The blower motor relay is a type of switch that helps a driver to control the speed of the fan that blows air through the cabin vents.
The blower motor (fan) in your vehicle’s HVAC system requires a good deal more power than the control circuit can carry. The fan speed selection switch or computer control operates on a low voltage circuit. But the blower motor needs more power and therefore operates on a higher-voltage circuit with more current.
Inside the blower motor relay are two electrical circuits. One that can accommodate the needs of the blower motor, and another that can receive the low-voltage signal from the climate controls. When you change the fan setting, the low-voltage control signal is used to trigger an electromagnetic switch on the high-powered side, switching the blower motor on or off.