chap40 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 40, the reader should
be able to:

•
•
•
•

Describe how the cranking circuit works.
Explain how to disassemble and reassemble

a starter motor and solenoid.
Discuss how to test the cranking circuit.
Describe how to perform cranking system
testing procedures.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
ampere-turns • armature
bench testing • brush-end housing • brushes
commutator-end housing • commutator segments •
compound motor (compound-wound) • compression
spring • counter- electromotive force (CEMF) • cranking

circuit
drive coil • drive-end (DE) housing • electromagnetic
switch
field coils • field poles
Continued
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KEY TERMS:
gear-reduction starters • ground brushes • growler tester
holding coil • hold-in winding • hot brushes
lap winding
main field housing • mesh spring • mica • movable pole
shoe

neutral safety switch • overrunning clutch
permanent-magnet field • plunger lever • pole shoes •
positive-engagement starter • pull-in winding
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
remote vehicle start (RVS)
series motor • shift fork lever • shunt motor • solenoidoperated starter • starter drive
through bolts
undercut
voltage-drop testing
wave winding

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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For any engine to start, it must first be rotated.
It is the purpose and function of the cranking circuit to
create the necessary power by converting electrical
energy from the battery into mechanical energy at the
starter motor and rotate the engine.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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CRANKING CIRCUIT
The cranking circuit includes mechanical and electrical parts 
required to crank the engine for starting. Early 1900s cranking 
force was the driver’s arm. Modern cranking circuits include:

1. The Starter motor. The starter is normally a 0.5­ to 2.6­
horsepower (0.4 to 2.0 kilowatts) electric motor that develops 
nearly 8 horsepower
(6 kilowatts) for a very
short time when first
cranking a cold engine.
Figure 40–1
A typical solenoid-operated starter.

Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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2.

The Battery. The battery must be of the correct capacity 
and be at least 75% charged to provide the necessary current 
and voltage for correct operation of the starter.

3.

The Starter solenoid or relay. The high current required by 
the starter must be able to be turned on and off. A large 
switch would be required if the current were controlled by 

the driver directly. Instead, a small current switch (ignition 
switch) operates a solenoid or relay that controls the high 
starter current.

4.

The Starter drive. The starter drive uses a small gear that 
contacts the engine flywheel gear and transmits starter motor 
power to rotate the engine.
Continued

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5. The Ignition switch  The ignition switch and safety control 
switches control the starter motor operation. 
Figure 40–3 A typical wiring diagram of a starter circuit.

Figure 40–2 Some columnmounted ignition switches
act directly on the contact
points, whereas others use
a link from lock cylinder to
ignition switch.

The engine is cranked by an 
electric motor controlled by a
key­operated ignition switch. 

Continued
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The ignition switch will not operate the starter unless the 
transmission is in neutral or park. Many manufacturers a neutral 
safety switch that opens the circuit between ignition switch and 
starter to prevent operation unless the gear selector is in neutral or 
park.
Neutral safety switches can be adjusted by loosening the hold­down 
screws and moving the switch slightly to be certain the engine will 
crank only with the transmission in the neutral and park positions.
Many manufacturers use a mechanical blocking device in the 
steering column to prevent the driver from turning the key switch to 
 start unless the gear selector is in neutral or park. Many manual 
transmission vehicles also use a safety switch to permit cranking 
only if the clutch is depressed.
Continued
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Watch the Dome Light
Whenever diagnosing any starter-related problem, open the door of the
vehicle and observe the brightness of the dome or interior light(s) while
attempting to crank the engine. Why?

•
•
•
•

The brightness of any electrical lamp is proportional to the voltage.
Normal operation of the starter results in a slight dimming of the dome
light.
If the light remains bright, the problem is usually an open circuit in the
control circuit.
If the light goes out or almost goes out, the problem is usually a
discharged or defective battery or a shorted or grounded armature of
field coils inside the starter.

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COMPUTER-CONTROLLED STARTING
Some key­operated and most push­button­to­start ignition systems 
 use the computer to crank the engine. The ignition switch start 
position on the push­to­start button is used as an input signal to 
the power train control module (PCM).
The ignition key can be turned to the start position, released, and 
the PCM cranks the engine until it senses that the engine has 
started. The PCM can detect that the engine has started by looking 
at the engine speed signal.
Normal cranking speed can vary between 100 and 250 rpm. If it 
exceeds 400 rpm, the PCM determines the engine started and 
opens the circuit to the “S” (start) terminal of the starter solenoid.
Continued
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Check That Extended Cranking May Be
Normal Operation
Some customers have complained that the engine cranks after they
release the ignition key and assume that there is a fault with the ignition
switch or starter circuit. If the vehicle is equipped with computer-controlled
starting, it is normal for the engine to crank until it starts and it may crank
longer than the customer thinks it should especially in cold weather.

Computer­controlled starting is almost always part of the 
system if a push­button start is used.
Before the PCM cranks the engine, the following conditions 
must be met:
The brake pedal is depressed.

