Why does blower motor resistor keep failing?
What is a blower motor resistor?
Many car makers use a blower motor resistor in conjunction
with a multiple speed switch to provide three or four blower motor speeds. The blower motor resistor is really a pack of resistors wired in series. See the image below. To run on the slowest speed, the power runs through all three resistors. For each higher speed, power flows through one less resistor. Some car makers run blower motor power through the fan speed switch first, then through the blower motor resistor and then to the blower motor and ground. Other car makers reverse that and run power to the blower motor first and terminate ground at the blower fan speed switch.
What causes a blower motor resistor to fail
By it’s very nature, resistors resist the flow of electricity, much like a flow restrictor in a pipe. However, as a side effect of reducing current flow, resistors generate heat. Over time, that heat can cause the resistor to fail naturally. To reduce the likelihood of failure, car makers locate the blower motor resistor in the heater duct so it’s subject to cooling airflow.
Worn blower motor causes repeat blower motor resistor failure
However, other factors can cause the blower motor resistor to fail prematurely. For example. as a blower motor ages and wears, it can draw more current—more current than the blower motor resistor was designed to handle. In that case, the blower motor resistor will fail repeatedly.
If you’ve replaced a blower motor resistor and that replacement has failed, it’s time to conduct an amperage test on the blower motor. If the amp draw exceeds manufacturer’s specifications, replace the blower motor.
High resistance in the electrical connector can cause repeat blower motor failure
Corrosion or poor contact in the blower motor resistor connector
can also cause an overheating condition. In some cases, the heat is so high that it melts the connector. This is common on late model GM vehicles. If you discover a melted connector, do NOT attempt repair. Purchase an upgraded pigtail and splice it into the harness along with a new blower motor resistor. Conduct an amperage draw test at the same time to rule out excessive current draw from the blower motor.
Water, leaves and debris in the airflow can cause blower motor resistor failure
Car makers design their HVAC systems to reduce the possibility of debris getting into the system. But they’re not always successful. If water spray enters the system, it can quench the hot resistor wires, causing them to break. If you notice the loss of one blower motor speed or all speeds soon after driving through rain, suspect water damage to the blower motor resistor.
©, 2018 Rick MuscoplatPosted on by Rick Muscoplat