Lesson 2: Charging

Lesson 2: Charging System

MACHINE ELECTRICAL
SYSTEMS
• Battery
• Charging System
• Starting System

Introduction

The charging system converts mechanical energy from the engine

into electrical energy to charge the battery and supply current to
operate the electrical systems of the machine. This lesson explains
the charging system and describes the charging system components.
Charging system testing is also covered.

Objectives

At the completion of this lesson, the student will be able to:

Explain the operation of the charging system by selecting the correct

response to questions on a multiple choice quiz.

Given a training aid or a machine and the appropriate tools, test the
charging circuit on the training aid or machine and correctly answer
the lab questions regarding charging circuit testing.

Given an alternator and a digital multimeter, test the electrical

components of the alternator on the bench and correctly answer the
lab questions regarding alternator testing.
Unit 4 4-2-2 Electrical Fundamentals
Lesson 2

Tooling

9U7330 Digital Multimeter

8T0900 AC/DC Clamp-on Ammeter
Variable DC Power Supply 0 - 30 VDC

References

Video "Testing the Alternator on the Engine SEVN1591

Video " 6V2150 Starting and Charging Analyzer SEVN9165

Service Magazine Article "Alternator/Generator Output Test on the

Engine" dated May 4, 1987
Unit 4 4-2-3 Electrical Fundamentals
Lesson 2

D.C. CHARGING CIRCUIT A.C. CHARGING CIRCUIT

REGULATOR IGNITION
SWITCH

REGULATOR

AMMETER AMMETER
GENERATOR

ALTERNATOR

GROUND
GROUND

BATTERY
BATTERY

Fig. 4.2.1 Charging Circuit

AC and DC Charging Circuits

The charging system recharges the battery and generates current

during operation. There are two kinds of charging circuits:

- DC charging circuits that use generators

- AC charging circuits that use alternators

Both circuits generate an alternating current (AC). The difference is

in the way they rectify the AC current to direct current (DC). DC
charging circuits have a generator and a regulator.

The generator supplies the electrical power and rectifies its current
mechanically by using commutators and brushes.

The regulator has three functions: It opens and closes the charging
circuit, prevents battery overcharging and limits the generators
output to safe rates.

AC charging circuits include an alternator and a regulator. The

alternator is really an AC generator. It produces AC current, like the
generator, but rectifies the current using diodes. Alternators are
generally more compact than generators of equal output, and supply a
higher current at low engine speeds.

The regulator in AC charging circuits limits the alternator voltage to a

safe preset level. Transistorized models are used in many of the
modern charging circuits.
Unit 4 4-2-4 Electrical Fundamentals
Lesson 2

ALT
+ R BATTERY LOAD

ALTERNATOR

ALT
+ R BATTERY LOAD

ALTERNATOR

ALT
+ R BATTERY LOAD

ALTERNATOR

Fig. 4.2.2 Charging Circuit in Operation

Charging Circuit Operation

Charging circuits operate in three stages:

- During starting the battery supplies all the load current

- During peak operation the battery helps the generator (or

alternator) supply current

- During normal operation the generator (or alternator) supplies all

current and recharges the battery

In both charging circuits, the battery starts the circuit when it supplies
current to the starting motor to start the engine (Figure 4.2.2, top
diagram). The engine than drives the generator (or alternator) which
produces current to take over the operation of the ignition, lights and
accessory loads in the whole system.

The center diagram in Figure 4.2.2 shows that the battery also
supplies current during peak operation when the electrical loads are
to high for generator (or alternator).

Once the engine is started, the generator (or alternator) provides the
current to the machine electrical systems (Figure 4.2.2, bottom
diagram). The generator supplies current as long as the engine is
running above the idle speed. When the engine is at idle or stops, the
battery takes over part or all of the load. However, an alternator will
continue to supply current during engine idling.
Unit 4 4-2-5 Electrical Fundamentals
Lesson 2

FIELD
CIRCUIT

FIELD
CIRCUIT

Fig. 4.2.3 Basic Generator

Generators

Generators in DC charging circuits will be covered briefly. The

generator is still found on some older machines. To service this
equipment, you should have a working knowledge of how the
charging system functions. The majority of this lesson will focus on
AC charging circuits which have replaced DC charging circuits in
late model machines.

The generator produces electrical energy using electromagnetic

induction. Electromagnetic induction is used to generate electricity
in the charging system. Electromagnetic induction occurs when there
is relative movement between a conductor and a magnetic field. As
the conductor cuts through the field a voltage is induced in the
conductor. This voltage causes current flow when the conductor is
connected to a circuit. The amount of output depends on the strength
of the magnetic field, the speed at which the magnetic field is cut and
the number of conductors cutting the field.

The basic generator has two components:

- Armature--rotating wire loop (conductor)

- Magnetic poles-- stationary magnetic field

As the armature rotates through the magnetic field of the poles,

voltage is generated. The ends of the armature loop are connected to
a split ring called a commutator. Brushes contact the commutator
and wires connect the brushes to a load. Current will flow since the
circuit is complete. To ensure a strong current and proper flow, wires
are wound around the magnetic poles and the wires are attached to
the brushes. The wiring is called the field circuit of the generator.
Unit 4 4-2-6 Electrical Fundamentals
Lesson 2

B A
S N S N
A B

FIRST HALF OF REVOLUTION SECOND HALF OF REVOLUTION

Fig. 4.2.4 Polarity Changing

At this point the basic generator produces an alternating current

because the armature reverses the polarity of the current and changes
the direction of current flow on each side of the loop as it rotates.

During the first half of the revolution, the top of the armature side A
cuts through the magnetic field first, while the bottom of side B is
first to cut through the field. Current flows toward side A and away
from side B. The conventional theory (+ to -) gives us the polarities
shown "+" for A and "-" for B.

During the second half of the revolution, the top of side B is the
leading edge, while the bottom of side A is leading. Now B is "-"
while A is "+." The armature loop ends reverse polarity during each
revolution and the result is alternating current.

AT STATIC "NEUTRAL POINT"

NO VOLTAGE IS GENERATED

GAPS BETWEEN
COMMUTATOR HALVES

Fig. 4.2.5 Generator Converts AC to DC

The commutator and brushes allow the AC current to flow to the load
in the same direction. Twice during each rotation, the armature is
vertical to the magnetic field as shown. The armature loop is not
passing through the field and no voltage is generated at this point.
This is the static neutral point.
Unit 4 4-2-7 Electrical Fundamentals
Lesson 2

The commutator is split into two parts with the open areas matching
the neutral point of the armature as shown. This means there is an air
gap as the commutator passes the brushes. Past this point the other
half of the commutator contacts the brushes. Since the coil is in the
same relative position as during the preceding one-half revolution,
current flow to the brush stays in the same direction. This results is
direct current.

