There are several devices that are labelled as solenoids. Solenoid can refer to a transducer device that converts energy into motion, or a pneumatic valve. In vehicle starter systems, it refers to an electromechanical or linear solenoid. A starter solenoid, also known as a starter relay, is essentially an electromagnet consisting of two copper wires wrapped around an iron plunger. When current passes through the coil, the plunger becomes magnetised and is drawn into the core. This engages the gears of the starter motor.
The solenoid starter system in a car is fairly simple. It consists of a battery from which the system's initial charge is derived, which is directly connected to the solenoid. Inside the solenoid are a pull-in coil (the larger coil) and a hold-in coil. The coils are wrapped around an iron plunger that, when the system is started, is moved into a core, where it makes contact with a linkage to the starter's drive gear.
When the car is started, voltage is sent from the battery to the solenoid. The pull-in coil is capable of drawing a lot of current and generating a large magnetic field, and is the primary drain on the battery. This creates the magnetic force that draws the plunger into the core and completes the circuit between the terminals of the battery. The movement of the plunger causes the starter's drive gear to move into mesh with the flywheel ring gear. This is what causes the motor to turn. The hold-in coil generates a much weaker magnetic field and holds the plunger in place. This saves battery power and reserves most of the battery's life for starting the car.
Most vehicle starting problems are a result of a lack of current moving through the large magnetic coil of a starter solenoid. When you hear a clicking sound but the engine does not turn over, the battery does not have enough life to magnetise the large coil and pull the plunger into place. Several things can cause this. In most cases, the battery does not have enough charge to power the solenoid because the headlights or other electrical component were left on and drained the battery's charge. Battery drainage can also be caused by short circuits in more severe cases. A lack of charge reaching the solenoid an also be caused by corroded battery terminals, loose cable connections or a damaged positive battery cable.