Transcript Chapter 16

Chapter 16
General Electrical System
Diagnostics and Service
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Electrical Problems
• Opens
– Are breaks in the circuit that stop current flow
• Shorts
– Are unwanted paths for current
• High Resistance
– Occurs when there is unwanted resistance in a
circuit
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Open Circuits
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Incomplete circuit
Loose wires or connectors
Broken wires
Voltage will be present to the open point
No current flow and no voltage drop
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Shorted Circuits
• A circuit with an unwanted path for current to
flow
• Can cause uncontrolled high current flow
• Caused by damaged insulation, loose wires
and connections, and improper wiring
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Short to Ground
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Shorted Circuits (Cont.)
• Short to ground before the load should open
circuit protection
– Reduced circuit resistance from bypassing the load
increases the current flow
• Short to power can cause unwanted circuit
operation
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High-Resistance Circuits
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Unwanted resistance in a circuit
Higher resistance decreases current flow
Corrosion at connectors is common
The corrosion becomes an additional load and
creates a voltage drop
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• The added
resistance in figure
B reduces the
voltage available
for the light
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Electrical Wiring Diagrams
• Also called schematics
• Does not show location or actual
appearance of components
• Symbols are used to represent components
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Electrical Diagrams (Cont.)
• Diagram may
illustrate one system
or one part of a
complex system
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Connector and Terminals
• Connectors are often
plastic and can contain
one or many terminals
• Terminals are located
inside the connector
• Connectors and
terminals are male or
female
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Electrical Testing Tools
• Multimeter
– Measures the voltage
available at any point
in the circuit
– Also measures
resistance and
current
– May perform other
test functions
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Electrical Testing Tools (Cont.)
• Lab Scopes
– Fast acting multimeters that measure and display
voltages within a specific time frame
– The voltage appears as a waveform or trace
– Upward movement means increasing voltage
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Scan Tools
• Plugs into vehicle
diagnostic connector
• Retrieves fault codes
• Displays sensor data
• Access freeze frames
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Electrical Testing Tools (Cont.)
• Circuit Testers (Test Lights)
– Are used to identify opens
or shorts
– Also used to show current
flow
– Can be non-powered or
powered
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Electrical Testing Tools (Cont.)
• Other Test Equipment
– Jumper wires are used to
bypass a switch or
connector
– Computer memory
savers are installed when
the vehicle battery needs
to be disconnected
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Using Multimeters
• Digital multimeters (DMM) most common
• Also called digital volt/ohmmeters (DVOM)
• Analog meters should not be used on
electronic equipment
• Meters used on HV vehicles should be rated
Category III (CAT III)
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Multimeters
• Display Area
• Range Selectors
– Many meters have
auto range ability
• Mode Selectors
– AC volts, DC volts,
Ohms, Amps, plus
other features
• Jacks/Test Leads
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Measuring Voltage
• A DMM can measure source voltage, available
voltage, and voltage drop
• Voltage is measured with the meter in parallel
to the circuit
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Measuring Available Voltage
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Testing for Voltage Drop
• Voltage drop must be tested with the
circuit energized
• Identifies unwanted resistance
• The voltage indicated is the amount of
voltage used between where the red lead is
connected and where the black lead is
connected
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Testing for Voltage Drop (Cont.)
• 0.1 volts per connection is acceptable in
non-computerized circuits
• 0.050 volts (50 millivolts) per connection is
acceptable in computerized circuits
• Excessive voltage drop is an indication of
high resistance
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Measuring Voltage Drop
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Knowledge Check
• Describe the difference between measuring
voltage and measuring voltage drop.
