How to Test a Heater's Blower Resistor

In a car heater, the blower resistor controls the air that comes out from the blower. The blower resistor is an electrical connection to the blower motor that is usually located in the passenger side of the car. If you are turning the air on to the highest setting, but nothing is blowing out, it is likely that you have a faulty resistor. To make certain the resistor is bad, you can manually test it and, if necessary, purchase a new resistor from an automotive-parts store to replace it.

Multimeter
Three jumper wires

Instructions

- 1
  Turn on your digital multimeter tool, and turn the dial to the “DC” setting.
- 2
  Locate the blower resistor in the passenger side of your car, below or behind the glove box. In some models, you may need to remove the entire glove compartment in order to access the blower resistor.
- 3
  Disconnect the resistor wire's electrical connector from the terminal in the bottom of the blower motor. Connect two jumper wires into the two terminals of the blower motor, where the resistor connector was previously plugged in.
- 4
  Connect the other end of one of the connected jumper wires into the one of the resistor’s electrical connector terminals.
- 5
  Connect a third jumper wire into the second terminal in the resistor’s electrical connector. Connect the two ends of the remaining jumper wires to your multimeter tool.
- 6
  Start your car, and turn your heater’s fan on to the lowest speed. Refer to your car’s owner manual to determine the maximum draw for your blower motor. For example, the maximum in many cars is 18 amps.
- 7
  Turn off the heater fan and turn off the car once you get your reading. If the reading exceeds the maximum amps allowed, you need to replace the blower resistor.

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