How Do Car Relays Work?by Don Bowman
The purpose of car relays is to keep the larger diameter, high amperage wires under the hood as opposed to running them into the interior of the car. These high amperage wires pose a higher threat of fire than common low amperage circuits. With the use of relays, the high amp circuits can remain under the hood or in the trunk and be actuated from the dash with the use of low voltage and amperage switches.
The relay is no more than a remote controlled light switch. For the sake of an illustration, a radiator electric cooling fan will be used. The electric radiator cooling fan is located in all front wheel drive cars. In most vehicles there are two or three relays directly related to the electric radiator fan circuit. They have, at the very least, a low speed relay and a high speed relay. The circuit runs from the computer to the coolant temperature sensor and back to the computer. The computer senses the temperature of the engine and then in response to this temperature will react by turning on either the low speed or high speed relay as it deems necessary to control the particular situation. The wires from the coolant temperature sensor circuit from the computer to the relays are small, low voltage, low amperage 16-gauge wires. The wires on the relay from the battery to the relay and then from another post on the relay to the fan are decidedly larger to carry much higher amperage. Usually these wires are 10 mm wires.
The relays are located in the fuse relay center on the driver's side fender well. There is a heavy gauge wire that goes from the battery to the fuse block to a buss bar. A buss bar is simply a main connection for incoming power and from which fuses are attached and then to the relays as the main source of power for the fans. This incoming power is directed to two posts on the relay, which are in a normally open condition unless connected through the switching of the relay. This is done by the computer sending a low voltage supply to the opposite post of the relay, causing the relay to close the terminal allowing power to flow through the relay to the fan.