In an electrical circuit, you may need to do an investigation, but energy is never lost. It is turned into either motion or heat. In the case of a motor, the electricity travels through coils of wires in the armature to generate a magnetic field. These coils create a load as the motor spins. The windings also have a resistance measured in ohms.
Resistance in a circuit changes some of the electrical energy into heat. The greater the resistance or load on a circuit, the more electrical energy that is transformed into heat. But, heat never likes to stay put.
The heat is not restricted to the load or component with the resistance. The heat can travel through the wires and warm up the wires and connectors. The connectors themselves can be a source of resistance and generate heat. This is what causes a lot of electrical components to burn out on a vehicle.
This is true for blower motor and blower motor resistor circuits. As blower motors begin to weaken or are clogged with debris, they draw excessive amounts of electrical current. The excessive draw causes more current flow in the circuit and blower motor resistor.
The heat eventually burns out the blower motor resistor and, in many cases, causes the electrical connector to melt. If the source of excessive draw or the electrical connector isn’t replaced, the likelihood of a repeat failure is high.
Blower Motor Resistors control the electrical current flowing from the fan switch to the blower motor, which allows the motorist to set the fan at different speeds. The fan speed can be changed mechanically using a rotating lever which selects a different electrical path of resistance in the blower resistor, or automatically by an HVAC control module.
Due to technological advancements on today’s automotive heating and cooling systems, blower motor resistors have high amounts of current running through their connectors producing heat that can melt the connector and resistor. Worn OE blower motors can create a demand that also damages the resistor or module as the excess current melts the wiring and plastic shroud, damaging the interface pins on the controller’s circuit board.
For diagnostics, technicians should use an inductive amp clamp to check that the blower motor current draw is less than 80% of the fuse rating on high. If the current is too high, replace the blower motor or the new resistor will fail too. The mating connector should also be inspected for signs of damage caused by excess heat.
The next time you diagnose a burned-out blower motor resistor, check the blower for excessive draw and the connector for damage. Also, take a look at the HVAC case drain to make sure it’s not plugged, and that water can drain properly and always make sure that the cabin air filter is replaced too. The clues are there to make your investigation a success.
This video is sponsored by The Network Academy.