Fig. 4.2.6 Voltage Regulator

Direct current systems will automatically provide more field current

as generator output increases. This increase in field current will
result in an increase in generator output. If left unregulated, this
continuous increase will result in current and voltage levels that will
destroy the generator, other electrical circuits and the battery.

The generator cannot control the amount of voltage it produces.

Therefore, an external unit called a voltage regulator is used in the
field circuit. It has a shunt coil and contact points to control the
strength of the magnetic field, thus limiting the voltage generated.

Alternator

An alternator operates on the same principle as a generator. It

converts mechanical energy into electrical energy. The alternator
could be called an AC generator. The difference between the
generator and alternator is in the way the alternator rectifies AC
current to DC current. The alternator rectifies current electronically
using diodes.

Alternators are generally more compact than generators and can

supply a higher current at low engine speeds. Since late model
machines include many electrical accessories, the alternator can best
supply the current output for the increased electrical loads.
Unit 4 4-2-8 Electrical Fundamentals
Lesson 2

B
LOAD ROTATING MAGNETIC
CIRCUIT FIELD
CHANGED
POLARITY

A
A

B
B

Fig. 4.2.7 Basic Alternator Operation

In the alternator, the magnetic field rotates inside the wire loop. This
rotating magnetic field is generated by a rotor. The wire loop, which
is stationary is the conductor.

Magnetic lines of force move across the conductors and induce

current flow in them. Since the conductors are stationary, they can be
directly connected instead of using brushes. This reduces heat and
wear.

Voltage will be induced in a conductor when a magnetic field is

moved across the conductor. For example, consider a bar magnet
with its magnetic field rotating inside a loop of wire. With the
magnet rotating as indicated, and with the S pole of the magnet
directly under the top portion of the loop and the N pole directly over
the bottom portion, the induced voltage will cause current to flow in
the circuit in the direction shown. Since current flows from positive
to negative through the external or load circuit, the end of the loop of
wire marked "A" will be positive polarity and the end marked "B"
will be negative.

After the bar magnet has moved through one-half revolution, the N
pole will have moved directly under the top conductor and the S pole
directly over the bottom conductor. The induced voltage will now
cause current to flow in the opposite direction. The end of the loop
wire marked "A" will become negative polarity, and the end marked
"B" will become positive. The polarity of the ends of the wire has
changed. After a second one-half revolution, the bar magnet will be
back at the starting point where "A" is positive and "B" is negative.

Consequently, current will flow through the load or external circuit

first in one direction and then in the other. This is an alternating
current, which is developed internally by an alternator.
Unit 4 4-2-9 Electrical Fundamentals
Lesson 2

STRONG WEAK
FIELD FIELD

ROTOR ROTOR

CONDUCTOR
AIR PATH- AIR PATH-
HIGH RELUCTANCE LOW RELUCTANCE

Fig. 4.2.8 Magnetic Lines of Force

How Voltage is Induced

Very little voltage and current are produced with a bar magnet
rotating inside a single loop of wire. When the loop of wire and the
magnet are placed inside an iron frame a conducting path for the
magnetic lines of force is created. Since iron conducts magnetism
very easily, adding the iron frame greatly increases the number of
lines of force between the N pole and the S pole.

A large number of magnetic lines of force are at the center of the tip
of the magnet. Therefore, a strong magnetic field exists at the center
of the magnet and a weak magnetic field exists at the leading and
trailing edges. This condition results when the air gap between the
magnet and field frame is greater at the leading and trailing edges
than at the center of the magnet.

The amount of voltage induced in a conductor is proportional to the

number of lines of force which cut across the conductor in a given
length of time. The voltage will also increase if the bar magnet turns
faster because the lines of force cut across the wire in a shorter time
period.

The rotating magnet in an alternator is called the rotor and the loop of
wire and frame assembly is called the stator.
Unit 4 4-2-10 Electrical Fundamentals
Lesson 2

A1A B1B C1C

C1

C1 A B

LOOP VOLTAGE
A

S
B 0° 120° 240° 360°

N
B1
B1 A1
A1 90° 120° 120° 30°
C C

ONE CYCLE

Fig. 4.2.9 Loop Voltage

In Figure 4.2.9 the single loop of wire acting as a stator winding and
the bar magnet acting as a rotor illustrate how an AC voltage is
produced in a basic alternator. When two more separate loops of
wire, spaced 120 degrees apart, are added to our basic alternator, two
more separate voltages will be produced.

With the S pole of the rotor directly under the A conductor, the
voltage at A will be maximum in magnitude and positive in polarity.

After the rotor has turned through 120 degrees, the S pole will be
directly under the B conductor and the voltage at B will be maximum
positive. Also 120 later, the voltage at C will be maximum positive.
The peak positive voltages at A, B C in each loop of wire occur 120
degrees apart. These loop voltages are also shown in Figure 4.2.9.

AC1
A1B

AC1
A1B

B1C
B1C
BA CB AC
PHASE VOLTAGE

ONE CYCLE

Fig. 4.2.10 Phase Voltage--Delta Stator

When the ends of the loops of wire marked A1, B1 and C1 are
connected to the ends marked B, C, and A respectively, a basic three
phase "delta" wound stator is formed (Figure 4.2.10). The three AC
voltages (BA, CB and AC) available from the delta wound stator are
identical to the three voltages previously discussed.
Unit 4 4-2-11 Electrical Fundamentals
Lesson 2

A B BA CB AC

B1

PHASE VOLTAGE
A1 C1
A
B
A1B1C1
C

ONE CYCLE

Fig. 4.2.11 "Y" Stator--Phase Voltage

When the ends of the loops of wire marked A1, B1 and C1 are
connected together, a basic three-phase "Y" wound stator is formed
(Figure 4.2.11). Each of these voltages consist of the voltages in two
loops of wire added together. Three AC voltages spaced 120 degrees
apart are available from the Y stator.

In delta windings each of the individual windings is connected to the

end of another winding (Figure 4.2.10). This creates parallel
connections in the delta stator which generally allows for higher
current output than the "Y" wound stator. In the "Y" wound stator
the windings are connected to form pairs of series connections
(Figure 4.2.11). The series connections generally provide higher
voltages but lower current output than the delta would stators.

