meter usage and circuit diagnosis -...

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CHAPTER 37

Meter Usage and Circuit Diagnosis

Introduction

• Electrical measuring tools to diagnose and repair electrical faults:– Digital volt-ohmmeters

– Oscilloscopes

Digital Volt-Ohmmeter (1 of 5)

• DVOM– Numerical reading on

digital display

– Likely to be first test tool selected

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• Basic DVOMs measure:– AC and DC voltage

– AC and DC amperage

– Resistance

• Most also measure:– Frequency

– Temperature

• Have dedicated diode test capability


• A variety of layouts and quality– Learn capabilities and how to use ones in shop.

– Most average-quality ones are “fused.”

• If amperage too high, fuse blows to protect meter.

• If not fused, meter is not protected.


• DVOMs and test leads have CAT ratings.– Each category designed to work safely on higher-

powered electrical systems

– Designed for high voltages

• Hybrids—usually CAT III or CAT IV

• Electric vehicles

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• When working on high-voltage systems:– Wear certified and tested rubber-insulated gloves.

– Use proper CAT-rated meter and leads.

– Use proper personal protective equipment.

Digital Volt-Ohmmeter—Use

• Can take many different measurements– Measures electrical voltage within circuits

(as voltmeter)

– Measures resistance of:

• Component

• Connector

• Cable

Digital Volt-Ohmmeter—Components (1 of 2)

• Two components– Main instrument body

– Test leads connecting to circuit

• Main instrument body has:– Function switch

– Digital display

– Sockets

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Digital Volt-Ohmmeter—Components (2 of 2)

• Leads—pairs of one red, one black– Basic leads—probe on one end, connector on other

– Wide variety of test leads and adapters

• Alligator clips

• Temperature probes

• Inductive current clamps

Digital Volt-Ohmmeter—Ranges and Scales (1 of 3)

• DVOMs read very small quantities in resistance measurements.– In range of millions of units

– Impossible to accurately measure with single range of scale

– Screen only displays four or five digits.

Digital Volt-Ohmmeter—Range and Scales (2 of 3)

• Symbols are substituted for some digits.– Electrical symbol (V, A, or Ω) is placed behind factor

symbol.

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Digital Volt-Ohmmeter—Range and Scales (3 of 3)

• Most DVOMs have both automatic and manual ranging capabilities.– Auto range—DVOM selects best range for value.

• Meter does not give flashing warnings if range changes.

Digital Volt-Ohmmeter—Min/Max and Hold Setting (1 of 4)

• Special settings assist in measuring rapidly changing values or freeze displays.

• Setting records in memory the minimum and maximum reading while connected to source.


• Used to measure vehicle battery voltage while engine cranks or battery charges– Current highest during cranking, but only fraction

of a second

– Battery lowest when cranking

– Min/max mode captures those numbers.

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• Sample rate—the speed at which DVOM can sample voltage– DVOM checks voltage at regular intervals.

– Transient voltage occurring between samples may not be recorded.

– Use other tools if quicker sample rates needed.


• Hold function allows display to freeze.– Display holds value until function or DVOM turned off.

– “Auto hold” capabilities on some DVOMs

• Useful when difficult to watch display while making connections

Digital Volt-Ohmmeter—Setting Up a DVOM (1 of 2)

• Need to know:– If measuring resistance, voltage, or current

– The expected reading

• Will help determine:– Connections to make

– Range to select

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Digital Volt-Ohmmeter—Setting Up a DVOM (2 of 2)

• To set up a DVOM:– Select leads and probes needed for task.

– Connect leads to DVOM.

– Use function switch to select measurement type.

– Select correct meter range (for manual).

– Connect leads to circuits being tested.

– Read meter display.

Digital Volt-Ohmmeter—Test Leads: Common and Probing (1 of 3)

• Meter near test lead terminals labeled with:– A (typically 10 A)

– mA

– Common

• Common to all functions of meter

– V/Ω

• Red lead does move, depending on function


• When various electrical signals are measured at same time on an oscilloscope, need more than just red lead.– Yellow, blue, and green test leads also act as probes.

– “Probing lead” is more accurate than “positive lead.”

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• Meter screen reads what probing lead touches.– If probing lead touches positive post, meter screen

will display “+” before reading.

– If it touches a negative post, will display “–.”

Digital Volt-Ohmmeter—Probing Techniques (1 of 3)

• Different types of probes: – Alligator clips

– Fine-pin probes

– Insulation piercing clips

– (High voltage needs special probes)


• Do not use excessive force when probing.

