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Chapter 26
Electrical Repairs
Objectives
• Use various kinds of electrical test instruments
• Find electrical problems
• Explain the operation of automotive electricalelectronic systems
• Describe the operation of computer systems
• Use scanners to find electrical-electronic
problems
Introduction
• Electrical systems, such as ignition, charging,
starting, lighting, and computer systems perform
needed functions for a vehicle
• Electrical system repair is an essential aspect of
repairing a collision-damaged vehicle
• Electrical repairs are becoming more common
due to use of computer-controlled systems
– These repairs are complicated by many computer
circuits running throughout the vehicle
Electrical Terminology
• Current is the movement of electricity through a
wire or circuit
• Voltage is the pressure that pushes the electricity
through the wire or circuit
• Resistance is the obstacle to current flow and a
conductor carries current to the parts of a circuit
• “Hot wires” connect the battery positive to the
components of each circuit
• Insulation stops the current flow and keeps the
current in the metal wire conductor
Electric Circuits
• An electrical circuit contains a power source,
conductors, and a load
• The load is part of a circuit that converts
electrical energy into another form of energy
• A series circuit has only one conductor path or
leg for current through the circuit
• If any part of the circuit is “opened”, all of the
series circuit stops working
Electric Circuits (continued)
• A parallel circuit has two or more legs or paths
for current
– One path can be closed and the other opened,
and the closed path will still operate
• A series-parallel circuit has both series and
parallel branches in it
• Ohm's Law calculates an unknown electrical
value when two values are known
Figure 26–2. Ohm’s Law is a simple formula for calculating
electrical values in a circuit.
Magnetism
• Magnetism involves the study of how electric
fields act upon ferrous objects
• A flux or magnetic field is present around
permanent magnets and current-carrying wires
• An electromagnetic field is a set of windings or
wires wrapped around an iron core
• Electric motors, solenoids, relays, and other parts
use this principle
Diagnostic Equipment
• Locating an electrical fault is not possible without
using diagnostic tools
• A multimeter (VOM) is a voltmeter, ohmmeter,
and ammeter combined into one case
• A digital multimeter (DVOM) has a number
readout for the test value
• An analog multimeter (AVOM) has a pointer that
moves across the face of a scale
• Use of an AVOM can damage sensitive
electronic components
Test Light, Jumper Wires
• Test light can determine if current is flowing
through a circuit
• Jumper wires are used to temporarily bypass
circuits or components for testing
Figure 26–4. Jumper wires can be used to bypass resistive parts or to connect
power directly to parts to check their operation. For example, if a motor runs
when connected directly to voltage, you would know the circuit might not be
providing power or a ground to the motor.
Using Multimeters
• A multimeter must never be connected to a circuit
in which current is flowing
• To measure resistance with a multimeter:
– Set the range selector switch on the highest range
– Connect the test leads to opposite ends of the
circuit
– Reduce the range setting until the meter shows a
reading near the middle of the scale
Checking Continuity
• A circuit remains closed and operational when it
has continuity
• The continuity check determines if the electrical
circuit has a complete path without any opens
– Set the range selector switch on the highest
resistance range position
– Connect the multimeter test leads to the opposite
ends of the wire or part being tested
– Read the meter
• An infinite reading shows an open circuit and a
zero reading shows continuity
Measuring Voltage
• The multimeter allows you to select either
alternating current voltage (ACV) or direct current
voltage (DCV)
• AC current is the current found in your home
• DC current is normally measured in an
automobile
• Some signals from sensors can be AC
• Current or amperage is sometimes measured to
check the consumption of power by a load
Measuring Voltage (continued)
• Modern ammeters have an inductive pickup that
slips over the wire or cable to measure current
• A high current draw indicates a low resistance
• A low current draw indicates a high resistance
Checking for Shorts
• When checking for a short between two adjoining
wires in a harness:
– Set the range selector as for a continuity check
– Connect the test leads to the opposite ends of the
adjoining wires
– The multimeter should read almost infinity
Wiring Diagrams
• Wiring diagrams are used to determine and
isolate problems
• Abbreviations are used on wiring diagrams so
that more information can be given
• Electrical symbols are graphic representations of
electrical-electronic components
• Wire color coding allows you to find a specific
wire in a harness or in a connector
Wiring Diagrams (continued)
• Circuit numbering specifies which part of the
circuit the service manual is referring to
• Wiring harness has several wires enclosed in a
protective covering
• Service manual gives illustrations with code
numbers for locating parts and connections
– May also give a part location diagram for finding
electrical parts
Electric Components
• A switch turns a circuit on or off manually
• A solenoid is an electromagnet with a movable
core or plunger
• A relay is a remote control switch
• A bad relay will often have burned points that
prevent current flow to the load
• Motors use permanent and electromagnets to
convert electrical energy into a rotation motion
Circuit Protection Devices
• Prevent excess current from burning wires and
components
• With an overload or short, too much current tries
to flow
• Without a fuse or breaker, the wiring in the circuit
would heat up
Fuses
• Fuses burn in half with excess current to protect
a circuit from further damage
• The cartridge fuse is found on most older
domestic vehicles and a few imports
• Domestic vehicles use blade or spade fuses;
European imports may use ceramic fuses
• A blown fuse has infinite resistance
– Fuse ratings are current at which a fuse will blow
• A fuse box holds various circuit fuses, breakers,
and flasher units for turn and emergency lights
Fuse Links
• Fuse links (fusible links) are smaller-diameter
wire spliced into the larger circuit wiring
– Ensures over-current protection
– Normally in engine compartment near battery
– Often installed in the positive battery lead that
powers the ignition switch
– Covered with a special insulation that bubbles
when it overheats
Circuit Breakers
