A/C Blower Motor Resister
A blower motors speed is adjusted with a blower motor speed resister. A blower resistor uses resistance to control the speed of the blower motor. In high speed the resistor is bypassed and power is feed to the motor directly from the switch. This is a typical blower resistor, specifications and test procedures vary between different manufacturers. 
Place one lead of the Ohmmeter on terminal 1 of the resistor.
Place the other lead on terminal 2 an check against specifications. If this circuit is open showing infinity on the Ohmmeter the blower resistor must be replaced.
Move the lead from terminal 2 to terminal 3 and check this reading against specifications. If this circuit is open showing infinity on the Ohmmeter the blower resistor must be replaced.
Move the lead from terminal 3 to terminal 4 and check this reading against specifications. If this circuit is open showing infinity on the Ohmmeter the blower resistor must be replaced.
By changing the resistance in the circuit the voltage and current are effected as well. The resistance gradually decreases and current increases as the selector is moved from Low to Medium to Medium-High. This increase in current as the resistor moves toward higher speeds increases the speed of the blower motor.
Remember resistance and current are proportional. Lowering resistance in a circuit increases the amperage and visa versa. Remember Ohms Law: changing any variable in the equation effects the others. (V = IR): voltage always equals current times resistance.