• Standard probe leads:– Basic straight metal probes

– Require both hands to hold in place

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• Leads with alligator clips– Can clip on circuit, freeing up hands

– Useful for connecting to larger terminals

• Back-probing—pushed through insulation from back of connector– Fine-pin probes used to reduce damage possibility

– Always reinsulate.

Digital Volt-Ohmmeter—Measuring Volts, Ohms, and Amps (1 of 7)

• Most common measurements:– Voltage

– Resistance

– Current


• For voltage measurements:– Connect probing lead to V/Ω terminal.

– Connect common lead to COM terminal.

– Select range or auto range.

– Probing lead is usually connected to positive side.

– Common lead is usually connected to negative side.

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• Most measure milliamps or 10 to 20 amps.– Select correct range.

– Connect red probe to A terminal.

– Connect black probe to COM terminal.

– May have a separate mA terminal to measure milliamps


• Measuring current– Connect DVOM in series with the circuit.

– Probing lead closest to positive terminal

– DVOM may have internal fuse that blows if current is too high.


• Measuring larger amperage– Connect clamps to measure high currents.

– Fasten current clamp around conductor to measure magnetic field strength in current flowing through conductor.

– Clamps instead of breaking into circuit to insert DVOM

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• Measuring component resistance– Remove component from circuit.

– Disconnect power.

• If not, may get false reading or damage the DVOM.


• Measuring resistance– Connect red lead to the V/Ω terminal.

– Connect black lead to the COM terminal.

– Select range or auto range.

– Red lead connected to one side

– Black lead connected to other side

Voltage Exercises

• Designed to explain use of DVOM in taking DC voltage measurements– Use of different ranges on the meter display

– Voltage drops in series circuits across equal and unequal loads

– Kirchoff’s voltage law—sum of series voltage drops equals the supply voltage

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Voltage Exercises—Ranges (1 of 2)

• DVOMs have capability for:– Auto range

– Manual range

• DVOMs have different range settings.– 6 V, 60 V, and 600 V

– 4 V, 40 V, and 400 V

Voltage Exercises—Ranges (2 of 2)

• A circuit with two resistors in a series with a 12-volt DC supply– Compare ranges by

measuring voltage drops across each resistor.

Voltage Exercises—Voltage Drop (1 of 12)

• Measured with a voltmeter

• Potential difference between two points in a circuit

• Sum of all voltage drops in a series circuit equals the supply voltage.

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• Voltage drop across all parallel circuit branches is the same.– Does not occur in all parts of the circuit

– Vast majority is across the component or load that does the work.


• Unwanted voltage drop may be excessive.– May be in other parts of circuit besides the load

• Example: Only resistance should be in headlight bulb

• Resistance also in cable and connectors

• If resistance high here, circuit efficiency is low


• Excessive voltage drop—fault in circuit

• To test for unwanted voltage drop:– Measure voltage drop across each circuit part and

add voltage drops together.

• In 12-volt system—total drop across each side should not exceed 0.5 (in 24-volt, 1.0 volt)

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• To measure voltage drop:– Set DVOM on the voltage range.

– Set function switch to “auto range volts DC.”

– Connect black lead to COM.

– Connect red lead to V/Ω.


• Can be measured across:– Components

– Connectors

– Cables

• Probing lead normally connected to circuit point where voltage needs to be checked


• Example: Voltage drop test on feed side of horn circuit– Connect black lead to positive terminal.

– Connect red lead to input wire of horn.

– When horn is activated, voltmeter reads amount of voltage drop in feed side.

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• If –4.2 volts, voltage is 4.3 volts less at input of horn than at positive battery post.– Drop more than 0.5 volts—excessive voltage drop

– Check wire by wire until voltage drop point is located.


• Same measurements can be done with DVOM leads reversed.– Red lead on positive battery post

– Black lead on input of horn

– Meter will read 4.2 volts, showing positive.


• Voltage in various parts of the circuit are measured with the switch in open position.

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• Series circuit of two resistors with a 12-volt battery and switch– Voltage in various

areas is measured with switch in closed position.


• Unwanted voltage drops in circuits – Corroded or bad chassis ground reduces voltage and

current available.

– Simple circuit with a bulb connected via a switch across a 12-volt circuit.

Voltage Exercises—Voltage Drop Across Multiple Loads (1 of 2)

• DVOM on voltage range– Select “auto range volts DC.”



