On Monday morning, just as I was beginning the drive home to Waterloo from Ottawa, about six hours in the sweltering heat, I discovered that the blower in my truck had stopped working properly. Between the time I gave the truck to my friend Jeff to drive for a day, and the time I got it back, something in the fan control broke. Jeff of course denies any wrongdoing, knowledge of, or involvement in this startling development. I think the facts speak for themselves.

Truck: 2001 Dodge Dakota Quad Cab.
Symptom: Fan only works at highest speed setting. In all lower settings, the fan just turns off.

I suffered with the A/C at full blast the whole drive home (better than turning it off), then hit Google to see if anybody else had encountered this. Turns out it is a somewhat common problem. The fan speeds are controlled by a high-power resistor in series with the fan motor. Apparently it is quite common for these resistors to burn out. This problem affects many makes of car, not just Dodge.

Fortunately, the resistors were quite inexpensive at my dealership’s parts department, and very easily replaced. The resistor comes as a module that is bolted into one of the air plenums in the passenger footwell. On the outside is a wiring connector, similar to the connector on halogen headlight bulbs (and similarly difficult to unplug). The resistors themselves are on a card that sticks inside the air plenum, presumably so the resistors will be cooled by the air flow.

The original appeared to be a card of fibreglass or mica or something, presumably with the resistive elements printed on it, and then painted black. Seemed very cheap to me, I’m not surprised it burned out. When I removed it, it was covered in moisture. And the paint was flaking off in one corner, which probably allowed moisture to seep in to the resistive element and corrode it. Tests with an ohmmeter showed that only two of the five pins appeared to have continuity with each other, the others were all open circuits.

The new resistor card cost me $13. It was somewhat different in design than the original. The replacement is using a ceramic core, with resistance wire wound around it, and dipped in epoxy. This is more like high-power resistors are supposed to be made. On the top is what appears to be a thermal fuse. The new module has continuity with all pins, the resistance to pin 1 from subsequent pins steadily increasing, up to pin 5 which had I think 2 or 3 ohms resistance.

I suspect that the fan speed selector provides power to one of pins 2 through 5, representing speeds from highest to slowest. Pin 1 is then used to supply the motor. Pins 2 through 5 appear to be a 2 or 3 ohm power resistor with multiple taps. The first tap (pin 2) is actually at 0 ohms resistance (pins 1 and 2 are shorted together), giving full voltage to the blower motor.

On the whole, this is actually quite an inefficient motor speed control. At speeds other than off and full-speed, considerable power is wasted as heat in the resistor. A more modern and efficient design would use a switch-mode transistor control. That would allow infinite variation in fan speed, waste less power, and probably last longer too (if implemented properly). It could probably be implemented on a plug-compatible module. That would be an interesting aftermarket part: an infinitely variable fan control. The hard part would be replacing the knob on the console.