To increase the output of the alternator some modifications to the

basic model are needed:

- increase the number of conductors in each of the phase windings

- increase the strength of the magnetic fields

- increase the speed of rotation

- magnetic field generation

Unit 4 4-2-12 Electrical Fundamentals
Lesson 2

RECTIFIER
A B R

B1
GRD BAT
A1 C1

C
BATTERY

Fig. 4.2.12 Three-Phase Rectification

Current Rectification

Even though the alternator seems complete, the current being

generated from it is still alternating. The electrical system requires
direct current. In order for the output of the alternator to be of any
value it must be converted from AC to DC.

The ideal device for this task is the diode. The operating principles
of diodes were covered in Unit 3. The diode is compact, will conduct
current in one direction only and can be easily installed in the
alternator housing.

Diodes are normally used in the alternator in two groups of three.

Since there are three phases or windings in the alternator, three
positive and three negative diodes are required. In systems that
require higher output, more diodes may be required.

A battery connected to the DC output terminal will have its energy

restored as the alternator provides charging current. The blocking
action of the diodes prevents the battery from discharging directly
through the rectifier.
Unit 4 4-2-13 Electrical Fundamentals
Lesson 2

A1 A B1 B C1 C

LOOP VOLTAGE
8

BA CB AC
16
1 3 5

PHRASE VOLTAGE
8

6 2 4 6

Fig. 4.2.13 "Y" Stator--Phase Voltage

For explanation purposes, the three AC voltage curves provided by

the "Y" type stator have been divided into six periods in Figure
4.2.13. Each period represents one-sixth of a rotor revolution, or 60
degrees.

A A

B B

BA BA BA BA
CURRENT

TIME

Fig. 4..2.14 "Y" stator period 1

During period 1, the maximum voltage being induced appears across

stator terminals BA. This means the current flows from B to A in the
stator winding during this period, and through the diodes as
illustrated in Figure 4.2.14.

Let's assume that the peak phase voltage developed from B to A is 16

volts. This means that the potential at B is 0 volts and the potential at
A is 16 volts. Similarly, from the voltage curves the phase voltage
from C to B at this instant is negative 8 volts. This means that the
potential at C is 8 volts, since C to B, or 8 to zero, represents a
negative 8 volts. At this same time instant the phase voltage from A
to C is also negative 8 volts since A to C, or 16 to 8, represents a
negative 8 volts. The voltage potentials are shown on the rectifier.
Unit 4 4-2-14 Electrical Fundamentals
Lesson 2

Only two of the diodes will conduct current, since these are the only
diodes in which current can flow in the forward direction. The other
diodes will not conduct current because they are reverse biased. The
voltages that exist at the rectifier and the biasing of the diodes
determine the current flow directions. These voltages are represented
by the phase voltage curves, which are the voltages that actually
appear at the rectifier diodes. Following the same procedure for
periods 2-6, the current flows can be determined.

D. C. CURRENT
BC BA CA CB AB AC BC

TIME

Fig. 4.2.15 DC Current Output

The voltage obtained from the stator-rectifier combination when

connected to a battery is not perfectly flat but is so smooth that the
output may be considered to be a non-varying DC voltage. The
voltage is obtained from the phase voltage curves and is illustrated in
Figure 4.2.15.

R
RECTIFIER
A B
GRD BAT

STATOR C

BATTERY

BA CB AC
16
1 3 5
PHRASE VOLTAGE

6 2 4 6

Fig. 4..2.16 Delta Stator and Phase Voltage

A delta type stator wound to provide the same output as a "Y" stator
will also provide a smooth voltage and current output when
connected to a six-diode rectifier. For explanation purposes, the three
phase voltage curves obtained from the basic delta connection for one
rotor revolution are reproduced here and are divided into six periods.
Unit 4 4-2-15 Electrical Fundamentals
Lesson 2

16
16 O
A B O 15

8 C

BATTERY

Fig. 4.2.17 "Y" Phase

During period 1 (Figure 4.2.17), the maximum voltage being

developed in the stator is in phase BA. The current flow through the
rectifier is exactly the same as for the "Y" stator since the voltage
potentials on the diodes are identical. The difference between the
Delta stator and the "Y" stator is that the "Y" stator conducts current
through only two windings throughout one period, whereas the delta
stator conducts current through all three.

Phase BA is in parallel with phase BC and CA. Since the voltage

from B to A is 16, the voltage from B to C to A also must be 16
because 8 volts is developed in each of these two phases (B to C and
C to A). Following the same procedure for periods 2-6, the current
flows can be determined.

Fig. 4.2.18 Alternator Components

Alternator Construction

As previously discussed, the magnetic field in the AC alternator is

created by the rotor assembly that rotates inside the stator. This rotor
consists of a rotor shaft, two rotor halves with fingers that will create
the many magnetic fields, a coil assembly and two slip rings.
Unit 4 4-2-16 Electrical Fundamentals
Lesson 2

When current is passed through the coil assembly, a magnetic field is

created in each of the rotor pole pieces. One set of fingers will
become north poles while the other set of fingers will become south
poles.

Since the rotor fingers overlap each other many individual flux loops
will be formed between the alternator north and south poles. Instead
of passing one magnetic field past each winding during one
revolution of the rotor, many fields will pass the windings, which will
increase the output of the stator.

Since the rotor must be supplied with current to create the magnetic
field, the coil assembly inside the pole piece is connected to slip
rings. These slip rings are provided so that brushes can be used to
provide current to the moving field. Slip rings are pressed onto the
shaft and insulated from it. The coil conductors are soldered to the
slip rings to form a complete circuit that is insulated from the shaft.

Because the rotor will be spinning at high speed, it must be supported

by bearings. The front and of the shaft has a bearing mounted in the
drive end housing assembly (Figure 4.2.18). Note the addition of
spacers to place the rotor in the correct position once the alternator is
assembled and to keep the fan from hitting the housing.

Since the generation of electricity creates heat, a fan is included to

provide a flow of air through the assembly for cooling. A pulley is
attached to the end of the rotor shaft and is driven by a belt.

Fig. 4.2.19 Alternator Components

The end housing supports the slip ring end of the rotor shaft and
provides a mounting surface for the brushes, rectifier assembly, stator
and regulator (if equipped). The drive end housing with the rotor and
the slip ring end housing with its components are assembled as a unit
with the stator held in between. This assembly is held together with
through capscrews.
Unit 4 4-2-17 Electrical Fundamentals
Lesson 2

The stator assembly is a laminated soft iron ring with three groups of
coils or windings. One end of each stator winding is connected to a
positive and a negative diode. The other ends of the stator windings
can be connected in either a "Y" type stator configuration or a delta
stator configuration.