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Measuring AC Voltage
• Can be displayed as root mean square
(RMS) and average responding
• Both methods will display the same for a
true sine wave
• RMS meters convert the AC signal into a
comparable DC voltage signal
• Average responding displays the average
voltage peak
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Measuring Current
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Tests the circuit under load
Low current indicates high resistance
DMM placed in series into the circuit
Check the meter rating before attempting to
measure current flow
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Inductive Current Probes
• DMM usually limited to
measuring 10A
• To read higher
amperages, an
inductive current probe
or clamp is used
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Measuring Resistance
• Tested with circuit off and disconnected
• Attempting to measure resistance on a
powered circuit may damage the meter
• Resistance can be checked and compared to
specifications
• Can be caused by loose, damaged, or dirty
connections
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Continuity Tests
• Many DMMs have an audible alert when
continuity is present
• Can be used when testing for an intermittent
problem
• Connect the DMM and move wiring and note
when the beeping stops
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MIN/MAX Readings
• Some DMMs have a MIN MAX function
• Displays minimum, maximum, and average
voltage recorded during testing
• Can be used to check for radio frequency
interference (RFI)
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Other Measurements
• Duty Cycle – measured as a percentage of a
devices on time
• Pulse Width – measured in milliseconds
(ms), the amount of time something is on
• Frequency – measured in Hertz, how many
times per second a signal occurs
• Temperature – determined with a
temperature probe
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Duty Cycle
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Pulse Width
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Frequency
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DMM Safety Guidelines
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Use only as designed to be used
Never use a damaged meter
Ensure battery is secured and enclosed
Inspect leads and replace if damaged
Never apply more than rated voltage
Use proper connections, mode, and ranges
Turn circuit off before measuring current
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Using Lab Scopes
• Displays any change in voltage as it occurs
• Screen divided into divisions of time and
voltage
• Time and voltage displays can be adjusted
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Analog vs. Digital Scopes
• Analog scopes display in real time
• Digital scopes, or digital storage oscilloscopes
(DSOs) convert signals into digital information
• Both can be dual trace
• Some DSOs have four channels
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Analog Scope
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Waveforms
• Represents voltage over time
• Any change in amplitude indicates a change in
voltage
• Scopes can display AC and DC voltage
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Scope Controls
• Select the best voltage and time base
setting
– If the time base setting is too long and the
voltage setting is too high, the waveform is too
small to read
– If the time base setting is too short and the
voltage setting is too low, the waveform is too
large for the screen
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Scope Controls (Cont.)
• Select the nearest voltage setting higher
than the expected voltage
• Start at the highest time base and move
down until the proper pattern appears
• Select the proper trigger setting for the
signal being tested
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Graphing Multimeters (GMM)
• Displays voltage, resistance, current, and
frequency as a waveform
• May included a signal and data recorder,
component tests, glitch capture, and audible
alarms
• Some include a library of known-good signals
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Graphing Multimeters (Cont.)
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Transferring Data to a PC
• Many DSOs and GMMs allow for data transfer
to a PC
• Allows for a library of waveforms and other
data
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Testing Basic
Electrical Components
• All electrical components can fail
• Think about what the component is
supposed to do then figure out how to test
the component
• Often, removing and testing on a bench is
the best way to test
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Checking Circuit
Protection Devices
• Ohmmeter
– Tests for continuity across the protection
device
• Test light or voltmeter
– Checks for voltage at both sides of the
protection device
– Checks for voltage drop across the protection
device
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Fuse Testing
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Fuse Links
• Wire with special insulation
• The wire melts to open the circuit
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Circuit Protection Devices
• Maxi-Fuses
– Replacing fuse links
– Easier to replace
• Circuit Breakers
– One type resets by removing power from
circuit
– Other type resets by pressing a button
• Thermistors
– High current flow increase resistance
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Switches
• Disconnect switch and check continuity
between terminals
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Relay Testing Tips
• Never use a test light to test a computercontrolled relay
• Verify the presence of voltage at both the
control and the controlled side of the
relay
• Verify the continuity of the relay coil
• Just because the relay “clicks” doesn’t
mean it is good
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Knowledge Check
• Explain how to use a DMM to test a fuse.
