A solenoid moves an iron core through the center of a wire coil when current flows through the coil. It is different to a relay which uses an electromagnet with a fixed iron core. Typical voltages used in solenoids are 12 and 24 volts. Some solenoids are controlled by pneumatic and hydraulic pressure instead of electricity.
Solenoids actuators have a spring in the base at the base of the iron core. When current flows through the solenoid, the electromagnetic field overcomes the spring resistance, and pulls the rod through the center of the solenoid. The spring pushes the rod back out when the current stops flowing. This push-pull action is used to control many common devices such as locks and valves.
Solenoids are used to control the starter motor in virtually every automobile. When the solenoid is energized by turning the ignition key, a conductor is pulled against two terminals, and a large current flows from the battery to the starter motor. It also moves the drive pinion to connect the starter motor gear with the engine flywheel. When the ignition key is released, the spring pushes the conductor off the terminals and stops the starter motor, and also disengages the pinion gear.
If a solenoid fails to move back into place when the coil current stops flowing, the spring in the base may be broken. If a solenoid makes a continuous clicking sound, the current is insufficient to create a field strong enough to keep the the spring compressed. This may be caused by a short in the coil that is preventing current from flowing through all the coil turns.