The blower motor resistor is a collection of resistors that control how much electrical power goes to the blower motor, which gives whoever is using the vehicle's controls a few options for fan speeds.
Usually the top speed a fan can run at has no resistor placed in in the electicity's path to and from the fan, and the other settings get resistance added for each slower speed. The slowest speed has the most resistance, and when the fan is not in use, the circuit is open.
Signs of a Faulty Blower Motor Resistor Unit
The easiest to diagnose issue would be one of the selectable speeds not working at all. This would mean that the resistor associated with that speed burned out or otherwise malfunctioned, and will need to be replaced. Sometimes, however, the whole resistor unit can fail and the diagnosis isn't as easy. In this case, testing can be performed to rule out other potential causes, such as a faulty switch, faulty relay, blown fuse, failed blower motor, or a melted electrical connector.
- Check for a fuse and if/when one is found, check its condition.
- To test other components, use a multimeter (or voltmeter) and confirm that the battery has about 12.6 volts. If the vehicle is designed to not allow the blower fan to run unless the engine is running, start the engine and check the battery again to check alternator voltage, which is typically 13.0 to 14.5 volts.
- Find the wire leading to the plug on the blower motor and disconnect it. Check the connector and the plug for melted plastic and burned terminals. Turn the fan on using the dashboard controls, setting it at full speed.
- If you know which wire is which, test the appropriate wire for voltage by putting the common lead of your multimeter on either the negative post on the battery or a good, reliable ground, and the positive lead on the positive pin/connector on the plug. If you don't know which wire is which, do the same, but test both connections. You should find a reading very close (0.5 volts lower, but it depends on the vehicle) to battery or alternator voltage.
- If a good power reading was found, power is getting to the blower motor connector. Remove the blower motor and inspect it for obvious damage. If damage is found, it is most likely the main issue, and the blower motor chamber should be checked for debris before installing a new motor. Otherwise, clamp the motor and safely provide 12V to the terminals. A good blower motor will spin now, and a faulty one will either do nothing or struggle.
- If both tests have passed, check the other wire on the connector by placing your positive multimeter lead on the battery positive post and the common lead on the second wire in the connector. If this fails, the wiring may be bad.
- If everything checks out with all other parts of the system on the full speed setting, but the problem was on one of the speeds, the only place left for defects is in the control switch itself, the blower motor, or their connectors. Remove the blower motor resistor and check each resistor pin for continuity. You will most likely find the fault here, but if you don't, the fault is in the switch.
Testing a Blower Motor Resistor Circuit
Assumptions: Most blower motors have two wires, a positive and negative/ground. If it has more, it is a more advanced design beyond the scope of this test, but it will still be tested in a similar way, you will just need to know which wires are which to prevent causing damage to other components while testing. It is also assumed that if you want to perform this diagnosis, you have a multimeter.