Distributor Cap
A removable plastic cover on the distributor that routes high-voltage current from the ignition coil to the individual spark plugs.
The distributor cap is a vital component of older mechanical ignition systems, serving as the interface that distributes high-voltage electricity to the spark plugs. Inside the distributor, a rotor spun by the camshaft sweeps past electrical contacts molded into the inside of the distributor cap. The cap has a central contact connected to the ignition coil and outer terminals connected to the individual spark plug wires.
As the rotor turns, it transfers the high-voltage spark from the coil to each terminal in the correct firing order. Distributor caps are typically made of durable plastic to insulate the high voltage, but they are subject to wear, moisture accumulation, and carbon tracking.
Over time, the metal terminals inside the cap can corrode or wear down, increasing electrical resistance. Cracks in the cap can allow moisture to enter, causing the spark to arc to ground, resulting in misfires, hard starting (especially in damp weather), and rough running. Replacing the cap and rotor is a standard part of a tune-up on older vehicles.