• Circuit breakers heat up and open with excess
current to protect the circuit
• Many circuits are protected by circuit breakers
• Each circuit breaker conducts current through an
arm made of two types of metal bonded together
• The arm heats up if it carries too much current
• As one metal expands farther than the other, the
arm bends, opening the contacts and breaking
the current
Electrical-Electronic Systems
• Ignition system produces an electric arc in a
gasoline engine to cause the fuel to burn
• Ignition coil is a step-up transformer that
produces high voltage needed to make the
current jump the spark plug gap
• Spark plug wires are high-tension wires that
carry coil voltage to each spark plug
• Spark plugs ignite the fuel mixture in the engine's
combustion chambers
Starting and Charging Systems
• Starting system has a large electric motor that
turns the engine flywheel
• Ignition switch connects battery voltage to a
starter solenoid or relay
• Starter solenoid, when energized, connects the
battery and the starting motor
• Starting motor is a large DC motor for rotating the
engine flywheel
• Flywheel ring gear meshes with the startermounted gear while cranking
Starting and Charging Systems
(continued)
• Charging system recharges the battery and
supplies electrical energy to the running engine
• Voltage regulator controls alternator output and
charging system voltage is 13 to 15 volts
• To check for spark
– Pull off one spark plug wire, install an old spark
plug; lay the spark plug on the engine ground
– When you crank the engine a bright spark should
jump across the spark plug gap
• To check for fuel, a pressure gauge on the
engine's fuel rail should read within specs
Electrical Problems
• Open circuit is an unwanted break in a circuit
• Short circuit is an unwanted wire-to-wire
connection in a circuit
• In a grounded circuit the current flows to ground
without flowing to the part
• Abnormal resistance is due to a bad connection
or partial short
• Diagnostic charts give possible causes for
electrical problems and symptoms
Batteries
• Battery stores electrical energy chemically
• Voltage spikes are voltage surges that can
destroy may microcircuits in electronic systems
• Good idea to remove the battery before doing
any kind of collision repair work
• Once the battery has been placed on a bench, it
should be checked for damage
• A voltmeter or a specialized battery tester checks
battery condition
Batteries (continued)
• Follow an eleven-step procedure for charging a
battery using a battery charger
• Avoid jump starting whenever possible
• Jumper cables connect two batteries when one is
“dead”
• Special care is necessary when charging or jump
starting to avoid damaging computer circuits
• Before reinstalling the battery after body work,
clean its terminals
Figure 26–13. Jumper cables or a battery charger must be connected carefully to
prevent sparks or electronic component damage. Connect positive to positive and
negative to negative in the proper sequence: red to red, then black to any metal
ground away from the battery.
Battery Safety
• Keep batteries away from the welding
operations, open flames, sparks, or heat sources
• Do not charge batteries with cracked case
– Do not smoke near batteries
• Ventilate the area around dead or damaged
batteries to avoid explosion
• Do not charge a frozen battery
• Electrolyte in the battery is a mixture of water and
sulfuric acid and must be kept in mind
Lighting and Other Electric Circuits
• Lighting system feeds electricity to the light bulbs
throughout the vehicle
• When lights fail to function, check the bulb first
– If the bulb is good, trace for an open feeding
current to the dead bulb
• Other electrical circuits (horn, power windows,
etc) use the same principles just discussed
Repairing Wiring and Connections
• When servicing electrical wiring, never tug on the
connectors
• Route wiring in the same location as the OEM
• Protect the electrical connectors from moisture
and corrosion by using dielectric grease
• Use the same size and type of wiring for repairs
Solder Repair
• Soldering uses moderate heat and solder to join
wires or other parts
• All copper wire joints should be soldered
• Use rosin-core solder, not acid-core, when
soldering electrical connections
• Acid fluxes create corrosion and can damage
electronic components
• Electrical tape can be used around wire splices
when they will not be exposed to moisture
Electrical Connector Service
• An automotive electrical connector includes two
plastic, snap together fittings
– Allow several wires to connect together securely
• Various connector designs are used on vehicles
– Each requires a different method for disconnection
• If needed, inspect electrical connectors when
trying to find opens
Computer Systems
• Basic computer system consists of sensors,
actuators, and a computer
• Sensors are devices that convert a condition into
an electrical signal
• Actuators are devices that move when
responding to signals from the computer
• The computer is a complex electronic circuit that
produces a known electrical output after
analyzing electrical inputs
Scanning Computer Problems
• Scan tool is the fastest way to use on-board
diagnostics to find electrical-electronic problems
• Scan tool “talks” to the vehicle's computers and
can tell you if it detects any problems
• Scanner will convert computer or electronic
control module (EMC) data into plain English
• Scan tools are removable cartridges that hold
information about the specific vehicle
• Menu allows you to scan different test functions
Figure 26–17. Numerous sensors provide inputs for the computer. The service
information for the specific make and model vehicle would be required.
Using the Scanner
• Connect the scan tool to a diagnostic connector
on the vehicle if an indicator light glows
• Turn off all accessories that could trip false
trouble codes
– Prepare tool for make and model being tested
• Select the information you want from the scan tool
• Correct the cause of the lower number code first
• Erasing trouble codes removes the stored codes
from the computer memory
Summary
• Current is the movement of electricity through a
wire or circuit, measured in amps
• Voltage is the pressure that pushes the electricity
through the wire or circuit, measured in volts
• Resistance is a restriction or obstacle to current
flow, measured in ohms
• Some diagnostic equipment used in finding
electrical faults includes the multimeter, test light,
and jumper wires
Summary (continued)
• Wires are often severed and electrical and
electronic components damaged during a
collision
• Scan tool helps find the location of circuit faults
• Scan tools display which circuits or components
are not operating within specifications