How To Test Blower Motor Resistor?

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Blower motor resistors play an important role in keeping you comfortable in the vehicle. They regulate the speed at which the blower motor rotates to keep the HVAC running. As a keen car owner or even a technician, you should always be  keen to ensure that the blower resistor is in a perfect working condition.

A bad blower resistor will result in the erratic operation of the motor. Sometimes the motor will even fail to function. On a positive note, you can easily test the state of your blower resistor. In this guide, I am going to take you through all the essential steps of testing a blower resistor.

What is a blower motor resistor and what does it do?

A blower motor resistor is an electrical component that  forms part of a vehicle’s heating, ventilation and air conditioning (HVAC) system. It uses its electrical properties to regulate or control the speed of the motor.

Just like other resistors, it achieves this by controlling the electric current that passes through the motor. Keep in mind that the motor is directly  connected to the blower fan which will determine the  volume of air to be circulated.

A blower motor resistor is located  directly next to the blower motor: This can be in the engine compartment or right beneath the dashboard. It comprises of a series of resistive  coils  which are connected to  the vehicle’s control module. This is the module that allows you to determine the speed of the blower fan.

How does a blower motor resistor work?

Before you even think of testing your blower motor resistor, it will be prudent to understand its working mechanism.

The first point of operation is when the driver adjusts the fan speed control switch. As we have stated, the blower switch is directly connected to the control module of the HVAC panel. The action by the driver will send signals to the blower motor resistor module.

The control module of the blower  motor resistor will receive signals from the  HVAC control panel and will respond by adjusting the amount of electric current  that goes through  the resistive coils.

The resistive coils  that are part of the blower motor resistor will regulate the amount of current that flows through them. This is the current that determines the actual speed of the blower motor.

As the current through the resistive coils keep changing, so  is the speed of the blower motor. The position of the fan speed controller will determine the speed  of the blower motor.

Once you turn this switch off, the  control module will cease sending current to the blower motor resistor and  the motor will eventually turn off stopping the movement of the fan.

You can use the voltage drop to get a deeper understanding of the blower resistor  working principle. When fully connected, the resistor will drop some voltage passing through it hence cutting down the volume of current that is delivered to the motor. Th lower the voltage drop across the resistor the lower the amount of current flowing through. You can use a voltmeter to ascertain this.

Testing the blower motor resistor: step-by-step guide

Now that you know what a blower motor resistor is and how it works, let’s look at how you can test it.

The most important toll that you will need for testing is the multimeter.

Locate the blower motor resistor

When it comes to testing, you first have to find the exact location of the motor resistor. It an be in the engine compartment or on the dashboard. The location will depend on the exact model and type of vehicle. For this step, it will be prudent to consult  your car manufacturer’s manual. Once you find the location, you can proceed with the next step of testing a blower motor resistor

Visual inspection

Now that you have found  the resistor, you can physically inspect it to ascertain its state. Check out for signs of physical damage such as corrosion, rust and overheating on the wires and connectors. If you notice any sign of physical damage, consider replacing that component with a new one. Most of the times you will have to buy a brand new blower motor resistor.

Test the resistance of the blower motor resistor using a multimeter

Visual inspection is not enough.  At this stage you will need a multimeter and set it to the resistance mode (ohms). Connect the probes of the multimeter to  the output terminals of the  resistor. A wiring diagram will come in handy at this stage if you are not sure of these  terminals.

Remove the blower motor resistor from its designated position so that you can access the terminals of the resistors. To do this, simply remove the screws  that are holding the resistor in place. Detach the clips  then pull out the resistor gently from its housing.

Find the resistance on each resistor coils that make up the whole unit. The value of the resistance will always vary, again depending on the model and specs of the vehicle. However, it should fall within a certain range. You should still consult your car manufacturer’s manual for the expected value.

If during your test you find out that the points display infinite resistance, chances are the blower motor resistor has failed. This is because the infinite value on the multimeter means the circuit is open.

You should be  extra cautious when handling  a blower resistor. It usually has some build-up current which  tend to stay there for long even after you have turned the vehicle off. This current can easily cause injuries when handled with bare hands.

Know the type of blower motor

Other than just testing the blower motor resistor, it is good to know the exact type of motor that runs your vehicle’s HVAC system. These motors are divided into two classes, single-speed motor and variable-speed motor.

A single speed blower motor blows the air at a constant speed. It features simple construction devoid of the control panel. You will not have the freedom to control the amount of air to be blown or how it should operate. This blower motor was common on older vehicles.

The variable blower motor can operate at different speeds. It has an elaborate control panel that allows you to select the speed you want depending on the intensity of air that you would like to get. For example, during winter, you will set the highest speed.

Symptoms of a bad blower motor resistor

To reach at appoint where you want to test your blower motor resistor, there are things that you might have experienced in the vehicle.

Remember that a bad motor resistor affects the overall functioning of the air conditioning system. You will not be able to control the level of conditioning that you should get. In a worst scenario, the HVAC system will completely fail to function.

Here are the symptoms of a faulty blower motor resistor:

No air

This is the first culprit of a damaged blower motor resistor. When you turn the AC on but everything remains silent, or no air comes through the vent. Chances are the motor is not getting any power due to a faulty resistor.

The blower works at high speed only

Image source CarThrottle

This is another common symptom of a bad blower motor resistor. It is stuck at high-speed setting. You cannot adjust it to low or medium settings and even if you do, no air comes from the vent.

Similarly, the blower may remain stuck at low-speed setting so you cannot adjust it to high or medium speed. If you constantly encounter this issue in your vehicle, take time to test the blower resistor motor.

A damaged blower switch can also cause this problem. So, if you find out that the resistor is okay, you should proceed to analyze the switch.

Reduced air/wind speed

Let’s say you have selected the highest speed on the controller but you get the opposite feedback. The speed and intensity of air blowing through the ventilator seems to be lower than expected. Chances are the motor resistor is not responding accordingly. The same applies when you have selected the lowest setting but you still get high-intensity stream of air.

While the above symptoms may point to bad or damaged blower motor resistor, you should be open to exploring more problems relating to the vehicle’s HVAC’s system.

Conclusion

At this pointy you know what it takes to test a blower motor resistor. Once you know the location of this electric component, you can di the diagnosis and find the right solution. Like everything related to electric components, you should take care when doing all the testing to avoid risk of electric shock. It will be best to seek help from professionals if you are not good at handling matters related to electricity.

If it comes to replacement, ensure that you buy high quality blower motor resistor from the best electronic components suppliers. Verify to ensure that the motor resistor is fully compatible with your vehicle’s HVAC system.

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