The ignition system can be one of the more mysterious parts of a vehicle. The vast majority of drivers have no idea why an engine starts up when they turn a small metal key in the ignition - it just works and most don't question it. But understanding how the starter operates is one of the handier pieces of knowledge when it comes to diagnosing engine issues, as it helps tie the whole operation together.
For example, many drivers have had an experience with a dead battery in the past. A battery with no charge means a dead car. However, most drivers also know that non-electric vehicles do not ultimately run off of a battery - it's the mechanical engine that does all the work. So how is the electric energy converted to mechanical power?
The answer is in the starter solenoid, sometimes also known as a starter relay. This is a small magnetic part that is housed within the starter motor, and it's the key to starting up a car. When a driver turns the key in the ignition, it triggers an electrical current from the battery to the solenoid. The solenoid, being magnetic, then triggers the shaft inside the starter motor to engage with the flywheel, which in turn causes the crankshaft to begin moving.
It's the crankshaft where the true mechanical energy comes into play, as this part causes the engine to start working. Once the fuel begins to combust, the engine does not need any help from the starter or ignition system anymore. Without this, however, the engine cannot get started in the first place.
The solenoid actually pulls double duty. It also functions as a relay between the battery and the motor. The solenoid causes heavy contacts to close within the motor, which triggers an even larger electrical signal from the battery. That current then causes the motor to begin turning, which provides additional power to the engine as it's starting up.
Drivers should keep in mind that the solenoid can degrade and eventually malfunction over time, requiring replacement. Fortunately, this isn't a difficult repair to make. Buy a replacement starter relay, then locate the old one by following the red "positive" cable away from the battery. Don't touch the solenoid, but first remove all the battery wires. It's then safe to remove the solenoid, then replace it with the new one. Reconnect the battery wires in the reverse order and your car should start right up.