MACK ELECTRICAL TROUBLESHOOTING
ELECTRICAL
TROUBLESHOOTING
SERVICE MANUAL
OCTOBER 1999
(NEW ISSUE)
8-212
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ELECTRICAL
TROUBLESHOOTING
SERVICE MANUAL
OCTOBER 1999
NEW ISSUE
© MACK TRUCKS, INC. 1999
8-212
Front.fm Page ii Tuesday, June 29, 1999 3:11 PM
ATTENTION
The information in this manual is not all inclusive and
cannot take into account all unique situations. Note that
some illustrations are typical and may not reflect the
exact arrangement of every component installed on a
specific chassis.
The information, specifications, and illustrations in this
publication are based on information that was current at
the time of publication.
No part of this publication may be reproduced, stored in a
retrieval system, or be transmitted in any form by any
means including electronic, mechanical, photocopying,
recording, or otherwise without prior written permission
of Mack Trucks, Inc.
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SAFETY INFORMATION
SAFETY INFORMATION
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Advisory Labels
SAFETY INFORMATION
Cautionary
manual. Information accented by one of these signal words must be observed to minimize the risk of
personal injury to service personnel, or the possibility of improper service methods which may damage
the vehicle or render it unsafe. Additional Notes and Service Hints are utilized to emphasize areas of
procedural importance and provide suggestions for ease of repair. The following definitions indicate the
use of these advisory labels as they appear through out the manual:
signal words
(Danger-Warning-Caution) may appear in various locations throughout this
Directs attention to unsafe practices which could result in damage to equipment and
possible subsequent personal injury or death if proper precautions are not taken.
Directs attention to unsafe practices which could result in personal injury or
death if proper precautions are not taken.
Directs attention to unsafe practices and/or existing hazards which will result
in personal injury or death if proper precautions are not taken.
An operating procedure, practice, condition, etc., which is essential to emphasize.
A helpful suggestion which will make it quicker and/or easier to perform a certain
procedure, while possibly reducing overhaul cost.
000001a
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Service Procedures and Tool Usage
Anyone using a service procedure or tool not recommended in this manual must first satisfy himself
thoroughly that neither hi s safet y nor vehi cle saf ety will b e jeo pardized b y the s erv ice method he s ele cts .
Individuals deviating in any manner from the instructions provided assume all risks of consequential
personal injury or damage to equipment involved.
Also note that particular service procedures may require the use of a special tool(s) designed for a
specific purpose. These special tools must be used in the manner described, whenever specified in the
instructions .
1. Before starting a vehicle, always be seated in the driver’s seat, place the
transmission in neutral, be sure that parki ng brakes are se t, and disengage
the clutch (if equipped).
SAFETY INFORMATION
2. Before working on a vehicle, place the transmission in neutral, set the
parking brakes, and block the wheels.
3. Before towing the vehicle, place the transmiss ion in neutral and li ft the rear
wheels off the ground, or disconnect the driveline to avoid damage to the
transmission during towing.
Engine driven components such as Power Take-Off (PTO) units, fans and fan
belts, driveshafts and other related rotating assemblies, can be very
dangerous. Do not work on or service engine driven components unless the
engine is shut down. Always keep body parts and loose clothing out of range
of these powerful components to prevent serious personal injury. Be aware of
PTO engagement or nonengagement status. Always disengage the PTO when
not in use.
REMEMBER,
SAFETY . . . IS NO ACCIDENT!
