The gear reduction obtained by having a small starter pinion gear drive the large flywheel gear is
usually about 12 to 1 or more. This means the rotational speed of the starter armature is about 12
times that of the flywheel when the engine is being cranked. The pinion gear on the armature shaft
meshes directly with the gear teeth on the flywheel. In some instances, however, a double
reduction is needed. Here the final gear ratio may be as high as 25 to 1 or even 40 to 1. With
double reduction, the gear on the armature shaft does not mesh directly with the teeth on the fly-
wheel, instead they mesh with an intermediate gear that drives the flywheel driving pinion. This
double reduction drive permits the use of a small starter motor to turn a fairly large engine.
If the overrunning clutch type drive is used, we must have a shift fork and linkage to shift the
pinion into mesh with the flywheel gear. As we have already said, this linkage may be operated
mechanically or electrically. If it is electrically operated, a unit called a solenoid is used.
A solenoid is an electromagnet with a movable core or plunger. It is mounted on top of the starter
motor. When the starter switch on the vehicle instrument panel is depressed (in some cases a key-
operated switch is used), the windings in the solenoid create an electric magnet.
When the shift plunger is in its released position, being held there by the contact plunger spring, no
current is flowing because the switch for the solenoid winding is open. The starter pinion is not
engaged with the flywheel. When the switch to the solenoid windings is closed, the solenoid coil is
an electromagnet. The electromagnet pulls the solenoid plunger to the left. This action shifts the
pinion into mesh with the flywheel and then closes the starter switch. Now current flows through
the starter motor causing the armature to rotate.
When the switch for the solenoid winding is opened, the spring pushes the plunger back. This
breaks the circuit to the starter and pulls the pinion back away from the flywheel.
Military tactical vehicles that are expected to ford water deep enough to cover the starter have
waterproof starters. Such starters are completely sealed so that no water can enter. Bearings are
lubricated on original assembly and need no attention between overhauls.