The blower motor in a vehicle is the electrically operated fan which moves air into the passenger compartment. Typically, the rotational speed (and, by extension, the volume of air that is moved) is electrical current dependent. The higher the amount of electrical current flowing into the blower motor, the faster the fan turns and the more air it moves. Controlling that current level is the function of the blower motor resistor.
Blower motor resistor basics
The blower motor resistor is typically found under the dashboard over on the passenger side of the vehicle in relatively close proximity to the blower motor itself. It is usually manufactured in one of two configurations. The first would be a set of wound coils mounted on a non-conductive tray or plate. The coils will be distinctly different heights, and may have different thicknesses in terms of the wire that is used. It is a relatively small physical package that measures approximately two inches wide by three inches long. The thinner coils offer less resistance to current flow, and correspond to higher fan speed. The thicker coils offer greater resistance to current flow and therefore correspond to a lower fan speed. The second type is a solid state type electronic printed circuit board mounted on a plastic tray or base plate, which also measures approximately two inches by three inches.
Replacing a blower motor resistor
The classic sign that a blower motor resistor is faulty and needs to be replaced is that the fan will only operate on one speed, regardless of where you set the selector switch. If there is no fan rotation at all, the first thing to check would be the dedicated fan/blower fuse in the vehicle's fuse/relay box typically located under the hood in the engine compartment. Some vehicles may have the corresponding fuse in the fuse block located in the passenger compartment under the dashboard. If there is a blown fuse, there will be no current flow to the blower motor and, as a result, no fan rotation.
Testing the blower motor resistor can be accomplished either with the resistor in-circuit or out-of-circuit. Both types of test will require a digital multi-meter (DMM). For testing the resistor in circuit, you will need to use the voltmeter portion of the DMM set in the 0-20 volts D.C. range. To test the resistor in-circuit, first check the fuse to ensure that there is current flow going to the resistor. If the fuse is good, then check the relay. If you first remove the relay, and then turn the ignition switch and the fan selector switch to the on position, when you replace the relay you should be able to hear an audible clicking sound. If not, then it's a bad relay that should be replaced. To test the resistor out-of-circuit, use the resistance part of the DMM. Any high resistance usually indicates a break in the wiring of the coils.