The rectifier assembly converts the AC current to DC current. Three

positive diodes and three negative diodes are mounted to the rectifier
assembly.

The alternator is designed to provide minimal clearance between the

rotor and stator to maximize the effects of the magnetic field. It is a
compact assembly capable of generating high current flow to satisfy
the needs of the electrical system.

The brushes are in contact with the copper slip rings to provide the
necessary current for production of the magnetic field in the rotor.
Since good contact is important for good conductivity, the brushes are
held against the slip rings by small coil springs.

There are two brushes, which are usually contained in a brush holder
assembly. This assembly can be easily attached to the slip ring end
housing of the alternator.

Fig. 4.2.20 Electro-mechanical Regulator

Regulating the alternator output

If the alternator were allowed to operate uncontrolled, it would

produce voltages too high to be used in the machine and would result
in damage to components. The regulator controls alternator output.

Current output is limited by the construction of the alternator and is

indicated as a maximum on the housing. For instance, a housing may
have a listing such as 12V 85A. This indicates that the maximum
output is 85 amperes and the alternator is designed for a 12 volt
system.
Unit 4 4-2-18 Electrical Fundamentals
Lesson 2

The regulation circuit controls the voltage output of the alternator by

changing the strength of the magnetic field produced by the rotor. It
does this by controlling the amount of current flow through the
brushes to the rotor coil.

The regulator is sensitive to the voltage of the battery and

consequently, to the electrical load being placed on the system. It can
then adjust the amount of current to the rotor to satisfy the demand.

If the battery voltage is low and the demand from electrical

accessories is high, the voltage regulator will increase the output of
the alternator to charge the battery and provide sufficient current to
operate accessories. When battery voltage is high and the electrical
demands are low, the voltage regulator will reduce output from the
alternator.

Alternator regulators can be of three different designs:

- electro-mechanical (older machines)

- electronic external regulators

- electronic integral regulators

Electro-mechanical regulators can be found on some older systems.

These regulators use relays to operate contact points. The double
contact voltage regulator controls alternator output by regulating the
amount of current flow to the rotor. Reducing current flow will
reduce the strength of the magnetic field and result in lower output
from the stator. This lesson will focus on electronic regulators found
in most machines today.
Unit 4 4-2-19 Electrical Fundamentals
Lesson 2

Fig. 4.2.21 Electronic Voltage Regulator

Electronic voltage regulators

Electronic voltage regulators perform the same function as the

electro-mechanical regulators. In the electronic regulator the field
circuit is switched on and off by electronic circuits, controlling
switching transistors. These electronic devices can be switched much
more quickly and carry more current than the contact points in the
electro-mechanical regulators. Higher output from the alternator can
be obtained because of greater current flow through the field circuit.

Electronic regulators use Zener diodes as part of the voltage sensing

circuit. These special diodes allow current to flow in reverse of
normal flow when a specific voltage across the diode is reached.
When the current flows back through the Zener diode the field
transistor is turned off and current flow is stopped in the field rotor.
The electronic components can switch on and off several thousand
times a second, this provides very smooth and accurate control of
alternator output.

Most electronic regulators are not adjustable. If they do not

accurately control the output of the alternator, they must be replaced.
Unit 4 4-2-20 Electrical Fundamentals
Lesson 2

ALTERNATOR

FIELD
REGULATOR
TERMINAL

STARTER GROUND
STARTING (IGNITION
MOTOR SWITCH)

OUTPUT

ALTERNATOR
INDICATOR R1 R2
LAMP
FIELD
DISCHARGE
R3 DIODE

TR1
R4

TR2
R5 ZENER
TRANSISTORIZED Rt
BATTERY DIODE
REGULATOR

R7 R8 R9

Fig. 4.2.22 Regulator Operation--During Engine Start-up

Electronic Regulator Operation at Engine Start-Up

When the starter switch is turned on, the circuit is completed (Figure
4.2.22). Battery current flows to the starter solenoid and the start key
switch as shown by the red lines. The key start switch directs current
flow to the alternator indicator lamp and the regulator.

As the current flows into the regulator, different voltage values

govern the course of the current. The voltage across resisters R7 and
R8 is below the Zener diode critical or breakdown voltage.
Therefore, the voltage felt at the base of TR2 is the same as the
voltage at its emitter. So the current cannot flow through TR2 (as
shown by the blue lines).

Thus the voltage difference in the emitter-base circuit of TR1 allows

current to flow from its emitter through its base and collector. The
collector current then goes on to excite the alternator field (vertical
red line). At the same time a slight amount of current flow travels to
the alternator ground as shown by the dotted red line.
Unit 4 4-2-21 Electrical Fundamentals
Lesson 2

ALTERNATOR

FIELD
REGULATOR
TERMINAL

STARTER GROUND
STARTING (IGNITION
MOTOR SWITCH)

OUTPUT

ALTERNATOR
INDICATOR R1 R2
LAMP
FIELD
DISCHARGE
R3 DIODE

TR1
R4

TR2
R5 ZENER Rt
BATTERY DIODE
REGULATOR
R7 R8 R9

Fig. 4.2.23 Regulator Operation--Transistor TRI turned on

Regulator Operation During Engine Operation

Regulator operation at the beginning of engine operation (Figure

4.2.23) is similar to the engine start-up period except that as the
engine speeds up the alternator field around the rotor generates
voltage to supply electrical loads.

However, the voltage values are still the same and transistor TR1 still
conducts the current to the alternator field as shown by the vertical
red line.
ALTERNATOR

FIELD
REGULATOR
TERMINAL

STARTER GROUND
STARTING (IGNITION
MOTOR SWITCH)

OUTPUT

ALTERNATOR
INDICATOR R1 R2
LAMP FIELD
DISCHARGE
R3 DIODE

TR1
R4

TR2 ZENER
R5 Rt
BATTERY DIODE
REGULATOR
R7 R8 R9

Fig. 4.2.24 Regulator Operation--Transistor TR2 Turned on

As the engine operates and load requirements begin to decrease, the

alternator voltage builds (Figure 4.2.24). This causes the voltage
across the resistors to also increase. Then the voltage across R7 and
R8 becomes greater than the Zener diode critical voltage. The Zener
diode immediately "breaks down" allowing current to flow in the
reverse direction. This "turns on" transistor TR2 and so current is
able to flow through TR2’s emitter, base and collector. When current
flows through TR2, the voltage at the base of TR1 is equal to or
greater than its emitter. This prevents current from flowing though
TR1 to the alternator field, which collapses the field reducing
alternator output and protecting the circuit.
Unit 4 4-2-22 Electrical Fundamentals
Lesson 2

The system voltage than drops below the critical voltage of the Zener
diode and it stops conducting, which turns off TR2 and turns on TR1.
Current again flows to the alternator field. This operation is repeated
many times a second. In effect, the two transistors act as switches
controlling the voltage and alternator output.

