On high speed the motor works with full 12 volts on the motor. For the lower speeds, resistors are included in the circuit to drop the voltage to the motor. The bigger the resistance the slower the motor runs. These resistors are internal to the fan motor unit. If you can dismantle the unit to get to the resistors these can be replaced. Spare ones should be available from rswww.com but you will have to find the resistance from the markings on them and obtain resistors of similar size and appearance to ensure that they will carry sufficient current for the motor. It might be simpler just to get a replacement motor unit from a scrap yard!
Phil Shaw, November 2005
