A resistor buys a newspaper. The vendor asks, “Why do you need a newspaper?” The resistor says “To stay current.”

Of all the vehicle components drivers are likely to take for granted, the blower motor resistor may be one of the most overlooked. Flip a switch, turn on the air conditioning or heater and air starts moving.

But what about when it doesn’t? Though the blower motor may often get the most attention (and a vicious rap or two on the top of the dashboard), behind the blower is a small component that has a big impact on the passenger compartment environment.

As the electrical signal to the blower motor resistor changes, it tells the blower motor to increase its speed and blow more air and vice versa.

Interestingly, though, when the fan is turned off, no power flows to the blower motor resistor.

When the fan is cranked up all the way the motor resistor is bypassed as well, so the flow of electricity doesn’t have to be modulated. It’s only between being completely off and totally on that the blower motor resistor has a job to do.

So, what don’t vehicle owners understand about a failed blower motor resistor?

First, the problem may be more complicated than just a failed resistor. Today’s complex heating and cooling systems put high amounts of current through the resistor connector, which can produce heat and melt the connector and resistor.

Worn OE blower motors may add to the problem. They can create a demand for extra current that can damage the resistor or module or melt the wiring and plastic shroud, damaging the interface pins on the controller’s circuit board.

If your customer complains of any of these HVAC system problems, the blower motor resistor should be checked.

• No Air. As obvious as it may sound, one of the most obvious complaints is a lack of air flowing through the vents. If there is no air moving at any speed, it’s a good sign that the blower motor resistor has failed.

• High speeds only. Because a blower motor resistor isn’t necessary when the fan is off at full blast, if the heat and air conditioning can only go from being off completely to being on at high power it’s almost guaranteed that the problem is a faulty blower motor resistor.

• In addition, when there is a wiring issue between the blower motor resistor and the blower motor itself, it’s possible that it can only work at low speeds.

• If the fan doesn’t turn off no matter what, regardless of whether it’s turned up, down or off, the blower motor resistor isn’t regulating the electrical current properly.

• Smoke coming from the vents or a burning electrical smell may be symptoms as well, especially if they accompany other problems.

Your customer may look online and decide that a blower motor resistor is relatively inexpensive – what’s not obvious is that the diagnostic and installation processes are likely not a DIY procedure.

Do your customers a favor and do the service for them. What they don’t know can hurt them. Thanks for watching.

This video is sponsored by The Network Academy.