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What does a blower motor resistor do

What does a blower motor resistor do

The fan motor resistor is responsible for controlling the speed of the fan motor. In the past, this was achieved by varying the fan motor voltage. In more recent applications, the fan motor resistor is actually a semiconductor module that sends a variable signal to the fan motor.

A conventional fan motor resistor is grounded and connected in series with the fan motor assembly. The fan motor resistor is made up of several resistors with varying resistance values. This controls the current flowing through the fan motor. Each fan stage is selected to be connected to one or more resistors in the fan resistor assembly. Modifying the resistors controls the current flowing through the fan motor, which in turn limits its speed. When high speed is required, the fan motor resistor is bypassed and the fan motor ground circuit is connected directly to ground. This causes maximum current to flow through the fan motor and maximum speed to be achieved.

Typical Fan Motor and Resistor Wiring
Note: Before testing, obtain a wiring diagram and system operation diagram for the fan motor and resistor for your vehicle application.

Testing Tips

Refer to the service information for specific test procedures and resistance values for ohmic resistance testing.
Check blower motor power
Check the grounding and resistance of the main switch.
Make sure the ground connection passes through each resistor pin to each pin on the switch and blower motor.
With the resistor disconnected, check that current flows through the switch to the resistor terminals.
Typical blower motor and resistor wiring.
Note on power supply: The blower motor resistor generates heat during operation. For this reason, the resistor is in the air flow of the blower motor, as the blower motor provides cooling to the resistor. If the blower motor operates without the resistor installed, the blower motor resistor may be defective. Blower motor overcurrent caused by a defective blower motor or clogged cabin air filter should also be considered or possibly replaced to avoid repeated failures.


Recently, blower motor speed has been adjusted by various methods. Most manufacturers have moved from simple resistive circuits to electronic control modules and solid state controllers.

The following guide is designed to help you understand how blower motor speeds are adjusted and how to test blower motor control circuits. We always recommend that you refer to the service information for the specific vehicle you are repairing.


Using an inductive current clamp, check that the current draw of the blower motor is not too high. A worn fan motor can cause the current to be too high. If the current is too high, replace the fan motor. This excessive current will cause the new fan motor controller to fail again. The guideline for maximum current at high speeds is approximately 15 to 20 amps. Check the application information for your particular vehicle.


Power Suggestion: This potentially recurring fault usually occurs during testing when we verify that our diagnosis is correct. It occurs when a currently installed or newly installed blower motor control unit starts without being fully installed in the correct position. The airflow generated by the blower motor through the heat sink prevents overheating of the blower control assembly electronics. This failure may occur immediately or may recur at a later time.


It is important that the vehicle itself has air flow to prevent the blower motor control assembly from overheating. The cabin air filter should be checked.

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