A blower fan motor resistor is a device in a car’s heating and air conditioning unit that controls the speed of the fan. There are 2 different types of these resistors. One is a wire type that has a different size coil for each fan's speed. And it has different resistances for each wire.
The next type is the printed circuit board. It has a fully integrated circuit. They are both located in the same position. That position is within the immediate proximity of the blower motor. They both are always a small unit, usually, they measure 2 by 3 inches. The resistor wire type has a series of different size coils and is mounted on a small flat plate that is nonconductive. The printed circuit board is mounted on a plastic base plate.
Step 1 - Locate the Blower Motor and Resistor
Locate the blower motor. Look around the outside and all around the motor to locate a 2 by 3-inch plate. It is held on by 2 or 4 screws. Depending on the vehicle type and the heating and air conditioning control panel, there will be an electrical connector plugged into it with 5 or 6 wires coming out of it.
Step 2 - Testing
Before replacing it, test the resistor with the voltmeter. Considering that there is a multitude of things that could cause the same symptoms, it is always recommended to test any electronic items before replacing them. It only takes a minute to test and this can verify that the part is bad. It can save money and time to repair the right part the first time.
Step 3 - Fan Speeds
Check the fan speeds by turning the fan control to all positions. Take note which speeds work and which do not work. If the fan only works on one speed, it is very likely that the resistor is bad.
Step 4 - Check the Fuses
If the fan speeds do not work, check the fuses. The fuses for the HVAC system in a car. are usually located under the hood in the fuse and relay center. If the fuse is good, take it out and turn the ignition key on. Use the voltmeter set at 20 volts DC. Make sure that there are volts going to the fuse. If you do not get a reading, check for an open circuit to the fuse.
Step 5 - Checking the Relay
To determine if there is power at the fuse, check the relay. The relay should have 2 of the 4 terminals that show power. With the fan and the key on, remove the relay and it directly back in. Be sure to listen for a click during this process. If you hear no click, replace the relay.
Step 6 - Checking the Blower Motor
On the blower resistor motor, check the voltage at the electrical connector to see is the fuse and relay are good. Make sure the fan and key are on during this process. There should be power going to two of the terminals of the blower motor resistor. Replace the resistor if you only have power to one. Replace the blower motor itself if 2 of the terminals have power. Unplug the connector and remove the screws to the blower motor resistor. Put the new one in its place. Screw it back into place and plug the wiring connector up.