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NOTES
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TABLE OF CONTENTS
TABLE OF CONTENTS
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TABLE OF CONTENTS
SAFETY INFORMATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iii
ADVISORY LABELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iv
SERVICE PROCEDURES AND TOOL USAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
DESCRIPTION AND OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
ELECTRICAL CONCEPTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Understanding Electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
VOLTAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Sources of Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
CURRENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Actual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Conventional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Types of Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
RESISTANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Resistance, Heat and Current Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
CIRCUIT TYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Series Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Parallel Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Series-Parallel Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
OHM’S LAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
EXPRESSING ELECTRICAL VALUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
DIAGNOSTIC TOOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6
Jumper Wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Multimeter (Volt-Ohm Meter) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Multimeter (Volt-Ohm Meter) Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
TROUBLESHOOTING METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Diagnostic Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Diagnostic Application s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Locating Shorts or Grounded Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
Circuit Continuity Checks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Checking Circuit Grounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
POWER DISTRIBUTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30
Battery-Powered Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Key-Powered Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Ground Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32
TYPICAL ELECTRIC EQUIPMENT PANEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
CIRCUIT BREAKERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
SAE Type 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
SAE Type 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
SAE Type 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Testing Circuit Breakers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
WIRE SIZES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
WIRE IDENTIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37
BATTERIES — GENERAL INFORMATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Types of Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41
Periodic Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41
Battery Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
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STARTING SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45
Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46
CHARGING SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52
Charging System Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53
MISCELLANEOUS CIRCUITS — DESCRIPTION/FUNCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . .55
Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55
Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56
Sending Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
TROUBLESHOOTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
TROUBLESHOOTING OF INSTRUMENT CLUSTER, GAUGES, SENDING UNITS,
SENSORS AND HORN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62
Gauge Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62
Specific Gauge and Sending Unit Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64
Speed Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72
Horn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .74
TABLE OF CONTENTS
REPAIR PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
COMMON ELECTRICAL PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .80
Correct Use of Tie Wraps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .80
Typical Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83
Chassis Electrical Sealant Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101
SPECIAL TOOLS & EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
RECOMMENDED ELECTRICAL TOOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .104
INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
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NOTES
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DESCRIPTION AND OPERATION
DESCRIPTION AND OPERATION
Page 1
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DESCRIPTION AND OPERATION
INTRODUCTION
Electricity provides the power necessary for
starting the engine and operating the various
lights and other auxi liary sy stems in stal led on t he
chassis. Diagnosing problems t hat can occu r in a
truck electrical system involves a basic
understanding of electrical concept s, and testing
and measurement procedures. The purpose of
this manual is to familiarize the technician with
basic electrical concepts and diagnostic
procedures. It is not intended to be vehicle
specific.
ELECTRICAL CONCEPTS
Understanding Electricity
Electricity is the movement of electrons through a
conductor. An electrical circuit can easily be
compared to a hydraulic (or pneumatic) circuit,
where hydraulic fluid (or compressed air) is
pushed through a conductor to an actuator that
performs a function.
1
1. Switch (Control)
2. Light Bulb (Load)
3. Electron Flow
Page 2
Figure 1 — Electrical Circuit
4. Battery (Voltage Storage & Source)
5. Alternator (Voltage Source — Electron Pump)
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DESCRIPTION AND OPERATION
2
Figure 2 — Hydraulic Circuit
1. Fluid Flow
2. Cylinder (Load)
3. Valve (Control)
A basic understanding of electricity begins with
an understanding of a few basic electrical terms
and concepts. They are:
r Voltage
r Current
r Resistance
r Circuit Types
r Ohm’s Law
4. Reservoir (Fluid Storage)
5. Fluid Pump
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DESCRIPTION AND OPERATION
VOLTAGE
The force that causes the electrons to move is
called “electromotive force.” Electromotive force
is more commonly known as voltage. Voltage is
the potential difference in electron pressure
between two points. The potential dif ference is an
excess of electrons on the negative side and a
lack of electrons on the positive side.
The movement of electrons requires:
r An excess of electrons on one side.
r A lack of electrons on the other side.
r A path between the two.
r A force capable of moving the electrons.
3
Figure 3 — Voltage (Electromotive Force)
1. Path for Electron Flow (Wire and Bulb Filament)
2. Negative Battery Terminal — Excess of Electrons
Page 4
3. Positive Battery Terminal — Lack of Electrons
4. Battery (Force That Moves Electrons)
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DESCRIPTION AND OPERATION
Sources of Voltage
Voltage can be generated by:
r Heat
r Friction
r Light
r Pressure
r Chemical Reaction
r Magnetism
The two sources of voltage available in a truck
electrical system are chemical reaction and
magnetism.
CHEMICAL REACTION
4
Voltage is created in a storage battery by
chemical reaction. The reaction that takes place
between the sulfuric acid/water (elect rolyte) and
lead plates inside the battery, produces a
potential difference in electron pressure between
the positive and negative terminals. As the free
electrons are drawn from the battery, the reaction
continues until the chemicals inside the battery
are exhausted.
The battery provides and stores the voltage
necessary for the starting system to crank the
engine. The battery also provides the additional
voltage needed when electrical demands exceed
the electron flow supplied by the charging
system.