When TR1 turns off, the alternator field current cannot drop
immediately to zero, because the rotor windings cause the current to
continue to flow. Before the current reaches zero, the system voltage
and regulator start current flow again. However, the decreasing field
current flow induces a high voltage which can damage the transistor.

The field discharge diode in Figure 4.2.24 prevents damage to

transistor TR1 by diverting high voltage from the transistor.

Fig. 4.2.25 Internal Regulator

Internal electronic regulators

Internal alternator regulators are mounted either inside or outside the

slip ring end housing of the alternator. This type of regulator
eliminates the wiring harness between the alternator and regulator
simplifying the system. This type of regulator is usually much
smaller than other types and uses electronic circuits known as
integrated circuits or "ICs." ICs are miniaturized electronics with
much of the circuit on one small chip. Integral regulators perform the
same function as the external electronic regulators and they do it in
the same way.

Some alternators with integral regulators have only one wire going to
them. This wire is the alternator output wire, the ground circuit is
completed through the housing to the engine block. Current for the
integral regulator is fed from the stator through a diode trio. The
alternator starts charging by using the small amount of permanent
magnetism in the rotor, this small amount of output is fed back into
the field which increases the output. This continues until full output,
determined by the regulator is reached.
Unit 4 4-2-23 Electrical Fundamentals
Lesson 2

RESISTOR

SWITCH
INDICATOR
LAMP
BATTERY
BAT.

DIODE
TRIO
R1
R2 ROTOR
TR2 (FIELD)

R3 C1 TR1
R4

RECTIFIED
STATOR BRIDGE

Fig. 4.2.26 "A" circuit field

Regulator circuits

There are two basic field circuit connections for an alternator--"A"

circuit and "B" circuit.

An "A" type circuit alternator (Figure 4.2.26) uses two insulated

brushes in the alternator. One brush is connected directly to the
battery, while the other brush is connected to ground with the
regulator and ignition switch or relay in series. The regulator is
located after the field, between the field and the alternator ground or
negative diodes. Full alternator output is obtained by grounding the
field windings. Some alternator have a tab in a test hole so that the
field is grounded by placing a screwdriver against the tab end and the
alternator frame. This type of circuit is used with integral regulators
and some external electronic regulators.

DIODES STATOR

ALTERNATOR FIELD

ISOLATION
DIODE
REGULATOR FIELD
KEYSWITCH TERMINAL TERMINAL
OUTPUT
TERMINAL

ALTERNATOR
INDICATOR LAMP

TR2

TRANSISTORIZED
TERMINAL

Fig. 4.2.27 "B" circuit field

"B" type circuits use a brush that is grounded inside the alternator
(Figure 4.2.27). The other brush is connected to the battery in series
with the regulator and the ignition switch or relay. In a "B" circuit
alternator the regulator is located before the field. The current flow is
usually from the regulator terminal of the alternator to the regulator.
After the regulator the current flows to the field coil in the rotor,
Unit 4 4-2-24 Electrical Fundamentals
Lesson 2

then to ground, and finally to the negative or return diode assembly.

Full alternator output is obtained by connecting the field terminal to
the regulator terminal or output terminal.

Fig. 4.2.28 Charge Indicator Light

Charge indicators

Charge indicators may be an ammeter, a voltmeter or a charge

indicator light. Ammeters may be installed in series if they are full-
current, shunt type or in parallel if the ammeter is the non-shunt type.

Voltmeters are more commonly used because they more accurately

indicate the operation of the system. They can be easily installed in
parallel with the charging system and provide information on both the
operation of the charging system and condition of the battery.

Charge indicator lights show general system operation. They will not
indicate high alternator output or high voltage conditions but will
show low output.

Charging System Testing

Accurate testing of charging systems begins with an understanding of

how the system functions. If your knowledge of the operation is
complete, you can logically determine the fault through visual
inspection and electrical testing.

Repair of the system may require replacement or repair of any of the

items included in the system. From the battery to the alternator.

All repairs should begin with a study or review of the Service Manual
for the machine upon which you are working.
Unit 4 4-2-25 Electrical Fundamentals
Lesson 2

When testing any electrical system a systematic approach will lead to

quicker repairs. Charging systems require the same troubleshooting
approach. Parts replacement without proper troubleshooting is not an
acceptable method of finding and repairing system faults.

Verify the complaint

Determine exactly what the complaint is, then verify that the fault is
occurring. Some common problems that occur in charging systems
are:

- the battery is discharged and the charging system is producing no

charge or low charge
- the battery is charging and the charging system is over-charging
- the alternator is noisy
- the charge indicator light stays on or fails to come on.

Define the problem

Begin with a thorough visual inspection. Check for:

- loose or corroded battery terminals

- loose or damaged ground connections at the engine and body
- loose, dirty connections at the alternator and regulator
- burnt fuse links or wires
- damaged, crimped, broken or cut wires
- evidence of shorts or grounds
- physical damage to the alternator or regulator
- damage to belts and pulleys
- odor of burnt electrical components

Determine whether the problem is electrical or mechanical.

Alternators are belt driven. These drive belts must be inspected for
tension, wear and damage to make sure that they are doing the job.
Inspect the belt for damage by checking the inside and outside
surfaces for cracking, chipping, glazing or missing pieces.

Inspect the alternator pulley for wear and any other pulleys that the
belt runs over. Premature belt failure is often caused by worn
pulleys. Inspect all pulleys for alignment. Usually a visual
inspection will show that they are not lined up correctly, but you may
have to check with a straight edge against the pulley.
Test the belt for proper tension. When adjusting belts or checking
belt tension make sure that you are not over-tightening or under-
tightening the belt. Incorrect tension will cause damage.
Unit 4 4-2-26 Electrical Fundamentals
Lesson 2

Noisy operation can be caused by worn belts, worn bearings or

internal problems such as the rotor rubbing on the stator, the fan
blades hitting the alternator or defective diodes or stators.

Mechanical problems can be corrected by replacing the faulty

components or repairing the defective unit as necessary. Electrical
problems will require more detailed testing.

Continue your inspection by performing a complete battery service.

Battery service and testing is covered in Lesson 1. A charging system
will not function efficiently if the battery is defective.

