Spark plug wires are a component found in many internal combustion engines. They typically consist of a conductive core surrounded by several insulating layers, as they are designed to carry very high voltages. In traditional ignition systems, spark plug wires connect the ignition coil to the distributor, and the distributor to the spark plugs. With distributorless ignition systems (DIS), plug wires will typically connect coil packs directly to the spark plugs. Certain systems, such as coil-on-plug, place the coil packs directly over the plugs and do not use spark plug wires at all.
Many internal combustion engines operate by igniting a mixture of fuel and air with the assistance of a high voltage spark. This spark is typically provided to the cylinder by a spark plug, though it is generated by an ignition coil or similar device. Coils can vary in design, but they generally operate by running voltage through a primary coil and then shutting it off. When the voltage to the primary is shut off, the magnetic field generated by the coil can collapse, generating significantly higher voltage in a secondary winding. If the ignition system has a distributor, this high voltage can be sent to it via a special spark plug wire, often known as the coil wire.
In a system with a distributor, the high voltage spark can then be transferred via a rotor to the correct spark plug wire. A rotor within the distributor is typically turned by the camshaft, allowing the spark to be timed correctly and sent to the right cylinder during its compression stroke. One spark plug wire is typically used per cylinder, though some applications use two spark plugs for each, and thus twice as many plug wires.
Distributorless systems operate in a similar fashion as far as spark plug wires are concerned. The method for generating the spark at the correct time is different, though each cylinder will usually have an associated coil pack and a spark plug wire that connects the two. Distributorless ignition systems can operate at substantially higher voltages than traditional systems, so extra care must be taken when dealing with spark plug wires in those applications. Severe injury or even death may be possible by handling a DIS spark plug wire incorrectly.
Spark plug wires are often routed near or even on top of very hot components, such as exhaust manifolds. The outer later of a spark plug wire is typically a heat resistant material, such as silicone rubber, though heat damage is still fairly common. If the protective insulating layers of a spark plug wire become damaged, the wire may short out to ground and cause a misfire condition. The gap in a spark plug can create enormous electrical resistance, so it may be possible for spark plug wires to short out even if there is no visible damage.