Common blower motor resistor failures including inspection and testing to ensure correct operation.
- 12V Multi-speed HVAC Systems
The GoTech technical services team receives many calls about inoperative blower motors and blower motors that only work on certain speeds. When a blower motor only works on the highest speed most likely the blower motor resistor has failed. A failed resistor will sometimes allow the blower motor to run on the highest speed position because the max speed is not resisted and will have a separate circuit through the resistor. Before replacing the resistor check the circuit fuse and wiring for possible damage. The failure of individual fan speeds may also indicate a failed blower resistor. Be sure to check the wiring and connector for damage first.
A DVOM (digital volt ohm meter) is necessary to test the blower motor circuits. Using a wiring diagram for the vehicle application and the test meter, test each circuit while cycling the fan switch at each of the fan speeds. You should see a change in the activated circuit as you cycle the fan switch. Fan resistors with integrated circuitry and vehicles with automatic climate control will require a more in depth understanding of circuit operation to test. It is possible in some cases the resistor connector can melt causing the failure, or you may have a situation where the resistor has been replaced recently and failed again. For this situation, the blower motor amperage draw should be tested to verify it has not exceeded the rated amperage specification causing excessive heat leading to melting and failure. For this test you will need to locate the fuse for the blower motor circuit.
The vehicle owner’s manual is a good place to find the location and correct rated amperage fuse required. Using a voltmeter and an amperage clamp, install the clamp around the harness leading to the blower motor, this will give you an accurate amperage reading of the blower motor. With the blower fan running on high speed the percentage should be less than half of the fuse rating. Allow the fan to run for an extended time for this test to allow the motor to reach full operating temperature. If the draw is over half the fuse AMP rating, the blower motor is the likely cause of the resistor failure. Replacement of all failed, damaged, and worn components are required to ensure the blower motor runs properly and to prevent future component failures or damage to the blower motor circuits.