What is a Blower Motor Resistor? Function, Symptoms, and How to Test It

A blower motor resistor controls the speed of the fan in your vehicle’s heating and cooling system. This small part lets you adjust how much air flows from the vents. When the blower motor resistor fails, you might notice the fan only works at one speed or stops completely. Many HVAC problems in cars come from issues with the blower motor resistor and the blower motor itself. If your heater fan only runs on the highest setting, the blower motor resistor could be the cause. Knowing how the blower motor resistor works helps you spot these signs early and keeps your car comfortable.

Key Takeaways

• A blower motor resistor helps control how fast the fan blows in your car’s heating and cooling system. Knowing what it does helps you stay comfortable when you drive.

• Some signs of a bad blower motor resistor are the fan only working at some speeds, no air coming out, or the fan stuck on high speed. Noticing these problems early can stop bigger troubles.

• You can use a multimeter to test the blower motor resistor. If you see infinite resistance, you should get a new resistor.

• Check your HVAC system often for signs of a bad blower motor resistor. Finding problems early can save you money and keep your car nice inside.

• If you do not feel sure about fixing the blower motor resistor, ask a professional for help. It is important to be safe when working with electrical parts.

Blower Motor Resistor Overview

What Is a Blower Motor Resistor

Every time you change the fan speed, you use the blower motor resistor. The blower motor resistor controls how much power goes to the blower motor. This changes how much air comes out of the vents. When you pick a new fan speed, the blower motor resistor sets the voltage for the blower motor. This part helps you get just the right amount of air. You can choose a soft breeze or strong airflow.

There are two main types of blower motor resistor assemblies in cars. Older cars have a wire coil blower motor resistor. Newer cars use electronic module resistors. Each type works in its own way. They also change how you control the fan speed.

Here is a quick look at how they are different:

Feature	Traditional Wire Coil Resistor	Modern Electronic Module Resistor
Mechanism	Uses coils for resistance	Uses transistors for control
Speed Control	Has fixed speeds	Has variable speeds
Design Complexity	Simple with fewer parts	Complex with more parts
Precision of Control	Not very precise	Very precise
Repairability	Easy to fix	Needs special repair

If your car is older, it likely has a wire coil blower motor resistor. This type gives you only a few fan speeds. Newer cars use electronic modules. These let you pick almost any speed. Modern blower motor resistor assemblies give smoother and more exact control.

Tip: If your fan only works on high or low, the blower motor resistor assembly might be broken.

Location in the Vehicle

The blower motor resistor is usually under the blower motor. This spot makes it easy to connect to the blower motor and HVAC controls. In many cars, you can reach the blower motor resistor by taking out the glove box. Some cars put the blower motor resistor in other places. Most keep it near the blower motor so you can get to it easily.

Knowing where the blower motor resistor is helps you fix problems. If you need to check or change the blower motor resistor assembly, look under the dashboard or behind the glove box. Always disconnect the battery before working near the blower motor resistor assembly.

Function of a Blower Motor Resistor

How It Controls Fan Speed

You change the airflow in your car by turning the fan knob. The blower motor resistor lets you do this. When you pick a fan speed, it changes how much electricity goes to the blower motor. This helps you choose soft or strong air.

• The blower motor resistor changes the electricity sent to the blower motor when you pick a fan speed.

• More resistance means less electricity, so the fan slows down and the air feels softer.

• Less resistance lets more electricity through, so the fan spins faster and pushes more air.

• Some blower motor resistors have a thermistor. This part stops the resistor from getting too hot by lowering the electricity if needed.

The amount of electricity the blower motor uses depends on the resistance. Lower resistance means the blower motor uses more electricity and spins faster. Higher resistance means the blower motor uses less electricity and spins slower. This system gives you different fan speeds, so you can make your car feel just right.

Tip: If your fan only works at one speed, the blower motor resistor might not be changing the electricity for the blower motor.

Voltage Regulation

The blower motor resistor helps control the voltage for the blower motor. It decides how much voltage goes to the blower motor. By changing resistance, it sets different voltages for each fan speed. More resistance lowers the voltage, so the fan runs slower. Less resistance lets more voltage through, so the fan runs faster.

Your car’s HVAC system has different fan speeds because the blower motor resistor can set many voltage levels. Each fan speed matches a different resistance. This lets you pick from several fan speeds and control the airflow.

The blower motor resistor is important for keeping you comfortable. You need steady airflow to keep the car at the right temperature. The blower motor resistor helps the blower motor send air through the vents. When you want heat, air goes over the heater core. When you want cool air, it goes over the A/C evaporator. The blower motor resistor controls the power to the blower motor, so you get the right amount of air.

• The blower motor resistor is needed to control the blower motor’s speed.

• This control changes airflow and temperature in the car.

• Good airflow and temperature keep you comfortable while driving.

• If the blower motor resistor fails, the heating and cooling system may not work right.

The electricity the blower motor uses changes with each fan speed. The blower motor resistor manages this, so the system stays safe and works well. If the blower motor uses too much electricity, it can get too hot or break. The blower motor resistor helps stop these problems by keeping the electricity at safe levels.

Note: Check the blower motor resistor and the blower motor’s electricity often to avoid sudden HVAC problems.

Bad Blower Motor Resistor Symptoms

When you notice problems with your car’s heating or cooling fan, you might be dealing with blower motor resistor symptoms. Recognizing these signs early helps you fix issues before they get worse. Here are the most common symptoms you should watch for:

Fan Only Works on Certain Speeds

You may turn the fan knob and find that only one or two speeds work. Sometimes, the fan runs only on the highest or lowest setting, while the middle speeds do nothing. This is one of the most common symptoms of a bad blower motor resistor. The blower motor resistor assembly controls how much power reaches the fan. When it fails, you lose control over some speeds.

