3208 – Starting System Components

Solenoid


Typical Solenoid Schematic

A solenoid is a magnetic switch that does two basic operations:

a. Closes the high current starting motor circuit with a low current start switch circuit.
b. Engages the starting motor pinion with the ring gear.
The solenoid switch is made of an electromagnetic (one or two sets of windings) around a hollow cylinder. There is a plunger (core) with a spring load inside the cylinder that can move forward and backward. When the start switch is closed and electricity is sent through the windings, a magnetic field is made that pulls the plunger forward in the cylinder. This moves the shift lever (connected to the rear of the plunger) to engage the pinion drive gear with the ring gear. The front end of the plunger then makes contact across the battery and motor terminals of the solenoid, and 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 now pushes the plunger back to the original position, and, at the same time, moves the pinion gear away from the flywheel.

When two sets of windings in the solenoid are used, they are called the hold-in winding and the pull-in winding. Both have the same number of turns around the cylinder, but the pull-in winding uses a larger diameter wire 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, and the rest flows through the pull-in windings to motor terminal, then through the motor to ground. When the solenoid is fully activated (connection across battery and motor terminal is complete), current is shutoff through the pull-in windings. Now only the smaller hold-in windings are in operation for the extended period of time it takes to start the engine. The solenoid will now take less current from the battery and heat made by the solenoid will be kept at an acceptable level.

Starting Motor

The starting motor is used to turn the engine flywheel fast enough to get the engine running.


Starting Motor
(1) Field. (2) Solenoid. (3) Clutch. (4) Pinion. (5) Commutator. (6) Brush assembly. (7) Armature.

The starting motor has a solenoid. When the start switch is turned to the START position, the solenoid will be activated electrically. The solenoid core will nowmove to push the starting motor pinion, by a mechanical linkage, to engage with the ring gear on the flywheel of the engine. The starting motor pinion will engage with the ring gear before the electric contacts in the solenoid close the circuit between the battery and the starting motor. When the circuit between the battery and the starting motor is complete, the pinion will turn the engine flywheel. A clutch gives protection for the starting motor so that the engine, when it starts to run, can not turn the starting motor too fast. When the start switch is released, the starting motor pinion will move away from the flywheel ring gear.

Magnetic Switch
A magnetic switch (relay) is used sometimes for the starting motor solenoid circuit. Its operation electrically is the same as the solenoid. Its function is to reduce the current load on the start switch and control current to the starting motor solenoid.

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