TIP AS-FUEL INJECTOR 4W8094 - Caterpillar

A solenoid is an electromagnetic switch that does two basic operations.
The solenoid closes the high current starting motor circuit with a low current start switch circuit.
The solenoid engages the starter motor pinion with the ring gear.
Illustration 1 g00285112
Typical solenoid schematic
The solenoid has windings (one or two sets) around a hollow cylinder. The cylinder contains a spring loaded plunger. The plunger can move forward and backward. When the start switch is closed and the electricity is sent through the windings, a magnetic field is made. The magnetic field pulls the plunger forward in the cylinder. This moves the shift lever in order to engage the pinion drive gear with the ring gear. The front end of the plunger makes contact across the battery and the motor terminals of the solenoid. The starting motor begins to turn the flywheel of the engine.When the start switch is opened, current no longer flows through the windings. The spring pushes the plunger back to the original position. The spring simultaneously moves the pinion gear away from the flywheel.When two sets of windings in the solenoid are used, the windings are called the hold-in windings and the pull-in windings. Both of the windings have the same number of turns around the cylinder. However, the pull-in winding uses a wire with a larger diameter in order to produce a greater magnetic field. When the start switch is closed, part of the current flows from the battery through the hold-in windings. The rest of the current flows through the pull-in windings to the motor terminal. The current then goes through the motor to the ground. When the solenoid is fully activated, current is shut off through the pull-in windings. Only the smaller hold-in windings are in operation for the extended period of time. This period of time is the amount of time that is needed for the engine to start. The solenoid will now take less current from the battery. The heat that is made by the solenoid will be kept at an acceptable level.