are some low resistance high wattage resistors mounted in the ductwork that are actually in series with the fan motor armature. They're mounted in the duct because they need the air blowing across them to cool them. This is how most automotive manufacturer's get away with using underrated resistors to do this. Switching to MAX bypasses one of them, and the rest are for the multi-speed selections in either NORM or MAX setting, and are progressivly bypassed as you switch from low to high speed setting for the fan. If any of the resistors fails, typically the fan won't work at all until you select a speed above the one that burns out. If you look, you'll see a wire harness going to a rather large plug in the duct work somewhere. If the fan isn't changing speeds when switching from NORM to MAX, something's hinky with the mode selector switch. I've never looked at my 92's AC/Heat controls (never had a problem) but older model cars tended to use both an electrical switch and a vacuum selector valve to control the dampers. If the wiper on the electrical portion is busted, then that would explain the no change in fan speed. Whether the dampers change positions or not, you WILL hear the difference. The fan speed nearly doubles on MAX. I would start looking at the fan speed control first, then possible problems with the vacuum switch or vacuum operators on the flaps. One other point of interest, is there is a pretty short line for the flap controller under the hood near the passenger side hinge. That one's famous for cracking. If you lose vacuum here, all the dampers are gonna go wonky. If the mode selector is strictly a vacuum selector valve, there will be a vacuum operated switch somewhere that isn't operating, either due to vacuum loss in the control circuits or crud in the mode valve. Either way, I'd find why the fan isn't speeding up first. That will probably fix the other problem with it.

Lemme know if you're still having problems. I have a '92 as well, and have the Haynes manual complete with electrical diagrams. I'll need a few more details about the truck though before I can scan and mark up what you need to check.

 

Did you not read what I posted above?
There is nothing different electrically between Norm and Max. Every thing you said electrically in the above on fan speed is not true.
Here is a diagram of the system electrically.

 

to subford, thanks for clearing that up, again.

to oremguy, buy a set of ford manuals. original manuals from the manufacturer is the best investment i ever made.

 

Well, it turns out it was the vacume line on the passenger side. It comes thru the firewall in a bundle and curves back to an actuator on a vent. It looked perfectly smooth and normal until I touched it. It had dried and it crumbled to dust (about 6 inches of it. I slipped a small piece of vacume hose over the top to splice/repair. The AC now works perfect. This truck is in great shape now! Just a small oil leak.
I'll figure it out. Thanks for the replies!