What Is a Blower Motor Resistor?

Your vehicle’s heating and air conditioning system consists of many parts working in unison to produce your preferred air temperature. Some of these parts will (hopefully) last throughout the life of your car, but others are more prone to wearing out and need replacement. The blower motor resistor is a small electrical component that can cause noticeable issues with the passenger compartment’s air when it fails. Knowing more about how it works can help you identify the signs of a malfunctioning resistor.

Resistance Is (Not) Futile

Resistors are used in electrical circuits to lower current by, you guessed it, resisting flow. Resistance is measured in ohms and can be anywhere from zero to OL (open loop). An OL reading on a multimeter likely indicates a full break in the circuit, meaning the resistance is too great to read from one test to another. The higher the reading, the more resistance there is. Static resistors are designed to supply current at a constant level under specific resistances. When they fail, they supply either too much or too little current to a component.

Motor Mouth

The heating and air-conditioning system uses several resistors to supply the desired airflow by sending power to the blower motor. On the dash, each setting effectively represents a different current flow: When you select a low setting, you’re sending less power to the blower motor, which is achieved by more resistance, and when you select a high setting, you are sending more power to the blower motor, which is achieved by less resistance. The more power supplied, the higher the fan speed and the stronger the airflow.

Blowing a Bunch of Hot Air

The blower motor resistor is a single part, often with several possible pathways for current flow that provide more or less resistance, depending on the demand from the dash setting. When it fails — usually due to corrosion or overheating — you may notice a few things:

• Air flows, but only at high speeds, and other settings no longer work.
• There may be no airflow at all, which could be a sign that the resistor has completely failed, the blower motor is bad, it has a broken module or a relay is failing.

Take time to determine the cause of your airflow woes before replacing parts. To test a resistor or motor, you’ll need to know the basics of electrical circuitry and have access to a multimeter and the proper electrical specifications for the involved circuits. Testing whether there’s power flowing to the blower in the first place is a good way to rule out a bad motor.

Replacement of each component varies in difficulty depending on the vehicle, but an experienced DIYer can do it at home comfortably.

Check out all the climate control products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on blower motor resistors, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Flickr.