MAGNETISM
5
Figure 4 — Chemical Reaction (Battery)
1. Terminal Post
2. Cell Partition
3. Intercell Connections
4. Plates and Separators
5. Element Rest
6. Positive Plate (Lead
Peroxide)
7. Negative Plate (Sponge
Lead)
8. Case
Figure 5 — Magnetism (Magnet and Conductor)
1. Conductor
2. Magnetic Field
3. Electron Flow
4. Conductor
5. Permanent Magnet
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DESCRIPTION AND OPERATION
Voltage is also generated when a wire is
physically passed through a magnetic field. This
process is called “induction.” As an example, an
alternator generates electricity when a magnetic
field (rotor) is passed over a coil of wire (stator).
Another example of voltage generated by the
principle of induction is the speed sensor used to
determine engine speed or vehicle speed. When
a toothed gear passes in front of a magnetic pickup, the magnetic field is broken and an electrical
pulse is generated.
6
Figure 6 — Speed Sensor
1. Speed Sensor 2. Speed Sensor
Connector (Integral)
Page 6
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DESCRIPTION AND OPERATION
CURRENT
Electrical current is the movement of electrons
through a conductor. Just as flow in a hydraulic
system is measured as the amount of fluid
flowing past a given point in a certain amount of
time (expressed as gallons per minute), electr ical
current is measured as the amount of electrons
moving past a certain point in a given amount of
time. Electron flow is expressed in amperes or
amps.
One AMP equals 6.25 trillion electrons flowing
past a given point in one second.
Actual
Actual current flow is the flow of free electrons
through a conductor. Current flow is the
movement of negatively charged electrons from
one atom to the next atom. The positi ve side of a
voltage source (which has a lack of electrons)
attracts the free electrons from the negati ve side
(which is giving up electr ons). Electr ons flow from
negative to positive.
7
Conventional
Conventional current flow describes a circui t
inside a battery. Atoms that gain or lose electrons
are called ions. Excess electrons do not move
through a battery, but are carried by ions. The
movement of ions inside a battery is from the
positive plates (or battery post) where free
electrons are given up, to the negative plates (or
battery post) where electrons are received. This
makes it appear as though current flow is from
positive to negative.
Conventional current flow is considered to be
from positive to negative.
8
Figure 7 — Electron Current Flow Through a Conductor
1. Copper Wire
2. Copper Atom
3. Voltage (Electron Push)
Figure 8 — Conventional Current Flow Through a Circuit
1. Battery 2. Migrating Positive Ions
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DESCRIPTION AND OPERATION
Types of Current
There are two types of current flow: Direct
Current (DC) and Alternating Current (AC).
DIRECT CURRENT (DC)
In a direct current circuit, electrons flow in one
direction only, from the negative terminal to the
positive terminal. Direct current, supplied by the
storage battery, is the type of current flow in a
truck electrical system.
9
1. Closed Switch
2. Lamp
3. Battery (Force to Move Current)
Figure 9 — Direct Current
4. Electrons flow in one direction only, from negative to
positive.
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DESCRIPTION AND OPERATION
ALTERNATING CURRENT (AC)
In an alternating current circuit, electron flow
changes direction at a fixed rate or cycle.
Alternating current is the t ype of current produced
by the charging system alternator. This type of
current however, is not compatible with a vehicle
electrical system. To be usable, it must be
converted (or rectified) into direct current. To
accomplish this, diodes are added to the circuit.
Diodes are used in an electrical system much li ke
check valves in a hydraulic or pneumatic syst em.
They allow current flow in one direction, and
block current flow when the c ycle rever ses (in t he
opposite direction).
10
1. Lamp (Uses DC Current)
2. Closed Switch
Figure 10 — Alternating Current
3. Alternator (Produces AC Current)
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DESCRIPTION AND OPERATION
RESISTANCE
Electrical current is the movement of electrons
from one atom to the next. Electrons, however,
resist being moved out of their shells. The atoms
of some substances (such as copper), give up
their electrons more readily than the atoms of
other substances (such as nickel). Atoms of
substances like rubber do not give up electron s
easily. Substances that readily give up electrons
are called “conductors.” Substances that resist
giving up electrons are called “resistors.”
Substances that do not give up electrons easily
are called “insulators.”
11
Resistance, Heat and Current Flow
Electron flow through a conductor or component
generates a certain amount of heat. A light bulb
illuminates when electrons flow through the
filament of the bulb. The thin filament inside the
light bulb offers such a great resistance to
electron flow that the filament heats up and
glows.
Wires used in an electric circuit are selected
according to the amount of current they must
carry. Thick wires have less resistance to current
than thin wires, and so are used to carry greater
amounts of current.
