Blower motor resistor
Ford blower motor resistor
The blower motor resistor controls the speed of the fan in the vehicle heating and air conditioning system. Not all cars have this part, you will more likely to find it in vehicles where the fan speed is adjusted by a manual switch.
The motor that runs the fan is called a blower motor. The resistor is needed to reduce the blower motor speed in lower speed settings (1,2,3 ).
Most cars have only one blower motor resistor. It's installed somewhere near the blower motor, often inside one of the air ducts. Minivans and SUVs with a separate heating and air conditioning unit for the rear passengers might have a second blower motor in the back. In this case, the second blower motor resistor might be installed in the rear A/C / heating unit.
The blower motor resistor is actually contains a three or four small resistors built on a circuit board or as coiled wires, see the diagram on the next page. A blower motor resistor doesn't need any maintenance; it only needs to be replaced if it fails. In most cars, it's covered by a basic 3-year warranty.
Blower motor resistor problems
This blower motor resistor from the 2008 Ford Escape
has failed due to corrosion.
As we mentioned earlier, a blower motor resistor is often installed inside one of the ducts in the heating and air conditioning system. This is done so the air flowing through the HVAC system cools down the resistor. However, the constant chage in temperature and humidity can cause the resistor to corrode. This is exactly what happen to the resistor in the photo. In this Ford Escape it stopped working because of circuit board corrosion.
Another reason a blower motor resistor may fail is when the blower motor itself has a mechanical problem or when a foreign object jams the fan blade. This leads to increased current through the resistor and causes it to overheat and fail.
The most common symptom of a failed blower motor resistor is when the fan works in "High" speed, but doesn't work in lower speeds. The highest speed setting may still work because in most cars in this setting, the current bypasses the blower motor resistor (see the diagram on the next page). In some cars, if the resistor is failed, the fan may stop working completely.
Read about blower motor resistor testing and replacement on the next page.