What Is a Blower Motor Resistor? A Complete Guide to What It Is and Its Uses

A vehicle’s air conditioning system consists of numerous parts that work together to ensure the system operates as expected. Such components include the blower motor and blower motor resistor. People can often confuse these two, but they serve different purposes in the AC system. This guide focuses on the blower motor resistor, differentiating it from the blower motor to provide a clearer understanding. Let us look into it below.

What is a Blower Motor Resistor?

A blower motor resistor is an electrical component that is part of a vehicle’s heating and AC system, whose job is to regulate the speed of the blower motor. This is done by controlling the electrical current flowing through the motor.

Blower Motor Resistor Circuit Diagram

An AC blower motor resistor is well explained with the circuit diagram below, so that you have a good understanding of what to expect and how it works. The resistor system typically consists of multiple resistors, each with a different resistance value. This is now what determines the fan speed settings, depending on the selected speed level.

How Does a Blower Motor Resistor Work?

We have already established what does a blower motor resistor do in terms of controlling speed, but how does it work?

In the older systems, the resistor was a series of wires that had varying resistance level. Therefore, the fan speed would depend on the selected resistor. This would then control the current flowing to the motor.

In modern systems, solid-state modules are used to achieve the same performance with greater efficiency.

Each time you select a lower fan speed, the blower motor resistor increases its resistance, thereby reducing the current flowing to the motor and, consequently, the fan speed. At higher speeds, the opposite occurs, where the resistance is reduced and more current flows to the motor, allowing it to spin the fan faster.

It is also possible to bypass the resistor. This is common on the highest fan speed setting. When the resistor is bypassed, the blower motor receives full power, allowing it to spin the fan at its fastest speed.

With this in mind, you can gain an understanding of the symptoms of a failed blower motor resistor and how to replace it.

Applications of Blower Motor Resistors

The main application would be in a vehicle’s heating and air conditioning system. Its function is to provide different resistance levels, allowing the motor to spin the fan at various speed levels.

This allows you to adjust the fan speed to your preference. It also comes with a thermal fuse in some cases to prevent overheating and protect the blower motor.

Right now, improvements in technology have made it possible to use solid-state modules that send a signal to the blower motor rather than using the old resistor winding setup.

Signs of a Failing Blower Motor Resistor

There are some symptoms that you may notice or hear, which can lead you to question whether the blower motor resistor is functioning correctly. So, before you do a blower motor resistor test, here are the notable signs of a failing blower motor resistor.

Blower Motor Stuck at One Speed

If the resistor fails to regulate the current flowing to the motor, then you will end up with a motor stuck at one speed. It is usually the highest, as it appears the resistor has been bypassed.

Inconsistent Airflow

Other than the AC blower motor resistor being stuck at one speed, it sometimes fails to provide consistent airflow. Sometimes you may experience airflow, but at different times, there is none. This fluctuation can point to a faulty motor resistor.

No Airflow

Yes, in some cases, there is no airflow at all. This is because you have a faulty resistor that prevents the blower motor from receiving any power. This means there will be no airflow from the AC vents.

Unusual Noises

A clicking, buzzing, or rattling noise each time you change the fan speed is a sign of a problem. It could be one of the components that includes a failing resistor. It could be due to arching each time you change the fan speed.

Burning Smell

When the resistor overheats, it can produce a burning smell emanating from the dashboard. This is a serious matter that warrants further investigation to determine the culprit.

Blown Fuses

When you keep experiencing blown fuses for the AC system, it’s time to investigate further. It is possible that the blower motor resistor is drawing excessive current, which can lead to blown fuses that are designed to protect the blower motor circuit.

Such symptoms of a bad blower motor resistor might warrant a visit to the mechanic to probe further. You can still do it yourself if you have all the relevant experience.

Causes of Blower Motor Resistor Failure

There are a couple of things that might lead to the failure of the blower motor resistor. This includes:

• Several factors can cause overheating. For example, having to use the blower motor with restricted airflow. This includes a clogged cabin filter, a blocked evaporator, or a dirty heater core, which leads to overheating.
• Corrosion occurs when moisture intrudes. When corrosion occurs on the resistor and its connections, it becomes evident how this can lead to failure.
• Electrical issues such as faulty wiring, bad ground connection, and faulty speed control module affect how the blower motor resistor works.
• Sometimes it could be as simple as age and wear. Like any other component, the blower motor resistor can experience normal wear and tear and might need replacement over time.

How to Check a Bad Blower Motor Resistor?

It is important to understand how to test a resistor of a blower motor so that you can know if it needs replacing or not. Here is how to do it.

Visual Inspection

Locate the blower motor resistor in the vehicle. It is usually near the blower motor under the vehicle’s dashboard.

Check if there are any signs of melting, burning, or damage to the resistor and its connector.

Also, look for signs of corrosion, loose pins, or damage around the wiring harness going to the blower motor resistor.

Multimeter Testing

This is a more effective method for determining whether the blower motor resistor is in good condition or not. First, set up the multimeter in the Ohms setting since we will be testing for resistance.

Look at the vehicle’s wiring schematics to identify the input and output terminals of the resistor. Connect the multimeter probes to the input and output terminals.

We will check for continuity in this case. If the multimeter shows infinite resistance, it means there is an open circuit and the resistor is faulty. You should replace it for it to work again.

Fuse Check

Locate the fuses that connect to the blower motor resistor. Using a test light, check if there is power still flowing to the resistor.

If the fuse is blown, simply replace it and test the blower motor resistor. If the fuse keeps blowing, there could be a short circuit somewhere that needs more investigation to deal with.

How to Replace a Blower Motor Resistor

Since you have now identified that the blower motor resistor has a problem, the next step is to replace it. So, what is the process on how to change a blower motor resistor? We clearly outline it below.

1. Disconnect the negative battery terminal as the first step for safety. This helps prevent an electrical shock.
2. Locate the blower motor resistor. It is normally found under the dashboard close to the blower motor. Some vehicles might require removing the glove box or panels to reach it.
3. Disconnect the wiring harness once you have located the resistor. There will be a clip or tab you have to release to disconnect the harness. A flathead screwdriver should do the job if needed.
4. Remove the old resistor from its place by removing the screws that hold it in place. Be cautious not to get burned in case it was overheating.
5. Go ahead to install the new resistor in its position. Align it with the screws you removed earlier. Tighten the screws making sure it is held in position.
6. Reconnect the wiring harness and make sure the locking tab is engaged. Proceed to reinstall any removed panels and reconnect the removed battery terminal.
7. Turn on the ignition and test the blower motor to see if it is working correctly at all speeds.

As you can see, it is usually a quick process that can be done in no time to help you gain control of the blow motor again.

Conclusion

The blower motor resistor is a key component of the vehicle’s AC system. This is because it helps with controlling the speed of the blower motor. That is how you end up with different levels of fan speed. It is basically switching from one resistor to another, which now restricts or allows more current to flow to the blower motor. You should also be able to identify if the blower motor has issues to ensure it is diagnosed properly and also replaced if necessary.