A blower motor relay is a switch that controls power to the blower motor. Relays use low amperage to control high-amperage circuits, and are typically used for devices exceeding 10 amps in load.
A blower motor uses a large amount of amperage. High amperage causes arcing across switch contacts when the switch is turned on and off, and high amperage also has the potential to create a dangerous amount of heat. An ordinary switch does not have the ability to sustain long-term high amperage use and creates a potential fire hazard within the passenger compartment of a vehicle.
A relay uses a low-amperage circuit to control a high-amperage circuit and allows a high-amperage circuit, such as a blower motor, to be used safely. Contained within the relay is a small electromagnet, a switch mechanism and a buffering device. Two circuits are contained within the relay; when the blower motor switch on the dash is activated, power is applied to the circuit containing the electromagnet. The electromagnet pulls the contacts of the switch mechanism together, completing the circuit that powers the blower motor.
As the contacts come together, the high-amperage voltage attempts to jump the gap between the contact points. This arcing quickly erodes the switch contacts if it is not controlled. The buffering device, usually a small capacitor, absorbs the electrical spikes that cause the arcing. The relay can be safely mounted in the engine compartment to keep high-amperage circuits inside of the passenger compartment to a minimum.