The Blower Motor Resistor: What Is It, What Does It Do, And What Happens When It Fails

The blower motor resistor is an essential component of a vehicle's HVAC (Heating, Ventilation, and Air Conditioning) system. This small but important component plays a crucial role in regulating the airflow and temperature inside your vehicle.

In this article, I'll explain what the blower motor resistor is, what it does, and how to tell if it is bad.

What Is A Blower Motor Resistor?

In older cars, the blower motor resistor is a physical resistor made up of wire coils of varying thicknesses or ohm values.

The coil-type resistor shown in the introduction image is a typical example of this type of resistor assembly.

In this type of blower motor resistor assembly, the different coils of varying thickness create a resistance difference that results in different amounts of current reaching the blower motor.

In newer cars, the blower motor resistor is typically an electronic component called a blower motor control module.

This module uses transistors instead of coils of varying thickness to control the flow of current to the motor.

In this type of blower motor resistor assembly, the transistors allow for more precise control of the motor speed and can also provide additional features, such as automatic climate control.

How Does The Blower Motor Resistor Work?

Both blower motor resistor assemblies that use coils of varying thickness and those that use transistors work by controlling the amount of electrical current that flows to the blower motor.

Here are some specifics: When you turn the blower speed switch to a specific speed in the HVAC control panel, the blower motor resistor assembly adjusts the amount of electrical current flowing to the blower motor to achieve the desired fan speed.

Resistor Assembly With Coils: The amount of current flowing to the blower motor in a resistor assembly with coils of varying thickness is regulated by routing the current through coils of different thicknesses, resulting in different levels of resistance.

When the current flows through more coils, the resistance is higher, and less current reaches the blower motor, which slows it down.

Conversely, when the current flows through fewer coils, the resistance is lower, and more current reaches the blower motor, which speeds it up.

Transistor-Based Resistor Assembly: In a transistor-based resistor assembly, the transistors regulate the current by switching on and off at a high frequency to control the average amount of current reaching the motor. This precise control of the current allows for more accurate fan speed control.

Symptoms Of A Bad Blower Motor Resistor

The blower motor resistor assembly, whether transistor-based or coil-based, experiences high heat temperatures as electrical current passes through it.

For instance, in the older style coil-based blower motor, the individual coils reduce the current flow to the blower motor by converting some of it into heat. This process will eventually destroy it.

Unlike the coil-based assembly, the transistor-based blower motor resistor assembly doesn't dissipate excess electrical current into heat but still generates heat due to its regulation of current flow to the blower motor using pulse width modulation (PWM).

Although the blower motor resistor assembly is designed for a long service life, it will eventually fail. Here are some of the most common signs that the blower motor resistor has failed in your vehicle:

  • The blower motor fan only works in certain speeds.
  • Blower motor fan only works in HI speed.
  • No air coming out of the vents when the AC or heater is turned ON.
  • Blower motor speed is erratic.

Check The Blower Motor Amperage Draw

If you're replacing a failed blower motor resistor, it's a good idea to check the amperage draw of the blower motor.

Why should you do this? Well, if the blower motor is worn out due to bad bearings or bad brushes (even if it appears to run fine), it will be using more current than the system is designed to handle.

This excessive current draw can eventually cause damage to the coil-based blower motor resistor assembly in a matter of days. In the case of the transistor-based blower motor resistor, it can instantly fry due to the excessive current.

For this reason, checking the blower motor amperage draw is essential as it helps prevent damaging a new blower motor resistor assembly due to excessive current draw.

Checking the amperage draw of the blower motor is a straightforward process. You just need a multimeter, and Ohm's law comes into play.

NOTE: You don't need an amp meter to check the amperage draw of the blower motor. Moreover, in most cases, it's not possible to use it because the blower motor needs to be running to measure its amperage draw. In cases where the blower motor resistor assembly is bad, the blower motor usually won't run, making it impossible to use an amp meter.

Here's how you can easily check the amperage draw of the blower motor:

  1. First, find the amperage rating of the fuse that provides current to the blower motor. This is critical information that you'll need to know.
  2. Disconnect the blower motor from its electrical connector.
    • In some cases, you may need to remove the blower motor to get better access to the connector terminals.
  3. Place your multimeter in Ohms mode.
  4. Measure the resistance across the blower motor male spade terminals.
  5. Take note of the resistance value.
  6. Divide the resistance value by 12.5 (this is your vehicle's battery voltage value).
  7. The result of this division is the amperage draw of the blower motor.

If the amperage draw you calculated earlier is below the amperage rating of the blower motor fuse, then you're good to go.

But if it's excessive, even if the blower motor seems to run fine, then you've got to replace the blower motor along with the blower motor resistor assembly to prevent damage to the blower motor resistor assembly.

Where Is The Blower Motor Resistor Located?

The location of the blower motor resistor can vary depending on the make and model of your vehicle but is usually located in the path of the airflow of the blower motor (helps to keep it cool, which in turn extends its lifespan).

The blower motor resistor can typically be found in one of two general locations:

  • Somewhere on the firewall in the engine compartment.
  • Somewhere behind or under the dashboard inside the vehicle cabin.

To find the exact location of the blower motor resistor in your vehicle, there are a few things you can do:

  • Consult your vehicle's repair manual.
  • If you don't have access to a repair manual, a quick Google search can also be helpful.

Forget about the owner's manual, it won't have this information.