A solenoid is an electric device that converts electricity to linear motion. Applying power to the coil creates an electromagnet that overcomes the force of the spring to move the plunger. Solenoids have various uses, including closing an electric circuit or activating a valve.
In an automobile, the starter solenoid connects the low power ignition switch to the higher power starter motor. Turning the key activates the electromagnet and moves the plunger. The plunger completes the starter circuit, providing enough power from the battery to turn the starter motor and ultimately the engine of the car, creating enough compression to start the car. This setup allows use of a shorter length of the heavyweight wiring required to power the starter.
Chrysler uses a solenoid valve to control the vacuum boost in some of its vehicles. While the plunger in this device activates just like the starter solenoid, the purpose differs. This solenoid activates a valve, moving it from one port to a second port with a higher vacuum source.
Solenoids also find use outside the automotive industry. Walt Disney discovered that using solenoids in audio-animatronics and recording the directions on magnetic tape created the movement required for attractions such as the Enchanted Tiki Room. In this case, the solenoid converted electric energy into motion to control puppets.