Heater Blower Motor Resistor Symptoms, Function & Location

Car Blower Motor Resistor

When the heating or air conditioning in your car is turned on, the fan turns on and forces air out of the car’s vents. The motor that turns the blower is called the blower motor. It is located in the dashboard, usually on the opposite side of the steering wheel or inside the engine compartment on the firewall.

The blower speed can be adjusted in almost every modern car. Some cars have blowers that have 4 to 5 fixed speed settings that can be selected, but in this case, the speed of the blower motor is controlled by a blower motor resistor.

Signs of a Bad Heater Blower Motor Resistor

1. No air blowing through vehicle’s vents

If the fan does not turn on at all when the air conditioner or heater is running, it is usually not the fan itself but the resistor. Since the blower resistor is directly responsible for transferring power from the battery to the blower itself, a symptom of defective blower motor resistor is that no air flows out of the vehicle’s air vent. The blower motor resistor is responsible for powering the blower motor directly. Therefore, if major problems in the heating and air conditioning system fail, as if the power supply to the motor was completely cut off due to overheating or corrosion, the motor will simply not work, which in turn will render the entire heating and air conditioning system unusable, as no air will flow through the vehicle’s air vents.

2. Blower motor only working on high speed

A common symptom of faulty blower motor resistor is when the heating and air conditioning blower only operates at high speed and cannot be adjusted to low speeds. This occurs because the faulty blower motor resistor is not able to control the current flow. It could be that the resistor is somehow bypassed and the current is fed directly from the switch. Excessive current can overheat the blower motor resistor and permanently damage it.
In this case, the heating and air conditioning system might still function normally by pushing the cold or warm air out of the outlet alongside the speed. The resistor must be replaced with a new one to restore its function.

3. Blower motor only working on low speed

Another common symptom of faulty blower motor resistance is that the blower motor only operates at low speed. This may be due to poor wiring between the blower motor resistor and the blower motor itself.

4. Blower motor doesn’t work in certain settings

Another symptom of a faulty blower motor resistor is that the blower motor cannot operate in some settings. This will cause the blower motor to operate incorrectly or not at all in one or more settings. It is very likely that the blower motor switch will no longer work. A proper diagnosis is highly recommended if you are uncertain of the issue.

5. Smoke coming out of the vents

If you notice smoke or a burnt smell from your air vents, this could possibly be due to the blower motor resistor overheating, which may have burned some wires inside the dashboard. It is best to stop the car immediately to investigate further damage and allow it to cool down.

What is a blower motor resistor?

Heater Blower Resistor

A blower motor resistor is an adjustable resistor. This electrical component is used to control the air conditioning system of a vehicle. It is the part that controls the fan speed of the fan motor according to settings that can be changed by turning the knob to the left or right, thereby increasing or decreasing the resistance of the electric current flowing to the rotating fan motor connected to the fan.

As part of our comfort and luxury, the blower speed is one of the most adjusted settings in a car, and so the resistance of the blower motor is constantly being put under stress, which can eventually lead to motor failure; too much humidity in the atmosphere can lead to an increase in resistance. For this reason, the blower motor resistor can cause problems with the entire heating and air conditioning system.

Although it is unusual for this motor resistance to deteriorate, with a few exceptions, almost all components fail at one point or another, after which they must either be repaired or completely replaced. Fortunately, in this case, the motor resistor is not a high-tech electronic component and can be replaced relatively easily and cheaply. In fact, even the diagnosis of a failed blower motor resistance is actually simple and requires no special skills or even basic knowledge, as the resistor serves only one purpose – to limit the blower to different speed settings. A failed blower motor resistor will usually cause symptoms that can be easily identified.

Solutions & Testing

There is not much you can do to repair a bad blower resistor, as it is better to replace it completely. If you feel comfortable replacing the resistor yourself, you can find out where the component is and how to test it.

Steps to check/test the resistor:
Locate the blower resistor and disconnect it from the rest of the circuit. Once the resistor is disconnected, it is often possible to determine whether the resistor has blown or not based on its condition. A blown resistor must be replaced with the correct alternative.
If the appearance is normal, it is necessary to check the resistance in each individual resistor. The resistors are all connected to a single common point. Switch the multimeter to resistance measurement mode, connect one probe of the multimeter to the common point and use the other probe to delimit the resistors at other points. If one of these resistors has an open circuit, the blower resistor needs to be replaced.

Heater Blower Motor Replacement Cost

The average cost of a blower motor resistance replacement is between $80 and $150, and labor costs are estimated between $35 and $80, with the price of the unit between $50 and $75. The estimate does not include taxes and fees.

Written by: Magnus Sellén

Founder, owner & main author of Mechanic Base. I have been repairing cars for more than 10 years, specialized in advanced diagnostics & troubleshooting. I have also been a drifting driver and mechanic for over 7 years.