I have an '86 Bronco II. The heater fan only runs on the "High" switch position.
I have taken the switch out and checked for continuity across poles at each switch position and there is continuity between one pole (common?) and one other pole in each of the switch positions. From this I assumed the switch was working and I replaced the fan motor.
It still only runs in the "High" position. (The replacement motor only had two leads but the parts guy said this was a multi speed motor)
As Mark said, they switch a resistor of higher value in series with the motor leads to get a slower speed. Unless you have a wiring error to that resistor, it would appear to be open.
Just make sure your blower motor turns freely. No leaves or bad bearings A little resistance on start-up from your blower motor may burn your new resistor
What actually goes in the resistor pack is a thermal fuse in series with the resistor chain for setting the lower speeds. It's placed on the resistor pack to prevent a fire hazard from overheat of the resistors if the fan stalls. They commonly fail in the winter when the fan might be frozen up from snow or slush enetring via the freash air intake. The circuit is bypassed in high speed, as there are no resistors in the circuit in that condition. At least you have the high speed...on my 94 Escort, if the thermal fuse goes, you lose all blower function, including high speed. I've had to replace several over the yeasrs in my Escort and 97 Taurus, always in the winter.
At any rate, the resistor pack is an easy fix, as long as you can get at it. In my Taurus and Escort, its mounted in the blower discharge ductwork behind the glove box. Held on with two self tapping hex head screws, and has a multi-pin connector that connects the wiring from the fan control switch and feeds the fan motor its voltage.
The resistor packs run about 10 to 12 bucks at Ford parts departments. If I knew what rating the thermal fuse was, I would just replace it. The resistors themselves ususally do not fail. Radio Shack used to stock thermal fuses, and I'm sure you can get them from applicance repair shops for not more than one or two bucks. They are also used in things like coffee makers. In a pinch, you could jumper around the thermal fuse until you can get a proper replacement unit. But be careful to ensure that the fan does not stall out!
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