504 Section 5 Electrical and Electronic Systems Technology
• Starting motor—The starting motor is a high-
torque electric motor for turning the gears at
the engine fl ywheel.
• Engine control module—The engine control
module (ECM) reacts to the voltage of the
starter button or ignition key to energize the
starter relay. See Figure 30-1.
Starting System Action
When you turn the ignition key or press the start
button, current fl ows through the ECM and starter
relay. This closes the relay and solenoid contacts,
connecting the positive battery cable directly to the
starting motor windings. The starter pinion gear is
pushed into and meshes with the fl ywheel ring gear.
The motor-driven pinion gear turns the fl ywheel
and crankshaft until the engine starts, or fi res, and
runs on its own power.
On many cars, when the engine starts, you have
to release the ignition key to disengage the 12-volt
starting motor. The ignition switch then cuts power
to the solenoid and starting motor. The starting
motor stops spinning, and the pinion gear is pulled
away from the fl ywheel gear.
Starting Motor Fundamentals
The starting motor converts electrical energy
from the battery into mechanical energy to crank the
engine. It is similar to other electric motors. All electric
motors (wiper motors, fan motors, fuel pump motors,
etc.) produce a turning force through the interaction
of magnetic fi elds inside the motor assembly.
Simple Electric Motor
As explained in earlier text chapters, a magnetic
fi eld is made up of invisible magnetic lines of force.
The lines of force fl ow between the poles of a perma-
nent magnet. They also fl ow around the outside of a
wire that is carrying current.
Since like charges (fi elds) repel each other and
unlike charges attract each other, magnetic fi elds
can be used to produce motion. Look at Figure 30-2.
Note how the lines of force in the magnet and the
lines of force around the conductor act upon each
other. This principle is used in electric motors.
To build a simple electric motor, you would start
by bending a piece of wire into a loop. When current
is passed through the wire loop (winding), a mag-
netic fi eld forms around the wire.
Pole shoes, or pole pieces, are needed to make
the loop of wire move. A magnetic fi eld is set up
between the pole shoes.
Changing Electricity into Motion
Electric current can be changed into a strong
rotating motion by placing the winding between the
pole shoes. When current passes through the wind-
ing, the magnetic fi eld around the winding and the
fi eld between the pole shoes act on each other. This
Battery
+ –
Electronic
control unit
Starter
solenoid
Starting motor
Starter
relay
Ignition
switch
Battery cable
Figure 30-1. Note the basic starting system operation. The
start switch energizes the engine ECM. If conditions are correct
for sensors and switches, the ECM will energize the starter
relay and solenoid. The solenoid then energizes the starting
motor. The motor turns the fl ywheel gear for engine starting.
Figure 30-2. Magnetic fi elds from magnets and the fi elds
around a current-carrying conductor can be used to produce
motion. Note how the fi elds interact.