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Stepped Resistors
• Test resistance or voltage after each resistor
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Variable Resistors
• Check resistance or
output voltage
• Both resistance and
voltage changes should
be smooth and
consistent during sweep
• Lab scopes can show
very small defects or
glitches
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Wiring
• Check insulation
– Broken, frayed, or damaged insulation can cause
short circuits
• Inspect ground connections
• Perform a voltage drop test on the wire
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Damaged Wiring
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Printed Circuits
• Flexible printed circuit boards are not
serviceable
• Never touch board surface
• Cleaned with commercial cleaner or by lightly
rubbing pencil eraser across surface
• Easily damaged when replacing bulbs
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Troubleshooting Circuits
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Gather information about the problem
Verify the customer’s complaint
Define the problem and when it occurs
Research all information to determine
possible causes
• Isolate the problem
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Troubleshooting Circuits (Cont.)
• Continue testing to pinpoint the cause of the
problem
• Locate and repair the problem
• Verify the repair
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Troubleshooting Logic
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Remember three types of circuit problems
Perform voltage checks
Take measurements at a connector
Use adaptors to front and back probe
connectors
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Using Wiring Diagrams
• One of the most important resources
• Shows relationships of circuits
• They illustrate:
– Wires by number or color coding
– Wire cross-section size
– Ground connections
– Wire connection points
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Using Wiring Diagrams (Cont.)
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Using Wiring Diagrams (Cont.)
• Diagrams show:
– Reference wire continuation
– Location of splices
– Terminal designations
– Component symbols
– Switches
– Fuse designations
– Relay information
– Circuit continuation
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Getting the Right Diagram
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Should be for the exact year, make, and model
Refer to the VIN
May have system specific diagrams
Some vehicles have only total vehicle
diagrams
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Tracing a Circuit
• Identify components, connectors, and wires
related to diagnosis
• Trace power through the circuit with
highlighters
• Identify power, ground, loads, and controls
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Tracing a Circuit (Cont.)
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Testing for Opens
• If possible, start testing at the load
component
• Check for voltage on the power side of the
component
– If voltage is present, check the ground side
– If voltage is not present, test all connections
working toward the battery
– If voltage is present at a connection, the open is
between it and the previously tested point
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Testing for Opens (Cont.)
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Testing for Opens (Cont.)
• If the power side of the circuit tests good,
test the ground side
– Use a jumper wire to bypass the ground side of
the circuit, and if the component works, check
all ground side connections
• If both the power and ground circuits test
good, the load component is open
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Knowledge Check
• Technician A says an open ground can cause a
component not to operate. Technician B says
an open ground can cause a reading of battery
voltage on the components ground wire. Who
is correct?
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Testing for Shorts
• Use an ohmmeter to check resistance of the
circuit
• If there is a short, the resistance will be less
than specified
• If the short is between two circuits,
components may operate when not intended
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Testing for Shorts (Cont.)
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Testing for Unwanted Resistance
• Typically caused by corrosion, loose or poor
connections, frayed or damaged wires
• Check voltage drop across the load
• Check sections of the circuit to find the
unwanted resistance
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Common Causes of
Wiring Problems
• Corroded terminals
• Loose terminals
• Frayed, broken, or oilsoaked wires
• Faulty insulation
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Wire Repair Considerations
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Type of repair required
Ease of access to the damaged area
Type of conductor
Size of wire
Circuit requirements
Manufacturer’s recommendations
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Wire Repair
• Wrapping with electrical tape or tubing
– Designed to insulate one circuit from another
• Solderless connectors
– Designed to be used in areas not subject to
weather, dirt, corrosion, or movement
• Soldering
– The best way to splice copper wires
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Replacing a Terminal
• Identify the type of terminal and locking
clips
• Remove the terminal
• Install and solder new terminal
• Reinstall the terminal into the connector
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Knowledge Check
• What type of solder should be used on
electrical wiring?
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