Starting System Components
A solenoid is an electromagnetic switch that does two basic
Closes the high current starter motor circuit with a low
current start switch circuit.
Engages the starter motor pinion with the ring gear.
The solenoid has windings (one or two sets) 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 starter
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 shut off through the pull-in windings. Now
only the smaller hold-in windings are in operation for the
extended period of time it takes to star 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.
Starter Motor (Typical Example)
(1) Brush assembly. (2) Field. (3) Solenoid. (4) Clutch. (5)
Pinion. (6) Armature.
The starter motor is used to turn the engine flywheel fast
enough to get the engine to start running.
The starter motor has a solenoid. When the start switch is
activated, the solenoid will move the starter pinion to engage it
with the ring gear on the flywheel of the engine.
Two engagement methods are used. Positive shift starter
pinions will engage with the ring gear before the electric
contacts in the solenoid close the circuit between the battery
and the starter motor. In positive engagement starters, the
electrical contacts in the solenoid close the circuit between the
battery and starter motor just before the pinion engages the
ring gear which causes the starter motor to rotate.