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 to 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
When this part burns out due to age or the blower motor which is pulling too much
amperage due to the age of its windings or the wear of the brushes it will short circuit not allowing
the lower speeds to work.
Testing a blower motor fan
The location of this resistor is 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
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.
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
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.
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
Article first published 2016-04-26