Blower Motor Resistor Testing

An A/C blower motor resistor uses resistance to control the speed of the blower motor. Many blower resistors control current to the blower motor in all speeds except high. In high speed the resistor is bypassed and power is fed to the motor directly from the switch.

Blower Resister

Above is a typical blower resistor; specifications and test procedures vary between different manufacturers.

  1. Place one lead of the Ohmmeter on terminal 1 of the resistor.
  2. Place the other lead on terminal 2 and check against specifications. If this circuit is open, showing infinity on the Ohmmeter, the blower resistor must be replaced.
  3. 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.
  4. 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.

This question is a great example of how resistance and current work in a circuit. The resistance gradually decreases and current increases as the resistor moves 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/current and vice versa.