The distributor cap and rotor distribute the high voltage from the coil to the correct cylinder. The coil is connected to the rotor, which spins inside the distributor cap. The rotor spins past a series of contacts, one contact per cylinder. As the tip of the rotor passes each contact, a high-voltage pulse comes from the coil. The pulse arcs across the small gap between the rotor and the contact (which don't actually touch) and then continues down the spark-plug wire to the spark plug on the appropriate cylinder.

Distributor caps pass the voltage from the ignition coils to the engine's cylinders to ignite the fuel-air mixture inside and power the engine.

It’s important to carefully inspect the distributor cap to see how the sparks are arcing. The internal and external surface of the cap should be clean. There should be no erosion on the surface and the firing points should be free of rust or corrosion. Check for any cracks or carbon trace in the distributor cap. A slight buildup of carbon inside the distributor can conduct electricity that affects performance. It can short out the coil voltage and cause a faulty connection inside the terminal of the distributor cap. The result is a misfire in the spark plugs.