– Current needs to flow for accurate measurement.

– Leads can be placed in either direction.

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Voltage Exercises—Voltage Drop Across Unequal Loads (2 of 2)

• Set DVOM on voltage range.– Select “auto range volts DC.”



– Measure voltage drop only when current flows.

– Leads can be placed in either direction.

Current Exercise (1 of 4)

• Exercise explains DVOM use for taking DC current measurements.– Discusses Ohm’s law and current measurements

– Demonstrates measuring current

– Shows magnetic fields around a conductor during current flow


• Current is the same in all parts of a series circuit.– Ammeter must be connected in series.

– To ensure all current flows through the ammeter:

• Circuit must be broken in two.

• Ammeter is connected to one of two broken ends.

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• Voltage measurement:– Select “auto range amps

DC.”

– Connect red lead to A socket.


• Current measurement:– Select “auto range amps

DC.”




• A circuit with a single resistor with a 12-volt DC supply– DVOM used to

measure both voltage and current

Current Exercises—Measuring Current (1 of 2)

• To measure current:– Select “auto range amps DC.”


– Connect black lead to COM socket.

– Select appropriate range if using manual.

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Current Exercises—Measuring Current (2 of 2)

• A circuit with two resistors in a series with a 12-volt DC supply– Connect DVOM to

various circuit parts to measure current flow.

Current Exercises—Current and Magnetic Fields (1 of 3)

• Example: Relay controlled by switch used to switch current through a resistor– Compass demonstrates a magnetic field is produced

around relay winding when current flows through it.


• To conduct experiment:– Set DVOM to measure “DC amps.”



– Select appropriate range if using manual.

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• Circuit with relay controlled by switch and single resistor with a 12-volt DC supply– Compass shows that when energized, relay winding

produces magnetic field.

– DVOM measures current flow.

Resistance Exercises (1 of 2)

• Exercises show how to use DVOM to measure resistance.– Check components/circuits against specs.

– Examples demonstrate:

• Measuring resistance

• How resistance affects current flow

Resistance Exercises (2 of 2)

• Current flow is inversely proportional to resistance.– The higher the resistance, the less current that

will flow.

– The lower the resistance, the higher the current flow.

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Measuring Resistance (1 of 3)

• For resistance measurements:– Select “auto range Ω.”



– If manual, select by starting at highest range and working down.


• A circuit with a lamp in series with a resistor and a 12-volt DC supply


• DVOM is used to measure resistance.– The measurement

to be expected from the circuit

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Resistance Exercise 1 (1 of 3)

• Measure resistance, voltage, and current. For resistance and voltage measurements:

• Select “auto range volts DC.”

• Connect red lead to V/Ω.

• Connect black lead to COM.

• Do measurements with component disconnected.


• For current measurements:– Select “auto range amps DC.”

– Connect red lead to the A socket.



• A circuit with a resistor and a 12-volt DC supply– DVOM measures

resistance, voltage, and current.

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• Measure resistance, voltage, and current. – For resistance and voltage measurements:




• Disconnect component before measuring.






• Circuit with resistor and a 12-volt DC supply

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• Measure resistance, voltage, and current. For resistance and voltage measurements:– Select “auto range volts DC.”



– Disconnect component before measuring.






• A circuit with a resistor and a 12-volt DC supply in a series with a LED– DVOM will measure resistance, voltage, and current

through R1, a 100-ohm resistor.

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• Measure resistance, voltage, and current. For resistance and voltage measurements:– Select “auto range volts DC.”



– Disconnect component before measuring.






• A circuit with a resistor and a 12-volt DC supply in series with an LED– Measures through R1,

a 10-kΩ resistor

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Series Circuit Exercises (1 of 2)

• Examples:– Demonstrate use of measuring voltage and current

– Describe how current flows

– Describe how voltage drop and current are affected by resistance

Series Circuit Exercises (2 of 2)

• Current flow is the same in all parts of a good series circuit.

• Sum of voltage drops across individual resistors is equal to supply voltage.

• Resistor additions affect current flow and voltage drops.

Series Circuit Exercise 1 (1 of 2)

• Measure voltage from series circuit.– To measure voltage drop:

• Set DVOM on voltage range.



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Series Circuit Exercise 1 (2 of 2)

• Can measure voltage drop across components, connectors, and cables

• Current flow for accurate measurements

• Can place leads in either direction

Series Circuit Exercise 2





• Current must be flowing for measurement.






