How Do Horn Relays Work?

Horn relays come in all sizes and shapes and are typically used on ATVs, cars, motorcycles and even boats. They are simple switching devices that take voltage from a main power source and switch it to a component, in this case a horn. They provide protection for the wires and circuits and are usually equipped with a protective fuse.

Horn Relay Outer Construction

Horn relays are electrical devices, small boxes that contain multiple terminal connectors, usually four terminals for automobiles, and three or four for motorcycles. The relay cases can be made of plastic or metal and can be plugged directly into a fuse box block for an automotive applications, or bolted to a motorcycle frame or bracket. Relays are sealed against weather and moisture to provide protection to the interior electronic components.

Horn Relay Interior Construction

An automotive horn relay typically has four terminals protruding from the case, and sometimes a fifth that is an additional power output. A Bosch-style terminal, with the same function as most others, for instance, has a No. 85 terminal for ground, No. 87 terminal output for power, a 30/51 terminal for power input and a trigger signal terminal (86). The trigger switch terminal has a magnetic contact inside the case that closes to complete a circuit once voltage is sent to it.

Horn Relay Wire Connections

A small gauge wire leads from the horn button or hub contact switch underneath the steering wheel cover to the relay trigger terminal. The input wire into the relay is a larger gauge and comes from the battery, with a fuse spliced into the wire. The output wire from the relay leads directly to the horn and is also made of a larger gauge wire. The ground terminal relay wire leads to a common ground wire inside the fuse block, which leads to a main ground source on the engine or chassis frame. Motorcycle relays, which have three terminals, have an exterior case that grounds when the case is bolted to a grounded bracket or the motorcycle frame.

Horn Relay Function

As soon as the horn button or horn hub is depressed, current is sent down the wire to the trigger terminal inside the relay. Magnetic contacts close, which completes a circuit between the power input wire and the power output wire. Power travels through the output wire to the horn diaphragm, which vibrates or pulses to produce a sound. Some horns are designed to complete a ground connection when depressed, allowing power through the circuit.