Blower Fan Motor High Speed Only

Does your blower fan work on the high speed setting only? This problem is a classic blower motor resistor problem which is common on older cars and trucks that use manual fan speed switch controls.

When the blower fan is switched onto high speed the circuit is a straight connection from positive and negative battery power, but when the motor is switched onto the lower speeds electrical power is routed through a series of resistors or a solid state assembly that drops the voltage down to 6 volts, 8 volts and then 10 volts (low, medium low, medium) which in turn makes the blower motor turn slower.

When this part burns out due to age or the blower motor is pulling too much amperage due to the age of its windings or the wear of the brushes it will short circuit the resistor not allowing the lower speeds to work.

Also see: Testing a blower motor fan

The location of this resistor is usually near the blower motor itself which cools the unit when it's in operation. Both of these parts are located in the heater plenum box either inside the vehicle on the passenger side, or under the hood mounted near the firewall on the passenger side. 

Lets Get Started

Step 1

To start testing turn the ignition key to the on position, (do not start the engine) then test the heater system fuse and replace it if needed. Some vehicles utilize a separate fuse to control lower fan speeds in the fuse box check it as well. Then use a grounded test light and locate the outgoing wire (usually orange or red) to the blower motor from the resistor while moving the switch to each fan speed and test for power. If there is no power on any of the lower speeds the resistor has failed and needs replacement.

Below is a typical wiring diagram for the blower motor circuit which is basically the same for all vehicles utilizing this system.

Step 2

Remove the fan resistor and inspect it for burn marks which is an indication of failure. If an area shows signs of extreme heat (burned) it needs to be replaced. Also inspect the resistor connector for heat damage at the terminals and replace or repair it as needed by splicing in a new connector. Anytime you replace this unit its a good idea to replace the blower motor as well to prevent premature failure due to high amperage draw. Learn more

If you have any questions about what you have just read please visit our forum where our mechanics have already supplied hundreds of answers about blower motor resistors.

 

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