Isolate the problem

Once you have defined what the problem is, you must isolate the
cause so that you can accurately make the necessary repairs.
Mechanical faults can be located by inspecting or listening.
Electrical faults require testing to locate the cause.

Charging System Tests

On machine charging system tests should be performed first to

determine whether the alternator must be removed and tested further.
On machine tests include :

- Alternator output test

- Regulator test

Bench tests will determine if the alternator must be repaired or

replaced. Bench tests include:

- Rotor field winding tests

- Stator tests

- Rectifier tests

- Brush tests

Show Video "Testing the Alternator on the Engine" (SEVN1591).

Distribute copies of Service Magazine Article
"Alternator/Generator Output Test on the Engine" dated May 4,
1987.
Unit 4: Lesson 2 -1- Electrical Fundamentals
Instructor Copy Lab 4.2.1

Instructor Copy: Lab 4.2.1

Alternator Output Test
Lab Objective: Given a machine or training aid, a digital multimeter and a clamp-on ammeter,
perform an alternator output test.

1. Place the positive lead of a digital multimeter on the B+ terminal of the alternator. Place the
negative lead of the digital multimeter on the negative terminal or frame of the alternator. Place
the clamp-on ammeter 8T0900 around the positive output wire of the alternator.

2. Turn off all electrical accessories. With the fuel off, crank the engine for 30 seconds. Wait two
minutes to let the starting motor cool. If the system appears to operate at the specifications,
crank the engine again for 30 seconds.

NOTE: Cranking the engine for 30 seconds partially discharges the batteries to perform the
charging test. If the batteries are already discharged, skip this step. Jump-start the engine or
charge the batteries as required

3. Start the engine and run at approximately half throttle.

NOTE: Full throttle approximates the required drive pulley speed of 5000 rpm.

4. Immediately check output current. When operating correctly, this initial charging current
should be equal to or greater than the full output current shown in the Service Manual.

Record the output current specification from the Service Manual: _________amps

Have the students locate the alternator output specification from the machine service manual.

5. The alternator output should stabilize within approximately 10 minutes at 1/2 throttle (possibly
longer, depending upon battery size, condition and alternator rating). When operating correctly,
the alternator output voltage is:

12V system: 14.0 ± 0.5V

24V system: 27.5 ± 1.0V

If the alternator is NOT performing within specifications, refer to the Fault Condition and
Possible Causes Chart in the Service Magazine article "Alternator Generator Output Test on the
Engine."

6. The charging current during this period should taper off to less than approximately 10 amps,
depending upon battery and alternator capacities. If the charging current does not decrease as
specified, refer to the Fault Condition and Possible Causes Chart.
Unit 4: Lesson 2 -1- Electrical Fundamentals
Student Copy Lab 4.2.1

Student Copy: Lab 4.2.1

Alternator Output Test
Lab Objective: Given a machine or training aid, a digital multimeter and a clamp-on ammeter,
perform an alternator output test.

1. Place the positive lead of a digital multimeter on the B+ terminal of the alternator. Place the
negative lead of the digital multimeter on the negative terminal or frame of the alternator. Place
the clamp-on ammeter 8T0900 around the positive output wire of the alternator.

2. Turn off all electrical accessories. With the fuel off, crank the engine for 30 seconds. Wait two
minutes to let the starting motor cool. If the system appears to operate at the specifications,
crank the engine again for 30 seconds.

NOTE: Cranking the engine for 30 seconds partially discharges the batteries to perform the
charging test. If the batteries are already discharged, skip this step. Jump-start the engine or
charge the batteries as required

3. Start the engine and run at approximately half throttle.

NOTE: Full throttle approximates the required drive pulley speed of 5000 rpm.

4. Immediately check output current. When operating correctly, this initial charging current
should be equal to or greater than the full output current shown in the Service Manual.

Record the output current specification from the Service Manual: _________amps

Have the students locate the alternator output specification from the machine service manual.

5. The alternator output should stabilize within approximately 10 minutes at 1/2 throttle (possibly
longer, depending upon battery size, condition and alternator rating). When operating correctly,
the alternator output voltage is:

12V system: 14.0 ± 0.5V

24V system: 27.5 ± 1.0V

If the alternator is NOT performing within specifications, refer to the Fault Condition and
Possible Causes Chart in the Service Magazine article "Alternator Generator Output Test on the
Engine."

6. The charging current during this period should taper off to less than approximately 10 amps,
depending upon battery and alternator capacities. If the charging current does not decrease as
specified, refer to the Fault Condition and Possible Causes Chart.
Unit 4: Lesson 2 -1- Electrical Fundamentals
Instructor Copy Lab 4.2.2

Instructor Copy: Lab 4.2.1

Regulator Test
Lab Objective: Given a machine or training aid, a digital multimeter, a clamp-on ammeter and a
variable Power supply, perform an alternator regulator test. This test does not cover all of the failure
possibilities, but is used to locate common alternator problems.

Note: Alternator output voltage is regulated by controlling field coil current. The regulator senses
the output voltage. If the voltage is low, the regulator allows field current to flow in the coil and
causes an increase in output voltage. When the upper limit is reached the field current is turned off.
The output voltage of a properly functioning alternator and regulator is:

12V System: 14.0 ± 0.5V

24V System: 27.5 ± 1.0V

The voltage should not vary more than approximately 0.3V during this process. The test measures
voltage when the regulator turns the field current ON and OFF and determines if there is an open
or short in the diode trio, field coil or regulator.

Directions: Select a 12V or 24V alternator for test

1. Connect a variable power supply positive lead to the alternator B+ and D+ terminals as shown
in fig: 4.2.29. Connect the negative lead to the alternator B- terminal or frame ground. Place
the clamp-on ammeter around the B+ lead on the alternator.

B
A

D V VPS

Fig. 4.2.29

2. Adjust the voltage on the variable power supply until the clamp-on ammeter indicates a current
draw. Record the turn-on voltage. ____________ volts

3. Did the alternator turn-on voltage meet the alternator specifications? ____________. If yes,
the lab is completed. If no, continue with the next step.

Regulator lab continued on next page.

Unit 4: Lesson 2 -2- Electrical Fundamentals
Instructor Copy Lab 4.2.2

Regulator Test Continued

NOTE: Before continuing the regulator test, have each student locate the field current specification
in the applicable service manual for the alternator under test. Record the specification below:
______________amps at ____________volts. Divide the voltage that first indicates field current by
the rotor field resistance as specified in the manual. The result indicates the correct field current
that should be read on the ammeter.