12
Figure 11 — Resistance in a Conductor
1. Less Resistance, Mo re
Current Flow
2. More Resistance, Less
Current Flow
The capacity of a substance to resist electron
flow is called “resistance.” Resistance is
expressed in ohms. All components in an
electrical circuit (light bulbs, motors, solenoids,
sensors, horns) add to the total resistance in a
circuit.
Figure 12 — Wire Size, Current Capacity and Resistance
Properly selected wires in a circuit have a low
resistance. If the resistance of a wire is too high,
circuit operation will be faulty in some way.
Examples of high-resistance conditions incl ude
partially cut wires and loose or corroded
connections. These types of faults can be
compared to a faulty hydraulic circuit where oil
flow is restricted by a kinked or leaking hydraulic
hose. With less oil flow, the hydraulic circuit will
not operate at full potential.
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DESCRIPTION AND OPERATION
CIRCUIT TYPES
The three basic types of circuits are series,
parallel and series-parallel.
Series Circuits
13
Parallel Circuits
14
Figure 14 — Parallel Circuit
Figure 13 — Series Circuit
Series circuits are the simplest of circuits. In a
series circuit, all the resistors are connected
together (end to end), to one voltage source.
There is only one path for electron flow. Series
circuits have the following characteri stics:
r The total resistance of the circuit is equal to
the sum of each resistor.
r Current flow (amperage) through each
resistor in the circuit is the same, and is
equal to the total amperage through the
circuit.
r The voltage drop across each resistor
equals resistance multiplied by the
amperage.
r The source voltage is equal to the sum of
the voltage drops across eac h resis tor in t he
circuit.
1. Branch 1 Amperage
2. Branch 2 Amperage
3. Branch 3 Amperage
4. 3.84 Amps (Total Amps)
5. Total Resistance
Calculation
6. Total Amperage
Calculation
A parallel circuit is one in which the resistors are
connected side by side, and there are several
paths for current flow. Parallel circuits, which are
the most commonly used circuits in truck
electrical systems are parallel circuits. The
following principles apply.
r Total resistance of the circuit is always less
than the value of the lowest resistor.
r Current flow (amperage) through each
resistor is different and depends on the
value of the resistor.
r The voltage drop across each resistor is the
same, and is equal to the source voltage.
r Total circuit amperage is equal to the sum of
the amperage through each branch.
r If one resistor in a parallel circuit is
disconnected, the remaining circuit still
operates.
If one resistor in a series circuit is disconnected,
the path for electron flow i s broken, and the entir e
circuit will not operate.
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DESCRIPTION AND OPERATION
To calculate total resistance in a parallel circuit:
15
Figure 15 — Calculating Resistance
To calculate total resistance in a parallel circuit
with only two branches:
16
Series-Parallel Circuits
17
Figure 16 — Calculating Resistance
Figure 17 — Series-Parallel Circuit
When series and parallel connections are used in
the same circuit, it is called a “series-parallel
circuit.” Calculating total resistance in a seriesparallel circuit involves simplifying the circuit into
a basic series circuit. To do this first calculate the
total resistance of the parallel branches. Then
add the result to the resist ance value of the seri es
part of the circuit. Once the circuit is brok en down
into a simple series circuit, amperage, total
resistance and voltage drops can be determined.
Series-parallel circuits are not used in truc k
electrical systems very often.
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DESCRIPTION AND OPERATION
OHM’S LAW
Ohm’s Law describes the relationshi p between
voltage, resistance and amperage. When any two
variables (voltage, amperage or resistance) are
known, the third variable can be determined
mathematically. Ohm’s Law states that voltage
(V) and amperage (I or A) are directly
proportional to any one value of resistance (R or
O), and amperage is inversely proportional to
voltage when voltage remains constant and
resistance changes.
The mathematical formula for Ohm’s Law is:
18
To use the Ohm’s Law circle, simply cover the
unknown variable, then perform the mathematic al
operation (either multiplication or division), using
the two remaining variables.
20
Figure 20 — Using the Ohm's Law Circle
To make it simple, the relationship between
voltage, resistance and amperage can be
described as follows:
Figure 18 — Mathematical Formulas for Ohm's Law
An easy way to remember Ohm’s Law is to use
the following Ohm’s Law circle:
19
r As voltage increases and resistance
remains constant, current increases.
r As voltage decreases and resistance
remains constant, current decreases.
r As resistance increases and voltage
remains constant, current decreases.
r As resistance decreases and voltage
remains constant, current increases.