The gear selector is in Park or Neutral.
The correct key fob (code) is present in the vehicle.
Continued
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Remote vehicle start (RVS) is a system that allows the driver to 
start the engine of the vehicle from inside the house or building 
from a distance of about 200 feet (65 meters).
The doors remain locked so the 
possibility of theft is reduced. 
This feature allows the heater or 
air­conditioning system to start 
before the driver arrives. 

Figure 40–4 The top button on this key fob is
the remote start button.

NOTE: Most remote start systems will turn off the engine after 10 
minutes of run time unless reset by the use of the remote.
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HOW THE STARTER MOTOR WORKS
A starter consists of a main field housing, one end of which is 
called a commutator­end (or brush­end) housing and the other 
end a drive­end housing.
The drive­end housing contains the drive pinion gear, which 
meshes with the engine flywheel gear teeth to start the engine.

The commutator­end plate supports the end containing the starter 
brushes. Through bolts hold the three components together.
See Figure 40–5.

Continued
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Figure 40–5 A typical starter motor.

Continued
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A starter uses electromagnetic principles to convert electrical 
energy (up to 500 amps) to mechanical power [up to 8 hp (6 kw)] to 
crank the engine.
The steel housing of the starter motor contains four electromagnets 
that are connected directly to the positive post of the battery to 
provide a strong magnetic field inside the starter. Current for the 
starter is controlled by a solenoid or relay controlled by the driver­
operated ignition switch.
The electromagnets use heavy copper or aluminum wire wrapped 
around a soft­iron core. The core is contoured to fit against the 
rounded internal surface of the starter frame. The soft­iron cores are 
called pole shoes. 
Continued
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Two of the four pole shoes are wrapped with copper wire in one 
direction to create a north pole magnet, the others wrapped to create 
a south pole.
When energized, these magnets create strong magnetic fields inside 
the starter housing. They are called field coils. The soft­iron cores 
(pole shoes) are called field poles.
Inside the field coils is an armature supported with bushings at 
both ends, which permit it to rotate. It is constructed of thin, circular 
disks of steel laminated together and wound lengthwise with heavy­
gauge insulated copper wire.
The laminated iron core supports the copper loops of wire and helps 
concentrate the magnetic field produced by the coils.

Continued
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The ends of the copper armature windings are soldered to the 
commutator segments. Current passing through the field coils is 
connected to the commutator of the armature by brushes that can 
move over the segments of the rotating armature. They are made of 
copper and carbon. Copper is a good conductor, and carbon added to 
starter brushes helps provide graphite­type lubrication needed to 
reduce wear of brushes and commutator segments.
The starter uses four brushes—two to transfer current from field 
coils to armature, and two for the ground return path for current 
flow through the armature. See Figure 40–6.

Two hot brushes are in holders, insulated from the housing. Two 
ground brushes primarily use bare, stranded copper wire 
connections to the brushes. The ground brush holders are not 
insulated and attach directly to the field housing.

Continued

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Figure 40–6 This series-wound electric motor shows the basic operation with only two
brushes: one hot brush and one ground brush. The current flows through both field coils, then
through the hot brush and through the loop winding of the armature before reaching ground
through the ground brush.

Current travels through
brushes into armature
windings, where other
magnetic fields are
created around each
copper wire loop in
the armature. 

The two magnetic
fields created inside
the starter housing
create force that
rotates the armature.
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HOW MAGNETIC FIELDS TURN AN ARMATURE
A magnetic field surrounds every conductor carrying a current. 
Field strength is increased as current flow (in amps) is increased.
Inside the starter housing is a strong magnetic field created by the 
field coil magnets. The armature, a conductor, is inside this strong 
field, with little clearance between armature and field coils.
The two magnetic fields act together, and their lines of force 
“bunch up” or are strong on one side of the armature loop wire and 
become weak on the other side of the conductor.
This causes the conductor (armature) to move from the area of 
strong magnetic field strength toward the area of weak magnetic 
field strength. This causes the armature to rotate. 
Continued
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Figure 40–7 The interaction of the magnetic fields of the armature loops and field coils creates
a stronger magnetic field on the right side of the conductor, causing the armature loop to move
toward the left.

Continued
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This rotation force (torque) is increased as the current flowing 

through the starter motor increases. The torque of a starter is 
determined by the strength of the magnetic fields inside the starter. 
Magnetic field strength is measured in ampere­turns. 
Figure 40–8 The armature loops rotate due to the difference in the strength of the magnetic field.
The loops move from a strong magnetic field strength toward a weaker magnetic field strength.

Continued
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If the current or number of turns of wire are increased, magnetic field 
strength is increased. 

The magnetic field 

of the starter motor 
is provided by two 
or more pole shoes 
and field windings. 
Figure 40–9 Pole shoes and
field windings installed in the
housing.

The pole shoes are 
made of iron and are 
attached to the frame 
with large screws. 
Continued
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This shows paths of
magnetic flux lines
within a four­pole
motor.
Figure 40–10 Magnetic
lines of force in a four-pole
motor.

The field windings
are usually made of
heavy copper ribbon
to increase current­
carrying capacity
and electromagnetic
field strength. 
Continued
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