ELECTRICAL TROUBLESHOOTING
SERVICE MANUAL
OCTOBER 1999 (NEW ISSUE)
8-212
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ELECTRICAL
TROUBLESHOOTING
SERVICEMANUAL
OCTOBER 1999 |
© MACK TRUCKS, INC. 1999 |
|
NEW ISSUE |
8-212 |
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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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SAFETY INFORMATION
Advisory Labels
Cautionary signal words (Danger-Warning-Caution) may appear in various locations throughout this 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 throughout the manual:
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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SAFETY INFORMATION
Service Procedures and Tool Usage
Anyone using a service procedure or tool not recommended in this manual must first satisfy himself thoroughly that neither his safety nor vehicle safety will be jeopardized by the service method he selects. 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 parking brakes are set, and disengage the clutch (if equipped).
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 transmission in neutral and lift 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Jumper Wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Multimeter (Volt-Ohm Meter) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Multimeter (Volt-Ohm Meter) Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 TROUBLESHOOTING METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Diagnostic Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Diagnostic Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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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TABLE OF CONTENTS
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
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
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DESCRIPTION AND OPERATION
INTRODUCTION
Electricity provides the power necessary for starting the engine and operating the various lights and other auxiliary systems installed on the chassis. Diagnosing problems that can occur in a truck electrical system involves a basic understanding of electrical concepts, 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.
Figure 1 — Electrical Circuit
1. |
Switch (Control) |
4. |
Battery (Voltage Storage & Source) |
2. |
Light Bulb (Load) |
5. |
Alternator (Voltage Source — Electron Pump) |
3. |
Electron Flow |
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DESCRIPTION AND OPERATION
|
|
Figure 2 — Hydraulic Circuit |
||
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|
|
|
|
1. |
Fluid Flow |
|
4. |
Reservoir (Fluid Storage) |
2. |
Cylinder (Load) |
|
5. |
Fluid Pump |
3. |
Valve (Control) |
|
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|
|
|
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A basic understanding of electricity begins with an understanding of a few basic electrical terms and concepts. They are:
rVoltage
rCurrent
rResistance
rCircuit Types
rOhm’s Law
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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 difference is an excess of electrons on the negative side and a lack of electrons on the positive side.
The movement of electrons requires:
rAn excess of electrons on one side.
rA lack of electrons on the other side.
rA path between the two.
rA force capable of moving the electrons.
Figure 3 — Voltage (Electromotive Force)
1. |
Path for Electron Flow (Wire and Bulb Filament) |
3. |
Positive Battery Terminal — Lack of Electrons |
2. |
Negative Battery Terminal — Excess of Electrons |
4. |
Battery (Force That Moves Electrons) |
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|
|
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DESCRIPTION AND OPERATION
Sources of Voltage
Voltage can be generated by:
rHeat
rFriction
rLight
rPressure
rChemical Reaction
rMagnetism
The two sources of voltage available in a truck electrical system are chemical reaction and magnetism.
CHEMICAL REACTION
Voltage is created in a storage battery by chemical reaction. The reaction that takes place between the sulfuric acid/water (electrolyte) 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
Figure 4 — Chemical Reaction (Battery)
1. |
Terminal Post |
5. |
Element Rest |
2. |
Cell Partition |
6. |
Positive Plate (Lead |
3. |
Intercell Connections |
|
Peroxide) |
4. |
Plates and Separators |
7. |
Negative Plate (Sponge |
|
|
|
Lead) |
|
|
8. |
Case |
|
|
|
|
Figure 5 — Magnetism (Magnet and Conductor)
1. |
Conductor |
4. |
Conductor |
2. |
Magnetic Field |
5. |
Permanent Magnet |
3. |
Electron Flow |
|
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|
|
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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.
Figure 6 — Speed Sensor
1. Speed Sensor |
2. Speed Sensor |
|
Connector (Integral) |
|
|
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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), electrical 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 positive side of a voltage source (which has a lack of electrons) attracts the free electrons from the negative side (which is giving up electrons). Electrons flow from negative to positive.
Conventional
Conventional current flow describes a circuit 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.
Figure 7 — Electron Current Flow Through a Conductor
1. |
Copper Wire |
3. Voltage (Electron Push) |
2. |
Copper Atom |
|
|
|
|
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.
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Figure 9 — Direct Current |
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Closed Switch |
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4. Electrons flow in one direction only, from negative to |
2. |
Lamp |
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positive. |
3. |
Battery (Force to Move Current) |
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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 type 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 like check valves in a hydraulic or pneumatic system. They allow current flow in one direction, and block current flow when the cycle reverses (in the opposite direction).
