Spark Plug Wire
An insulated cable that carries high-voltage electricity from the ignition coil or distributor to the spark plugs.
A spark plug wire (also known as an ignition cable) is a heavily insulated wire that carries high-voltage electrical current (ranging from 20,000 to over 50,000 volts) from the distributor cap or ignition coil to the spark plugs. This high voltage is necessary to create the electrical spark across the spark plug gap to ignite the air-fuel mixture in the cylinders. While many modern vehicles use "coil-on-plug" ignition systems (which mount an ignition coil directly on top of each spark plug, eliminating spark plug wires), older vehicles and many trucks still rely on ignition cables.
Spark plug wires are constructed with a conductive core (often carbon-impregnated fiberglass) wrapped in thick silicone or rubber insulation and fiberglass braiding. This heavy insulation is critical to prevent the high voltage from leaking or "arcing" to nearby metal engine parts before reaching the spark plug.
Due to the extreme heat in the engine bay, vibration, and age, the insulation on spark plug wires can break down, crack, or dry-rot. When this happens, electrical current will jump through the cracked insulation to the engine block. This causes engine misfires, rough idling, hesitation during acceleration, reduced fuel economy, and a Check Engine Light. In dark conditions, you can sometimes see blue sparks jumping from worn wires, a clear sign of arcing.
Replacing spark plug wires is a routine tune-up service. It is critical to replace them one at a time to avoid mixing up the firing order, and to apply dielectric grease to the boots to prevent them from sticking to the spark plugs or distributor terminals.