• The fan may operate only at certain speeds.

• You might notice the loss of speed control function.

• Sometimes, only the highest or lowest speed works.

If you can’t adjust the fan speed as you want, check the blower motor resistor assembly first.

No Airflow

You might set the fan to any speed and get no air from the vents. This can happen when the blower motor resistor stops sending power to the blower motor. Sometimes, a bad motor bearing causes the blower motor to draw too much current, which overheats and damages the resistor. Other problems, like a clogged cabin air filter or blocked air ducts, can also stop airflow, but a faulty blower motor resistor is a frequent cause.

• No air comes from the vents, even when the fan is on.

• The blower motor does not receive power.

• The HVAC fan control may be lost at all speeds.

If you notice no airflow, check for obstructions, but also inspect the blower motor resistor assembly.

Fan Stuck on Full Speed

You may find the fan stuck on the highest setting, no matter what speed you select. This happens because the blower motor resistor fails to regulate the current. When the resistor burns out, the system often bypasses it, sending full power to the blower motor. The fan then runs at maximum speed all the time.

• The fan operates at a single speed, usually the highest.

• You cannot lower the fan speed.

• The resistor fails to control the current.

A fan stuck on full speed is a clear sign of a bad blower motor resistor.

Intermittent Operation

Sometimes, the fan works, then stops, then starts again. You may notice intermittent fan speed or the fan running at random speeds. This can happen when the blower motor resistor does not send the right voltage to the blower motor. Loose wiring connections can also cause this problem. If you see the fan acting up, you should check the blower motor resistor assembly and the wiring.

• The fan speed changes on its own.

• The fan stops and starts without warning.

• You may hear unusual noises from the blower motor.

Intermittent operation often points to a failing blower motor resistor or loose wires.

What Causes Blower Motor Resistor Failure?

Several things can damage the blower motor resistor. Overheating is a major cause. When you run the fan at low speeds for a long time, the resistor gets hot. Too much heat can burn out its parts. Moisture and humidity inside your car can cause corrosion. Corrosion damages the resistor’s contacts and circuit boards, breaking electrical connections. Electrical shorts and overloads from sudden voltage spikes also harm the resistor. These problems can burn out the resistor element and stop it from working.

Cause	How It Damages the Resistor
Overheating	Burns out internal components
Corrosion	Destroys electrical connections
Electrical Short	Burns or damages the resistor element
Overload	Places excessive stress on the resistor

Tip: Regularly check your HVAC system for signs of a bad blower motor resistor. Early detection keeps your car comfortable and prevents bigger repairs.

Testing and Replacing a Blower Motor Resistor

Tools Needed

You need some simple tools to check and change the blower motor resistor assembly. Get these tools before you begin:

• Multimeter (checks voltage and resistance)

• Phillips and flathead screwdrivers

• Socket set

• 12 V test light

• Gloves and a flashlight

Tip: Wear gloves to keep your hands safe. Use a flashlight to see better in small spaces.

How to Test with a Multimeter

You can check the blower motor resistor with a multimeter by doing these steps:

1. Find the fuse for the blower motor resistor in your car.

2. Use a 12 V test light to see if power goes through the fuse.

3. Take out the blower motor resistor assembly and unplug the power.

4. Test the resistor using the 12 V test light.

5. Make sure the blower motor’s ground wire is still good.

When you test the blower motor resistor, look at the resistance numbers:

Resistance Reading	Indication
Infinite Resistance	Blower motor resistor is broken
Values outside specified range	Possible problem with motor windings

Note: If you get infinite resistance, you need to replace the blower motor resistor.

Troubleshooting Tips

You can fix many problems if you know how to check the blower motor resistor and look for common trouble:

• Use your repair manual to find the blower motor resistor assembly.

• Check for rust or signs of too much heat.

• Use your multimeter to test for open circuits.

• Look for blown fuses, bad wires, or a blower fan that is hard to turn.

If you feel unsafe or uncomfortable while driving, ask a mechanic for help. Trying to replace the blower motor resistor yourself could make things worse.

Replacement Steps

Follow these steps to replace the blower motor resistor:

1. Unplug the car battery so you do not get shocked.

2. Take out the glove box or panel to reach the blower motor resistor assembly.

3. Unplug the wires from the old resistor.

4. Remove the screws that hold the resistor.

5. Put in the new blower motor resistor assembly and screw it in.

6. Plug the wires back in and reconnect the battery.

Make sure the new resistor fits your car. The usual cost to replace a blower motor resistor is between $135 and $186. For a Chevrolet Silverado 1500, the price is about $109 to $170.

Remember: Throw away old blower motor resistors the right way. They have harmful parts. Recycling helps keep the earth clean.

Knowing about the blower motor resistor helps you find problems early. This can stop big repairs that cost a lot of money.

• If you find problems soon, your HVAC system stays safe. You also save money on repairs.

• Doing regular checkups makes your system work better. It also helps parts last longer and break less often.

Dirt and dust can make the blower motor stop working. Clean the blower wheel every 3 to 5 years. This helps your system work for more years.

Risk Type	Description
Overheating	Not fixing problems can break important parts.
Fire Hazard	If you smell something burning, there could be a fire risk.
HVAC Function	Bad air and poor temperature control make driving less safe and comfy.

If you are not sure how to fix things, look for guides online. You can also ask a pro for help.

• Tips for Changing a Blower Motor

• How to Test a Blower Motor Resistor

 

 

 

 

 

Written by Jack Elliott from AIChipLink.

 

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