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What Are the Best Tips for Relay Wiring?

By Paul Reed
Updated May 16, 2024
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Relays are electronic switches used to power accessories or electrical systems in vehicles. A relay can be a solid-state integrated circuit, with no moving parts, but even in the 21st century many are reliable, inexpensive mechanical switches. They provide power to many systems without having to install heavy-duty wiring throughout the entire vehicle, which reduces cost and weight. Relay wiring can be improved by choosing good locations to place them, and providing adequate wiring size for the power needed by the accessories.

A typical relay looks like a small box, perhaps 1 inch (2.5 cm) on each side, with four connectors or pins at the bottom. Relay wiring is often installed in wiring blocks, where the relay plugs into sockets that match the pin shapes, and wires are connected from the sockets to the vehicle systems. This permits a part to be replaced easily if it fails by unplugging it and plugging in a new one.

The four pins on the bottom of the relay are often numbered, and each is connected to a different wire. One connects to the vehicle ground, which returns any unused power to the battery. One pin is connected to the switch, such as the ignition key, that activates the relay. The other two are connections to the battery and from the relay to the vehicle systems or accessories.

When drivers turn the key to start their car or truck, a signal is sent from the ignition switch, but the wiring may be quite thin. This is possible because the main power relay, which supplies the entire vehicle with power, is located in the engine compartment near the battery. The wire from the battery to the relay may be quite thick to supply all electrical needs, but the wire from the ignition key can be thin because its only purpose is to supply a small electrical current. When the key is turned, the current activates a switch inside the relay, and power from the battery flows to the vehicle through the relay wiring.

Installing relay wiring is not difficult, but some tips can help make the job easier. Placement is important so heavier wiring lengths are kept to a minimum, both to save money and reduce power losses. A wire taken from the battery positive terminal should be wired directly to the relay pin, and another to the accessory, using the shortest length of wire needed. This may place the relay closer to the battery or next to the accessory, depending on the vehicle.

The switch that controls the relay can be installed anywhere on the vehicle, with wiring supplied from an existing power source or another accessory power wire. Power requirements for the activation switch are small, so smaller wire sizes can be used and located in the vehicle as needed to hide them. Using a relay block for installing multiple units can make the job easier than wiring them separately.

Relay wiring is useful for extra lighting, louder horns that require additional power, or navigation systems. Using separate relays allows these systems to be operated separately from the vehicle ignition system. This reduces power loads on the ignition system wiring and can reduce electrical problems.

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