A starter solenoid mounts directly to the starter. Although similar to the starter relay, the solenoid converts electrical energy into linear motion. The solenoid contains a set of windings that, when energized, cause movement of the solenoid's plunger, which then engages the starter.
The starter solenoid supplies the high current needed to run the starter motor and also provides the force needed to engage the starter.
Proper starter solenoid and starter operation rely on a healthy battery and sound cable connections. Make sure the connections are clean and tight at every oil change. A faulty starter solenoid can cause the starter to not engage or lose engagement, and overall poor starter performance. Each spring, prior to travel season, it's wise to have your car's starting system tested as part of a comprehensive starting, charging and battery test. This test will determine whether your car's starter solenoid is working as it should, along with the rest of the starting circuit. If you suspect a problem with the starter solenoid or the starter, have it investigated as soon as possible by a professional service technician.