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Figure 10 — Alternating Current |
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Lamp (Uses DC Current) |
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3. Alternator (Produces AC Current) |
2. |
Closed Switch |
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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 electrons 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.”
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.
Figure 11 — Resistance in a Conductor
1. Less Resistance, More |
2. More Resistance, Less |
Current Flow |
Current Flow |
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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 include 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 |
Parallel Circuits |
The three basic types of circuits are series, parallel and series-parallel.
Series Circuits
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 characteristics:
rThe total resistance of the circuit is equal to the sum of each resistor.
rCurrent flow (amperage) through each resistor in the circuit is the same, and is equal to the total amperage through the circuit.
rThe voltage drop across each resistor equals resistance multiplied by the amperage.
rThe source voltage is equal to the sum of the voltage drops across each resistor in the circuit.
If one resistor in a series circuit is disconnected, the path for electron flow is broken, and the entire circuit will not operate.
Figure 14 — Parallel Circuit
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Branch 1 Amperage |
5. |
Total Resistance |
2. |
Branch 2 Amperage |
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Calculation |
3. |
Branch 3 Amperage |
6. |
Total Amperage |
4. |
3.84 Amps (Total Amps) |
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Calculation |
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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.
rTotal resistance of the circuit is always less than the value of the lowest resistor.
rCurrent flow (amperage) through each resistor is different and depends on the value of the resistor.
rThe voltage drop across each resistor is the same, and is equal to the source voltage.
rTotal circuit amperage is equal to the sum of the amperage through each branch.
rIf one resistor in a parallel circuit is disconnected, the remaining circuit still operates.
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DESCRIPTION AND OPERATION
To calculate total resistance in a parallel circuit: |
Series-Parallel Circuits |
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Figure 15 — Calculating Resistance
To calculate total resistance in a parallel circuit with only two branches:
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 resistance value of the series part of the circuit. Once the circuit is broken down into a simple series circuit, amperage, total resistance and voltage drops can be determined. Series-parallel circuits are not used in truck electrical systems very often.
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DESCRIPTION AND OPERATION
OHM’S LAW
Ohm’s Law describes the relationship 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:
To use the Ohm’s Law circle, simply cover the unknown variable, then perform the mathematical operation (either multiplication or division), using the two remaining variables.
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:
Figure 20 — Using the Ohm's Law Circle
To make it simple, the relationship between voltage, resistance and amperage can be described as follows:
rAs voltage increases and resistance remains constant, current increases.
rAs voltage decreases and resistance remains constant, current decreases.
rAs resistance increases and voltage remains constant, current decreases.
rAs 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 (volts) in the series circuit?
Figure 21 — Finding Voltage (Series Circuit)
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 circuit?
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 resistance (ohms). Divide 12 volts by 8 amps of current. What is the total resistance (ohms) in the series circuit?
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).
It is not practical to express these large or small electrical values in pure numeric form, and it is not possible for a meter to display these values.
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 actually 12,300 ohms, not 12.30 ohms.
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 |
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Examples |
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mega |
M |
1,000,000 (or 1 x 106) |
5 MΩ (megaohms) = 5,000,000 ohms or 5 x |
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106 ohms |
kilo |
k |
1,000 |
(or 1 x 103) |
12.30 kΩ (kilo-ohms) = 12,300 ohms or 12.3 x 103 |
milli |
m |
0.001 |
(or 1 x 10-3) |
48 mA (milliamperes) = 0.048 ampere or 48 x 10-3 |
micro |
μ |
0.000,0001 (or 1 x 10-6) |
15 μA (microamperes) = 0.000,015 ampere or |
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15 x 10-6 |
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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 that will be needed are:
Multimeters are available with a variety of functions. All multimeters measure voltage, current and resistance. Some meters can perform additional functions such as quick continuity checks, capacitance checks and diode tests.
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 and cause serious injury.
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 multimeters are recommended because of their accuracy, ease of use, circuit protection capabilities, and are required for troubleshooting circuits containing solid state components or digital circuitry.
Figure 25 — Digital Multimeter (Volt-Ohm Meter)
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Digital Display Screen |
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Milli/Microampere Lead |
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Function Selector |
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Input |
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Switches (continuity |
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Amperage Lead Input |
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check, display hold, |
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Volt-Ohm Lead Input |
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range change, etc.) |
7. |
Function Selector Dial |
3. |
Common Lead Input |
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