4. If the ammeter indicates zero amps, the probable faults is the field coil or regulator is open. If
the ammeter reading was too high, the field coil is probably shorted. If the turn-on voltage is
not within specification, the regulator is probably malfunctioning. Turn-on voltage
specifications are:
12V System: 14.0 ± 0.5V
24V System: 27.5 ± 1.0V

5. If the measurements in steps 3 and 4 are correct, proceed to step #6. If they are not correct, the
alternator and/or regulator is defective.

6. Adjust the variable power supply to the turn-on voltage measured in step #2. Slowly increase
the voltage until the ammeter reads zero amps. This is turn-off voltage. Record the turn-off
voltage: Turn-off voltage = _____________volts.

7. The difference between the turn-on and turn-off voltages must be no more than 0.3V. If the
voltage is higher than 0.3V, the regulator is malfunctioning. Record the difference._______
Volts. Also, the ammeter reading should drop quickly to zero amps. If not, the regulator is
faulty.

8. If the alternator and regulator meets all the test requirements and it still fails to operate properly
complete the rotor field, stator and rectifier tests.
Unit 4: Lesson 2 -1- Electrical Fundamentals
Student Copy Lab 4.2.2

Student Copy: Lab 4.2.2

Regulator Test
Lab Objective: Given a machine or training aid, a digital multimeter, a clamp-on ammeter and a
variable Power supply, perform an alternator regulator test. This test does not cover all of the failure
possibilities, but is used to locate common alternator problems.

Directions: Select a 12V or 24V alternator for test (Handout the alternator specification sheet for
the alternator being used in the lab)

1. Connect a variable power supply positive lead to the alternator B+ and D+ terminals as shown
in Fig: 4.2.29. Connect the negative lead to the alternator B- terminal or frame ground. Place
the clamp-on ammeter around the B+ lead on the alternator.

B
A

D V VPS

Fig. 4.2.29

2. Adjust the voltage on the variable power supply until the clamp-on ammeter indicates a current
draw. Record the turn-on voltage. ____________ volts

3. Did the alternator turn-on voltage meet the alternator specifications? ____________. If yes,
the lab is completed. If no, continue with the next step.

4. If the ammeter indicates zero amps, the probable faults is the field coil or regulator is open. If
the ammeter reading was too high, the field coil is probably shorted. If the turn-on voltage is
not within specification, the regulator is probably malfunctioning. Turn-on voltage
specifications are:
12V System: 14.0 ± 0.5V
24V System: 27.5 ± 1.0V

5. If the measurements in steps 3 and 4 are correct, proceed to step #6. If they are not correct, the
alternator and/or regulator is defective.

Regulator lab continued on next page.

Unit 4: Lesson 2 -2- Electrical Fundamentals
Student Copy Lab 4.2.2

Regulator Test continued

6. Adjust the variable power supply to the turn-on voltage measured in step #2. Slowly increase
the voltage until the ammeter reads zero amps. This is turn-off voltage. Record the turn-off
voltage: Turn-off voltage = _____________volts.

7. The difference between the turn-on and turn-off voltages must be no more than 0.3V. If the
voltage is higher than 0.3V, the regulator is malfunctioning. Record the difference._______
Volts. Also, the ammeter reading should drop quickly to zero amps. If not, the regulator is
faulty.

8. If the alternator and regulator meets all the test requirements and it still fails to operate properly
complete the rotor field, stator and rectifier tests.
Unit 4: Lesson 2 -1- Electrical Fundamentals
Instructor Copy Lab 4.2.3

Instructor Copy: Lab 4.2.3

Rotor Field Winding Test

Lab Objective: Given a multimeter, perform the rotor field winding test as outlined in the appropriate
service manual for the alternator being tested.

Note: Instructor should perform these tests prior to teaching this unit.

Rotor Field Winding Continuity Test

1. Set the multimeter to the 200 ohm scale. Touch the meter leads to each slip ring on the rotor.

Record the results.______________ ohms

Using the appropriate service manual, have students locate the specified resistance values.

2. Is the measured resistance values within specification? ____________

3. If the resistance value is not within specification, briefly explain the most probable cause. __

______________________________________________________________________.

Rotor Field Winding Ground Test

4. Set the multimeter to the 20M ohm scale. Touch the meter leads between each slip ring and the
rotor shaft.

Each reading should be greater than 100,000 ohms.

Resistance measured: _______________________ ohms

Resistance measured: _______________________ ohms

5. If the resistance value is not within specification, briefly explain the most probable cause. __

______________________________________________________________________.

NOTE: If the results of the Rotor Field Winding test are not within specification, the probable
cause is an open or shorted field coil.
If the resistance value is less than 100,000 ohms in the Ground Test, the most probable cause is a
grounded rotor.
Unit 4: Lesson 2 -1- Electrical Fundamentals
Student Copy Lab 4.2.3

Student Copy: Lab 4.2.3

Rotor Field Winding Test
Lab Objective: Given a multimeter, perform the rotor field winding test as outlined in the appropriate
service manual for the alternator being tested.

Rotor Field Winding Continuity Test

1. Set the multimeter to the 200 ohm scale. Touch the meter leads to each slip ring on the rotor.

Record the results.______________ ohms

Using the appropriate service manual, have students locate the specified resistance values.

2. Is the measured resistance values within specification? ____________

3. If the resistance value is not within specification, briefly explain the most probable cause. __

______________________________________________________________________.

Rotor Field Winding Ground Test

4. Set the multimeter to the 20M ohm scale. Touch the meter leads between each slip ring and the
rotor shaft.

Each reading should be greater than 100,000 ohms.

Resistance measured: _______________________ ohms

Resistance measured: _______________________ ohms

5. If the resistance value is not within specification, briefly explain the most probable cause. __

______________________________________________________________________.
Unit 4: Lesson 2 -1- Electrical Fundamentals
Instructor Copy Lab 4.2.4

Instructor Copy: Lab 4.2.4

Stator Winding Test

Lab Objective: Given a multimeter, perform the stator test as outlined in the appropriate service
manual for the alternator being tested.

Note: Instructor should perform these tests prior to teaching this unit.

Stator Winding Continuity Test

1. Set the multimeter to the 200 ohm scale. Touch the meter leads to each pair (3 pairs) of stator
leads. Record the results below:
1st pair _________________ ohms
2nd pair ________________ ohms
3rd pair ________________ ohms

Using the appropriate service manual, have students locate the specified resistance values.

