The blower motor resistor is used to control the speed in a vehicle that has a manual HVAC system. The speed is controlled by adding resistance to the circuit when in lower speed settings. The blower motor is one of those parts that commonly causes problems for vehicle owners, but luckily, it is not an expensive fix. You will find the blower motor resistor by the blower motor, and those GMCs with a rear HVAC system have resistors in the rear blower motor, as well as the front.
How it Works
The resistor is bypassed in a vehicle where the fan speed is at its highest because the blower motor is then powered by the fan switch itself. You may find the blower motor working even with a bad resistor if you have the fan at its highest speed.
When the fan switch is set at a lower speed, the current to the blower motor is reduced by how many resistors it has, and that correlates to how many speeds it has. There are equal parts resistors to fan speed options less the highest speed.
You may find that the fan works in “High” speed, but not at lower speeds. This is a symptom of a failed or failing resistor. In some cases, a failed resistor prevents the fan from working at all. If your fan blade is not spinning freely due to an obstruction, it can overheat the blower motor resistor. Make sure there are no obstructions in the fan blade.
Test the Blower Motor Resistor
You will find the exact procedures for testing the blower motor resistor in the service manual for your specific GMC model. The service manual will give you the exact specifications between the terminals and the resistance. For instance, you need to know the ohms necessary before you test your resistor in order to determine if it is performing within the acceptable range for your vehicle.
When you remove the resistor, you can check for obvious corrosion or other damage that would interfere with its ability to perform.
- Locate the blower motor and the blower motor resistor. You will need to remove the cover box that is held in place by bolts.
- Remove the blower motor resistor cover.
- Disconnect the wires that are connected to the resistor. One will be a pigtail and may be hard to reach. You may need to use a screwdriver to release the tab that connects it. Make sure that these wires are in good shape before going any further. You may need to replace a connection that as been damaged.
- Connect all the wiring to your replacement resistor and test it before putting the cover back on it. Run the fan and check the fan speeds. If everything is working as it should, then replace the cover.
Your fan should now work as it is supposed to, but if this problem happens again, you may need to replace the blower motor itself. Sometimes they wear out and may require more amps to run. The additional amps will cause the resistor to fail.
Should you run into trouble or find that you need more help, then contact your local GM dealership for help.