As the name suggests, a distributor cap is a cup-shaped piece of thick plastic that fits over the rotor at the end of the distributor in an automobile. It also helps to transport the spark, or voltage, to the cylinders from the coil. Though they are found in most older cars, many newer cars no longer have manual distributors. The caps are made of hard plastic with small metal parts and should be periodically changed throughout the car's life.
Inside the cap are small copper contacts, which rest almost touching the metal of the rotor. On the outside of the distributor cap, over the contacts, are plug holes. The center hole connects the coil to the cap via a wire. The holes circling the center plug connect to the spark plug wires. The number of plugs and contacts for the spark plugs depends on the cylinders of the car's engine: four, six, or eight. The caps are normally held in place by two screws or clamps on either side.
To start the ignition process, the coil sends a spark through the wire connecting it to the distributor cap. This spark reaches the copper contact inside the cap and jumps to the metal rotor. The rotor, being turned by the camshaft, passes close enough to each of the spark plug contacts to allow the spark to arc between the two. The energy from the spark travels from the contacts, along the spark plug wires, to the spark plugs, which spark to ignite the compressed gas in their respective cylinders.
Since distributor caps are often subjected to both heat and vibration, they periodically need to be replaced. Most often, they will crack. Sometimes the cracks are not obvious, but often they can be seen by even non-professionals. Carbon deposits that build up on the cap over time will also cause it to not function properly. Difficulty starting, particularly when the temperature is cold, stalling or backfiring and knocking noises are all indications the device might be damaged.
To change a distributor cap, the old one is removed and the spark plug wires are connected to the new cap in the same order. If they are not connected in the same order, the car simply will not start. Then the cap is fitted back onto to the car, over the rotor, and screwed or clamped back in place.
Mechanics can, of course, replace this component for a fee, but many people find it is more cost-effective to do it themselves. Although it is not difficult to do, it requires attention to detail and patience, since caps are not always located in an easily-accessible location. Putting one on incorrectly can potentially damage the rotor as well and end up costing the do-it-yourself car owner more money.