2. Is the measured resistance values within specification? ____________

3. If the resistance value is not within specification, briefly explain the most probable cause. __

______________________________________________________________________.

Stator Winding Ground Test

4. Set the multimeter to the 20M ohm scale. Touch the meter leads between each stator lead and
the stator frame.

Each reading should be greater than 100,000 ohms. (each pair of stator leads)

Resistance measured: _______________________ ohms

Resistance measured: _______________________ ohms

Resistance measured: _______________________ ohms

5. If the resistance value is not within specification, briefly explain the most probable cause. __

______________________________________________________________________.

NOTE: If the results of the Stator Winding test are not within specification, the probable cause
is an open or shorted stator.

If the resistance value is less than 100,000 ohms in the Ground Test, the most probable cause is a
grounded stator.
Unit 4: Lesson 2 -1- Electrical Fundamentals
Student Copy Lab 4.2.4

Student Copy: Lab 4.2.4

Stator Winding Test
Lab Objective: Given a multimeter, perform the stator test as outlined in the appropriate service
manual for the alternator being tested.

Stator Winding Continuity Test

1. Set the multimeter to the 200 ohm scale. Touch the meter leads to each pair (3 pairs) of stator
leads. Record the results below:
1st pair _________________ ohms
2nd pair ________________ ohms
3rd pair ________________ ohms

Using the appropriate service manual, have students locate the specified resistance values.

2. Is the measured resistance values within specification? ____________

3. If the resistance value is not within specification, briefly explain the most probable cause. __

______________________________________________________________________.

Stator Winding Ground Test

4. Set the multimeter to the 20M ohm scale. Touch the meter leads between each stator lead and
the stator frame.

Each reading should be greater than 100,000 ohms. (each pair of stator leads)

Resistance measured: _______________________ ohms

Resistance measured: _______________________ ohms

Resistance measured: _______________________ ohms

5. If the resistance value is not within specification, briefly explain the most probable cause. __

______________________________________________________________________.
Unit 4: Lesson 2 -1- Electrical Fundamentals
Instructor Copy Lab 4.2.5

Instructor Copy: Lab 4.2.5

Rectifier Test
Lab Objective: Given a multimeter, perform the rectifier test as outlined in the appropriate service
manual for the alternator being tested.

NOTE: Instructor should perform these tests prior to teaching this unit.

Directions: Perform the Positive Diode Check.

1. Set the multimeter to the diode check function. Connect the meter leads between each positive
diode and the B+ stud. The positive diodes are black.

2. Record the meter reading. ______________. Reverse the leads and record the reading ______

3. Briefly explain the readings: _____________________________________________________

____________________________________________________________________________.

What should a serviceable diode read? _____________

_________________________________________________________________________.

Directions: Perform the Negative Diode Check.

4. Set the multimeter to the diode check function. Connect the meter leads between each positive
diode and the B+ stud. The negative diodes are silver.

5. Record the meter reading. ______________. Reverse the leads and record the reading ______

6. Briefly explain the readings: _____________________________________________________

___________________________________________________________________________.

What should a serviceable diode read? __________________________________________

_________________________________________________________________________.

NOTE: In both checks the meter should read a voltage drop of approximately 400 millivolts to 900
millivolts in one direction and OL: in the other.

If the meter reads 400 millivolts to 900 millivolts in both directions the diode is shorted.

If the meter reads OL in both directions the diode is open.

Unit 4: Lesson 2 -1- Electrical Fundamentals
Student Copy Lab 4.2.5

Student Copy: Lab 4.2.5

Rectifier Test

Lab Objective: Given a multimeter, perform the rectifier test as outlined in the appropriate service
manual for the alternator being tested.

Directions: Perform the Positive Diode Check.

1. Set the multimeter to the diode check function. Connect the meter leads between each positive
diode and the B+ stud. The positive diodes are black.

2. Record the meter reading. ______________. Reverse the leads and record the reading ______

3. Briefly explain the readings: _____________________________________________________

____________________________________________________________________________.

What should a serviceable diode read? _______________________________________________

_________________________________________________________________________.

Directions: Perform the Negative Diode Check.

4. Set the multimeter to the diode check function. Connect the meter leads between each positive
diode and the B+ stud. The negative diodes are silver.

5. Record the meter reading. ______________. Reverse the leads and record the reading ______

6. Briefly explain the readings: _____________________________________________________

_______________________________.
What should a serviceable diode read? ______________________________________________

___________________________________________________________________________.
Unit 4: Lesson 2 -1- Electrical Fundamentals
Instructor Copy Lab 4.2.6

Instructor Copy: Lab 4.2.6

Brush Tests
Lab Objective: Given a multimeter and a ruler perform the alternator brush test as outlined in the
appropriate service manual for the alternator being tested.

NOTE: Instructor should perform these tests prior to teaching this unit.

Directions: Perform the Brush Continuity Check.

1. Set the multimeter scale to the 200 ohm range.Touch one meter lead to the positive brush and
the other to the terminal. Record the measurement below:

Resistance measured: _____________ ohms

Directions: Perform Brush Ground Check

2. Set the multimeter to the 20M range. Touch one meter lead to the positive brush and the other
to the terminal. Record the measurement below:

Resistance measured: _____________ ohms

Directions: Perform Brush Length measurement.

3. Using a ruler, measure the length of the brushes on the longest side.

Length measured: __________mm ______________in.

Length measured: __________mm ______________in.

Use the appropriate service manual for determining proper brush length. Replace brushes if
necessary..

NOTE: In the Brush Continuity test, the resistance between the positive brush and the terminal
should be approximately .1 to .3 ohms.

In the Brush Ground check, the meter should read in excess of 100,000 ohms.
Unit 4: Lesson 2 -1- Electrical Fundamentals
Student Copy Lab 4.2.6

Student Copy: Lab 4.2.6

Brush Tests
Lab Objective: Given a multimeter and a ruler perform the alternator brush test as outlined in the
appropriate service manual for the alternator being tested.

Directions: Perform the Brush Continuity Check.

1. Set the multimeter scale to the 200 ohm range.Touch one meter lead to the positive brush and
the other to the terminal. Record the measurement below:

Resistance measured: _____________ ohms

Directions: Perform Brush Ground Check

2. Set the multimeter to the 20M range. Touch one meter lead to the positive brush and the other
to the terminal. Record the measurement below:

Resistance measured: _____________ ohms

Directions: Perform Brush Length measurement.

3. Using a ruler, measure the length of the brushes on the longest side.

Length measured: __________mm ______________in.

Length measured: __________mm ______________in.

Use the appropriate service manual for determining proper brush length. Replace brushes if
necessary.

4. Briefly explain the readings: _____________________________________________________

_______________________________. How long should a brush be, and what is a serviceable

length?_______________________________________________________________________

____________________________________________________________________________.