A/C Blower Motor Resister

AC fan speed control on control panel.

A blower motor’s speed is adjusted with a blower motor 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 illustration shows a typical blower resistor, specifications and test procedures vary between different manufacturers.

A/C Blower Motor Resister

Testing:

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 affected as well. Resistance 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.