A/C Blower Motor Resistor

AC fan speed control on control panel.

The blower motor resistor adjusts the blower motor's speed. It uses resistance to control the speed of the blower motor. At high speed, the resistor is bypassed, and power comes 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 and check against specifications. If this circuit is open, showing infinity on the Ohmmeter, replace the blower resistor.

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, replace the blower resistor.

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, replace the blower resistor.

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.