If your car’s blower has only 4 or 5 different speed settings, it likely uses a simple electric resistor to control the fan’s speed. The resistor limits electrical current so that the blower operates at the correct speed. If the resistor fails, generally, the blower will only work on select speed settings.
There are also some instances where the blower motor may not work at all.
Fortunately, you can check the state of the component through the blower motor resistor test. With just a multimeter, you can find out if it’s time to get a replacement.
Blower Motor Resistor Test
If you can hear the blower fan running but can’t make it change its speed, you probably have an issue with its resistor. Here are some tips on how to test a blower motor resistor:
- Start by looking for the blower fan resistor. A repair manual can tell you where its location is in your vehicle’s make and model.
- Inspect the resistor visually. Look for obvious signs of corrosion and burns from overheating.
- Check the electric resistance of each of the resistor’s terminals with a multimeter. Set the tool to read ohms (a measurement of resistance) and connect one of its probes to the common output terminal of the resistor. Use the other probe to measure the resistance between the output terminal and each of the resistor’s input terminals. Measure the resistance between one pair of terminals at a time.
- If any of the points show infinite resistance, the blower motor resistor has failed. Infinite readings denote an open circuit.
Be careful when handling the blower resistor. Since electric currents pass through the resistor for long periods, the device can build up significant heat—enough to burn your fingers or worse.
What is a Blower Motor Resistor?
The blower motor resistor usually sits outside either the heater box or the HVAC plenum that distributes air in the interior space. It’s typically wired into the HVAC control circuit somewhere between the blower control knob and the blower motor itself.
The resistor controls the fan speed of the blower motor and the resulting change in air pressure produced by the movement of the blower fan. It accomplishes this by regulating the electric current that powers the motor.
What Does the Blower Motor Resistor Do?
The resistor limits current, thereby reducing voltage, to control the speed setting of the blower motor.
More current equals a higher blower speed, and vice versa.
Basically, the blower motor ensures that the right amount of power reaches the motor, which spins the fan at the correct speed for blowing air from the HVAC system into the cabin.
It’s also worth noting that most vehicles have a circuit that bypasses the resistor when the blower motor is on the high-speed setting. In this situation, battery power is sent directly to the blower motor.
What’s more, many modern vehicles have a mini-computer, referred to as a blower motor control module, instead of a traditional resistor.
What are the Symptoms of a Bad Blower Motor Resistor?
Every time you fiddle with the fan speed of your car’s air conditioning or heating system, you change the current that flows through the blower motor resistor on the way to the blower motor.
A bad or failing blower motor resistor hampers the efficiency of the heating and air conditioning system. The most common symptoms include:
The blower fan gets stuck on one speed setting
A working resistor can shift between speed settings according to the user’s preference. However, a failed resistor can cause the speed to get stuck on a single speed setting.
While the blower motor will still run, you cannot change its speed. This can prove annoying in situations where you want to adjust the temperature of your car’s interior to a more comfortable level.
A blower stuck on a high-speed setting will keep blasting you with hot or cold air when you don’t need it, while one that’s stuck on the low setting can be equally as frustrating.
The blower fan cannot access certain speed settings
Sometimes, a failed resistor can still set more than one speed setting. However, it may also lose the ability to access other speeds. The blower motor will run fine at the available settings, but it won’t operate correctly if you select a disabled speed.
This symptom isn’t limited to a bad blower motor resistor. A broken blower motor switch can also stop the blower from changing to certain speed settings. If your blower motor cannot change to certain speeds, check both the resistor and the switch on your dashboard.
No air comes out of the vents
In most cases, since the bypass circuit is still intact, the blower motor will continue to work on high-speed with a failed resistor.
There are, however, instances where a bad resistor will prevent the current from reaching the blower motor altogether. Without any power, the motor won’t run. As a result, the blower fan stays still and silent, and no air blows out of the vents.
This symptom can also stem from a failed blower fan or several other issues. If a blower motor resistor test clears the resistor and the wiring, the problem may lie in the fan’s motor.
Causes of a Bad Blower Motor Resistor
Resistors have it rough. They always have electricity flowing through them, and the voltage can change sharply as the vehicle’s owner adjusts the strength of the blower fan.
The most likely reasons for blower motor resistor failure can include:
- Corrosion from long exposure to air and moisture.
- Overheating—if a motor finds it harder to move because of jamming or worn bearings, it can raise the resistor’s temperatures to dangerous levels.
- Normal wear due to time and use.
What is the Cost of a Replacement Blower Motor Resistor?
You can still drive your car despite a faulty blower motor resistor. However, you’ll probably find it uncomfortable to have the blower running at one speed setting the entire time.
Depending on the manufacturer and model, a blower motor resistor can go anywhere between $3 and $335 on CarParts.com. The replacement parts come in individual units, sets of 2 resistors, or as part of a blower motor resistor kit.