How do I know if I need to replace the blower motor or the blower motor resistor?
One way to tell that the blower motor resistor is faulty (rather than the blower motor itself) is when the fan runs on some speed settings but not on others. If the blower motor fails, it will usually fail to function at all, whereas a bad resistor is often intermittent or partially impaired.
What causes a blower motor resistor to fail?
Blower motor resistors fail for several reasons, including overheating, excessive current draw from the blower motor, or corrosion. To promote cooling, the blower motor resistor is often mounted in the stream of cool air entering the HVAC system from outside the vehicle. Unfortunately, this also means that the resistor is exposed to moisture, salt spray (during winter driving), dust, and other airborne contaminants drawn into the HVAC system. All of these contribute to corrosion of the resistor coils and possible overheating.
Why does the fan in my car only work on high?
The blower motor resistor changes the amount of electrical current that flows to the blower motor. “High” speed is the normal amount of current, and the lower speeds are due to designated degrees of electrical resistance. If the resistor fails, it is possible for the electrical signal to bypass the resistor in its normal “high” state. This results in a fan speed that only runs on high and is unresponsive to other settings.