Blower Motor Resistor : Working & Its Applications

We know that resistor is a passive electronic component commonly used in different electrical and electronic circuits. There are different types of resistors available that are used based on the requirement. Among them, the blower motor resistor is one of the types. In cars, heating & air conditioners are used to produce the required air temperature together. The Blower resistor plays an important role in these systems. This article discusses an overview of the blower motor resistor and its working.

What is Blower Motor Resistor/Define Blower Motor Resistor?

A blower motor resistor is an essential component in the heating & air conditioning systems of cars. The arrangement of this resistor can be done within the ductwork or near the car blower either under the hood or the dash. The main function of a Blower resistor is to control the speed of the fan that supplies air toward the compartment of a passenger in the car. It is mainly designed to produce a voltage in direct proportion to current, therefore, allowing the resistor to fewer speeds. If this resistor fails, then it causes some problems within the compartment of a passenger.

Blower Motor Resistor
Blower Motor Resistor

How does a Blower Motor Resistor Work?

The blower motor resistor works by controlling the speed of the blower motor. Previously, this control was simply done by changing the supplied voltage toward the blower motor. At present, this resistor is used to transmit a varied signal toward the motor.

This resistor is inbuilt with different resistors including different resistances. The typical resistor is connected in series through the motor assembly.

So, this controls the current flow throughout the blower motor. Each fan speed range in the blower resistor assembly can be connected to one or a number of resistors. So, the resistances change controls the flow of current throughout the motor to limit the speed at motor rotates.

Once the maximum speed is requested, then the resistor goes around & the ground circuit of the motor can be connected to the ground directly. This permits the high current throughout the blower motor & maximum speed takes place.


Blower Motor Resistor Circuit Diagram & Working

The wiring diagram of the blower motor resistor is shown below. The following diagram shows how the resistor is connected to a car. The resistor has the highest fan speed setting that will be bypassed within this car & the blower motor can be power-driven through the switch of a fan. Thus, even if this resistor is defective, then the motor within some cars may operate continuously at a maximum speed.

Blower Motor Resistor Circuit Diagram
Blower Motor Resistor Circuit Diagram

In the following diagram, the fan switch is set to position ‘1’ in the diagram, then the flow of current throughout the motor can be decreased by three resistors like R2, R3 & R4 which are connected in series. In the above diagram, the current flow is specified from voltage supply to motor then fan switch.

Similarly, when the fan switch is set to position ‘2’ then two resistors are connected in series, and in setting ‘3’, only a single resistor will be there. Once resistance is included in a circuit, then the flow of current within the circuit will be decreased. When numbers of resistors are connected within series, then overall resistance will be increased & equivalent to the amount of the independent resistances.


The specifications of a blower motor resistor include the following.

  • The type of motor used in this is the DC series motor.
  • The Blower motor resistor weight is approximately 1.5lbs.
  • Reliable & high quality.
  • The maximum power used by the blower motor is 27 kW.
  • They have superior finish & long-lasting nature.
  • It includes four poles.
  • The rated speed is 1750 RPM.
  • The surface gets cooled by a DBR fan.
  • Vertically mounted.
  • It includes four brush holders.
  • Class H insulation.
  • Armature temperature in motor is 80 ºC.
  • Commutator temperature is 85 ºC.
  • The maximum absolute bearing temperature is 90 ºC.

What Causes a Blower Motor Resistor to Fail?

  • The faults of this resistor mainly occur in cars. The most frequently occurred faults are, once the heater fan simply works at maximum speed setting & doesn’t function at fewer speeds.
  • In cars, a faulty resistor will stop the working of a heater fan completely.
  • This resistor mostly fails because of overheating or corrosion.
  • In some cases, the mechanical resistance toward the motor rotation can cause an extreme electric current that can overheat & break the resistor prematurely. For instance, this happens once the blade of the fan is jammed or the bearings of this motor are exhausted & it doesn’t turn freely.
  • Once a blower control module gets damaged or does not work, the blower motor doesn’t function at all so it can cause the motor to run continuously even ignition is OFF.
  • The bad resistor symptoms mainly include no air will come from vents, working on less speed or high speed, the fan won’t be turned off, burning smell, bad ground, faulty wiring or faulty switch, blown circuit breaker or fuse, failure of the thermal fuse, burned connectors, etc.

How to Test a Blower Motor Resistor?

The testing of a blower motor resistor can be done by using a multimeter. Before that, we need to check the resistor visually. So, check the surroundings for clear evidence of rust & hot burns. Change the blower resistor if you notice evidence of overheating connectors, however, make sure you merge the wires properly.

So if you want to test a blower motor resistor quickly then use a multimeter to gauge the resistance of resistors. For that, locate the multimeter to read ohms & connect one of its probes to the common output port of the resistor. Calculate the resistance in between the two terminals like input & output through probes.

At a time, you can calculate the resistance in between two terminals but you need to identify the common terminal & also which of the terminal will control which speed.

How do you Recognize if a Resistor is Bad?

The bad blower motor resistor can be identified through the following symptoms.

  • A bad blower motor resistor can disturb the process of the whole cooling & heating system.
  • Only single speed from the blower motor.
  • Different airflow settings will not work properly.
  • No air will come from vents.


The applications of a blower motor resistor include the following.

  • This resistor is used to control the speed of the blower motor within a car.
  • This resistor controls the flow of current throughout the blower motor.
  • These resistors are used in automobiles.
  • Actually, this resistor is a solid-state module that transmits a varied signal toward the blower motor.

Thus, this is all about an overview of the blower motor resistor. So replacing this resistor mainly includes different steps like placing the blower motor resistor, detaching the wires, and remove the resistor, and installing a new blower motor resistor. Here is a question for you, what is the antilock braking system in automobiles?