CH16 general electrical system diagnostics and service STUDE

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Chapter 16

General Electrical System Diagnostics and Service

Shorted Circuit

• 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

Short to Ground

High-Resistance Circuits

• Unwanted resistance in a circuit

• Higher resistance decreases current flow

• Corrosion at connectors common

• The corrosion becomes an additional load and creates a voltage drop

• The added

resistance in figure

B reduces the

voltage available for the light

Electrical Wiring Diagrams

• Also called schematics

• Does not show location or actual appearance

of components

• Symbols are used to represent components

• Diagram may illustrate one system or one

part of a complex system

Electrical Testing Tools

• Circuit Testers (test lights)

– Are used to identify opens or shorts

– Can be non-powered or powered

Measuring Available Voltage

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

Testing for Voltage Drop

Measuring Voltage Drop

Measuring Current

• 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

Inductive Current Probes

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

• Connect the DMM and move wiring and

note when the beeping stops

DMM Safety Guidelines

• 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

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

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

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.

Fuse Links

• Wire with special insulation

• The wire melts to open the circuit

Circuit Protection Devices

• Disconnect switch and check continuity between terminals

• Check insulation

– Broken, frayed, or damaged insulation can cause short circuits

• Inspect ground connections

• Perform a voltage drop test on the wire

Troubleshooting Logic

• Remember three types of circuit problems

• Perform voltage checks

• Take measurements at a connector

• Use adaptors to front and back probe

connectors

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

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.

Testing for Opens (Cont’d)

• 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

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

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

Common Causes of Wiring Problems

• Corroded terminals

• Loose terminals

• Frayed, broken, or oil-soaked wires

• Faulty insulation

Wire Repair Considerations

• Type of repair required

• Ease of access to the damaged area

• Type of conductor

• Size of wire

• Circuit requirements

• Manufacturer’s recommendations

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