Whenever I switch my A/C to high it stays on low & it sounds like there is a small vacuum leak. If I turn the truck off the vacuum system leaks down pretty quickly. It also does this when I turn the HVAC switch to the OFF position. All the other positions hold vacuum pressure (Normal A/C, Vent, Heat, Defrost), even overnight, as it used to for Off & High A/C. I have checked the vacuum lines in the engine compartment & they seem okay. Are there any vacuum lines inside (I don't think so, but there may be) ? I think the switch is electric & I have a wiring problem (short or open), or I missed a leak. It appears I'll have to break down & buy the books (DVD's now ?) for both electrical & mechanical. I could also have a vacuum operated component (diaphram) that is bad.
The A/C works great (just on low though) and the vacuum pump is good (my brakes work, and you can hear the mixing plate moving and changing the airflow to the proper vents).
Any help would be greatly appreciated. It's already getting into the 90's down here, and I'd rather fix it than have to turn it in & rent a car.

 

Sounds like you blower fan resistor is shot. It is the rectangular item that your blower fan plugs into. It is about 2" X 3.5". Look on the heater core/evaporator core box under the dash. It is held on by two or four screws.

 

Your problem is electrical, not vacuum related.

It is most likely the speed selector switch, not the resistor pack.

If the resistor pack were to open, the fan would not run at all. If the switch opens, the fan runs in LOW only.

For the 97, the service docs are on CD, not DVD. www fordcds com

Steve

 

Thanx guys, I'll check it out & let you know what I find. I should have mentioned that all of the speeds on the fan work, but only the "low" speeds.

 

I second this. I had to replace this part on two vehicles and they were doing what you mention (low in all instances or works in some positions and not others). It was a relatively cheap part and simple to replace in both instances.

 

In order for the resistor assy to cause this, it would have to suffer three separate failures at the same time.

The switch assy will cause this with only a single failure, namely, the COMMON terminal/arm of the switch.

I'd suggest actually looking at the F150 circuit schematics and thinking about it for a bit.

Steve

 

Um, have you had thid exact problem before? I have, where 3 of 4 fan speeds were inop(all switch positions blow on low), and I also incorrectly assumed the fan or switch was the problem. In both cases it was the fan resistor. I don't understand why a blown resistor allows it to work at all, but it does. That's where I'd put my bet.