Starter power begins at the battery and solenoid. Solenoids are nothing more than electromagnets-heavy-duty switches (relays) we energize with the ignition switch-that get power from the battery to the starter and ignition coil. When you turn the key to start, the "S" lead fires the solenoid, which makes contact between the battery and starter. At the same time, power passes through the "I" lead to the ignition coil. Solenoids fail for two basic reasons: They can stick in the "on" position and continue cranking our engines, or they just don't work at all due to internal failure. They cannot be serviced, only replaced.