An engine wiring harness is a bundle of wires that distributes electricity throughout a vehicle's engine. It is the network that connects the fuse box to the various components of the engine that require electricity to operate.
Modern vehicles have an abundance of accessories and components that are powered by electricity. The primary source of this electricity is the vehicle's battery. When the engine is running, the battery is charged by a part of the engine called the alternator. Before it is distributed, the electricity travels through the fuse box, where fuses on each circuit prevent components from receiving too much power in the event of an electrical disturbance. Different wiring harnesses then distribute the electricity to the engine and throughout the vehicle.
The engine harness is typically a large bundle of individual wires protected by a shell of insulation and conduit which helps to protect it from the elements. It is often asymmetrical, with the large end close to the fuse box and becoming smaller as it branches out over the engine. Since the various automobile manufacturers have different engine designs which also change over time, engine wiring harnesses are often unique to the year, make and model of a vehicle. Because of its proximity to the engine as well as contaminants on the road, engine wiring harnesses can degrade and require repair or replacement.