As the rotor rotates inside the cap, it receives the high voltage from the ignition coil, then passes it to the nearest connection, which is a metal projection in the cap, which is connected to a spark plug. The distributor cap should be checked to see that the sparks have not been arcing from point to point within the cap. The inside of the cap must be clean. The firing points should not be eroded, and the inside of the towers must be clean and free from corrosion.