508 Section 5 Electrical and Electronic Systems Technology
the driving direction. They slide back and release
when driven in the other direction (freewheeling
direction).
Spiral grooves on the armature shaft and the
inside diameter of the clutch force the shaft and pin-
ion drive assembly to turn together. They also allow
the pinion drive assembly to slide on the armature
shaft. Refer to Figure 30-12.
Starter Solenoid
The starter solenoid is a high-current relay. It
makes an electrical connection between the battery
and the starting motor. The starter solenoid is simi-
lar to other relays but is capable of handling much
higher current levels. See Figure 30-13.
The starter solenoid may be located on the start-
ing motor or away from it. When mounted on a
body panel (away from starter), the solenoid sim-
ply makes and breaks electrical connections. When
mounted on the starter, it also slides the pinion gear
into the fl ywheel ring gear.
With the ignition key in start position, a small
amount of current fl ows through the solenoid
windings. A magnetic fi eld is produced that pulls
the solenoid plunger and disc into the coil wind-
ings. This causes the solenoid disc to touch both of
the high-current terminals, completing the battery-
to-starter circuit. A current of 150–200 amps fl ows
through the solenoid to the starter.
When the ignition key is released, current is dis-
connected from the solenoid windings. The mag-
netic fi eld collapses, and the plunger is free to slide
out of the windings. This opens the disc-to-terminal
connection. The open connection stops current to
the starter, and the starter motor shuts off.
Reduction Gearing
The planetary gearset is used to provide reduc-
tion. A reduction starter is sometimes used to fur-
ther increase the rotating force applied to the engine
fl ywheel. It uses an extra set of gears to improve the
gear reduction, as in Figure 30-14.
The 12-volt starter pinion gear drives an idler gear.
The idler gear drives a larger gear on the overrun-
ning clutch assembly. This allows higher armature
Brake disc
Shift lever
Shift lever in
rest position for
braking action
Armature
shaft
splines
Meshing
spring
Pinion
gear
Overrunning
clutch
Starter Engaged Starter Disengaged
Solenoid
windings
or coil
Ignition
bypass
Starter
terminal
Plunger
contact disc
Battery
terminal
Plunger
Ignition
switch
terminal
for starting
Starter
solenoid
Overrunning
clutch assembly
Flywheel
engaging
gear
Clutch gear
Idler gear
Pinion gear on
starter shaft
Armature
Felt seal
Figure 30-12. The pinion gear assembly is splined to the
armature shaft. This makes the pinion gear assembly turn with
the shaft. It also lets pinion gear slide on the shaft for engagement
with the fl ywheel gear. Note how this unit also has a disc brake for
stopping the pinion after disengagement. (Robert Bosch)
Figure 30-13. Study the construction of a 12-volt starter
solenoid. One small terminal connects to the ignition start
switch. Larger terminals connect to the battery and starting
motor. The plunger movement pulls the disc into contact with
two battery contactor terminals to activate the starter. (Ford)
Figure 30-14. A reduction starter uses extra gears to increase
motor torque. A gear on the armature shaft turns the idler gear.
The idler turns a larger gear on the clutch for gear reduction.
The exposed gear turns the fl ywheel gear. (Honda)