It is not usually necessary to use Ohm’s Law
when troubleshooting an electrical problem, but
understanding the relationship between voltage,
resistance and amperage makes the job much
easier.
Figure 19 — Ohm's Law Circle
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DESCRIPTION AND OPERATION
Given the values for current (amps) and
resistance (ohms) shown in Figure 21, use Ohm’s
Law to determine the value for voltage (volts).
Multiply 4 amps of current by 6 ohms of
resistance. What is the total voltage (volt s) in the
series circuit?
21
Figure 21 — Finding Voltage (Series Circuit)
22
Figure 22 — Finding Amperage (Series Circuit)
Given the values for current (amps) and voltage
(volts) shown in Figure 23, use Ohm’s Law to
determine the value for res istance (ohms). Di vide
12 volts by 8 amps of current. What is the total
resistance (ohms) in the series cir cuit?
23
Given the values for voltage (volts) and
resistance (ohms) shown in Figure 22, use Ohm’s
Law to determine the value for current
(amperage). Divide 18 volts by 36 ohms of
resistance. What is the total current flow
(amperage) in the series circui t?
Figure 23 — Finding Resistance (Series Circuit)
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DESCRIPTION AND OPERATION
EXPRESSING ELECTRICAL
VALUES
In many instances, the numerical values used to
express amperage, voltage and resistance, are
either very large or very small. For example,
resistance in a circuit may be millions of ohms, or
current (amperage) may be in the milliampere
range (a few thousandths or millionths of an
ampere).
In these cases, it is more practical to express
values as multiples or submultiples of the basic
values. The values are based on the decimal
system of tens, hundreds, thousands and so on,
with a prefix to designate the value. For small
units (submultiples), “milli” and “micro” are used.
For large units (multiples), “kilo” and “mega” are
used. As an example, 5,000,000 ohms is written
as 5M ohms. When measuring the resistance of
an unknown resistor and the multimeter is
displaying 12.30K, the value of the resistor is
It is not practical to express these large or small
actually 12,300 ohms, not 12.30 ohms.
electrical values in pure numeric form, and it is
not possible for a meter to display these values.
It is important to know and understand these
prefixes. The following table lists the most
common prefixes used to express large or small
electrical values.
ELECTRICAL VALUES
Prefix Symbol Relation to Basic Unit Examples
mega M 1,000,000 (or 1 x 10
kilo k 1,000 (or 1 x 103)12.30 kΩ (kilo-ohms) = 12,300 ohms or 12.3 x 10
milli m 0.001 (or 1 x 10-3) 48 mA (milliamperes) = 0.048 ampere or 48 x 10
micro µ 0.000,0001 (or 1 x 10-6)15 µA (microamperes) = 0.000,015 ampere or
6
)5 MΩ (megaohms) = 5,000,000 ohms or 5 x
6
ohms
10
15 x 10
-6
3
-3
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DESCRIPTION AND OPERATION
DIAGNOSTIC TOOLS
Most electrical test procedures require taking
measurements of voltage, current flow
(amperage), resistance and continuity. Some
important diagnostic tools t hat wil l be needed ar e:
Jumper Wire
A jumper wire is used to bypass an open circuit
by providing an alternate path for current flow. It
is a short length of wire with either alligator clips
or probes on each end, and provides a quick
means of bypassing switches, suspected opens,
and other components. Adding a 5-amp fuse to
the jumper wire is recommended to protect the
circuit being tested.
Never connect a jumper across a load, such as a
motor that is wired between hot and ground.
Doing so would introduce a direct short that could
result in a fire a n d c a us e se rious inju ry.
Multimeters are available with a variety of
functions. All multimeters measure voltage,
current and resistance. Some meters can perform
additional functions such as quick conti nuity
checks, capacitance checks and diode tests.
25
24
Figure 24 — Jumper Wire
Multimeter (Volt-Ohm Meter)
Probably the most valuable tool needed for
diagnostics is the multimeter, which is used to
take accurate measurements of voltage,
amperage and resistance. Digital mult imeters are
recommended because of their accuracy, ease of
use, circuit protection capabilities, and are
required for troubleshooting cir cuits containing
solid state components or digital circuitry.
Figure 25 — Digital Multimeter (Volt-Ohm Meter)
1. Digital Display Screen
2. Function Selector
Switches (continuity
check, display hold,
range change, etc.)
3. Common Lead Input
4. Milli/Microampere Lea d
Input
5. Amperage Lead Input
6. Volt-Ohm Lead Input
7. Function Selector Dial
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