What is a blower motor resistor and where is it located?

The blower motor resistor, also commonly referred to as an A/C blower motor resistor or a heater blower motor resistor, is an important component to your vehicle’s heating, ventilation, air conditioning (HVAC) system. While a blower motor is what actually forces the hot or cold air through ducting and your car’s interior vents into the cabin, the resistor is what regulates the speed of the blower fan by reducing voltage going to the fan. It allows you to vary the settings of the blower motor so that you can lessen the amount of air that comes into the cabin, as opposed to only being able to have it blasted at you. The blower motor resistor works between a vehicle’s heater and A/C controls and the blower motor and is typically attached under the dash to the heater ducting near the blower motor to help keep it cool while in use.

Basically, there are two types of blower motor resistors—one that uses coil wires and one that uses transistors and a circuit board. The wire-type, which is more common in older cars, uses a different size coil of wire for each fan speed with different resistances, while the transistorized blower motor resistor, found in more modern cars, consists of a printed circuit board with a fully integrated circuit.

How do I know my blower motor resistor needs to be replaced?

On most vehicles, the highest blower motor speed setting bypasses the blower motor resistor to give the fan full vehicle voltage (typically between 12 and 14.4 volts). If the blower motor still works on high, but will not work on any other speed, it usually points to a bad blower motor resistor. Since the part gets hot and draws a lot of power from the electrical system, a failed blower resistor may also melt part of the wiring harness when it fails. When a blower motor starts to go bad, it can draw more power than the resistor is rated for and cause it to fail. Lastly, a malfunctioning heater core can leak coolant into the car and damage the blower motor and resistor as well.

Can I replace a blower motor resistor myself?

Replacing a bad blower resistor can usually be done by a novice do-it-yourselfer. The repair typically requires only a screwdriver or wrench to unbolt it from the HVAC ducts. To access the motor resistor, you usually have to remove the glove compartment. Then simply unbolt the and disconnect the resistor. In the case of a burnt or melted connector, the replacement will be a bit more involved. While soldering and shrink wrapping the new connector is the best method, crimping the new pigtail in place will also suffice.