• Allow current flow for voltage drop measurement.

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• Measure voltage from series circuit.– Select “auto range volts DC.”



– Allow current flow for voltage drop measurement.


• To measure voltage drop:– Set DVOM on voltage range.

– Select “auto range volts DC.”



– Allow current flow for measurement.


• To measure voltage drop:– Set DVOM to “DC amps.”



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Parallel Circuit Exercises (1 of 3)

• Exercises explain DVOM use in measuring volts, amps, and ohms in a parallel circuit.– Commonly used in electrical systems

– Understanding relationship between voltage, amperage, and resistance helps diagnose electrical faults.


• Examples demonstrate:– How to measure volts, amps, and ohms

– How current flows and voltage drops in a parallel circuit


• Laws for parallel circuits– Resistance goes down when more parallel paths

are added.

– Current flow from individual legs add up in parallel.

– Voltage stays the same at all common inputs.

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Parallel Circuit Exercise 1 (1 of 2)

• Measure voltage from parallel circuits.– Set DVOM to measure voltage.




– Allow current flow for voltage drop measurement.


• Three resistors connected in parallel across a 12-volt supply


• To conduct this exercise:– Set DVOM to “DC amps.”



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Parallel Circuits Exercise 2 (2 of 2)

• Circuit with a single resistor and a 12-volt DC supply– DVOM can be

connected in various circuit parts to measure flow.






• Two resistors in parallel– Additional resistor in

parallel causes an increase in circuit current flow, decrease in total circuit resistance.

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• Three resistors in parallel– The additional resistors

in parallel cause an increase in total circuit current flow, decrease in total circuit resistance.





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• Four resistors in parallel– Additional resistors in

parallel cause an increase in circuit current flow, even more decrease in total circuit resistance.

Series-Parallel Circuit Exercise (1 of 6)

• Exercises explain DVOM in measuring current and voltage in series-parallel circuits.– Found in dash light dimmer and similar circuits

– Used when unwanted resistance shows up in parallel circuits


• Example demonstrates:– Measuring voltage and current

– How current flow and voltage drop are affected by resistance

• To analyze and calculate current flow and voltage drop, consider total resistance.

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• Voltage drop across parallel branch is the same for all resistances in the branch.

• Sum of current flow in each branch equals total parallel circuit current flow.

• Exercises examine how resistor additions affect the circuit current and resistance.


• Voltage and current measurements taken from circuit formed by resistors:– R1

– R2

– R3

– R4


• To measure voltage drop:– Set DVOM on voltage range.




– Allow current flow for measurement.

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• To measure current:– Set DVOM to read DC amps.



Variable Resistors (1 of 2)

• Exercises explain how DVOM measures voltage and amperage with a variable resistor and a potentiometer.

• Understand relationship between voltage, resistance, and current as variable resistor is adjusted.

Variable Resistors (2 of 2)

• Example demonstrates:– Measuring voltage and current

– How current flows

– How voltage drop and current are affected by potentiometer wiper

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Variable Resistors Exercise (1 of 3)

• Variable resistor used as a potentiometer– To measure voltage:





• For current measurements:– Select “auto range milliamps DC.”



– If manual, select appropriate range.


• Example:– Circuits with a 250-Ω variable resistor as a voltage

divider with a 12-volt DC supply

• Variable resistor continuously variable

• Voltage V1 a divider output from variable resistor VR1

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Capacitors (1 of 2)

• Exercises: – Explain DVOM use in current and voltage in circuit

with a capacitor connected in a series with a lamp

– Show how voltage and current change as the capacitor charges

Capacitors (2 of 2)

• Examples demonstrate:– Measuring voltage and current

– How current flow and voltage drop change as a capacitor charges

• As capacitor charges, voltage drop increases and current flow decreases.

Capacitors Exercise (1 of 3)

• Capacitor connected in a series with a lamp– For voltage measurement:




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• For current measurements:– Select “auto range milliamps DC.”




• Capacitor connected in series with a lamp with 12-volt DC supply– DVOM measures

voltage across capacitor and lamp when switched on.

Waveforms and Scope Testing—Oscilloscopes (1 of 5)

• Provide a graph on screen of voltage values (waveform)

• Have:– Screen

– Control knobs

– Test sockets

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• Designs and specifications vary by manufacturer.

• Designed to generate electronic measurements– Specifically designed for automotive work

• Special ranges and leads


• Oscilloscopes – Analog—displaying waveforms as they occur

– Digital (DOS)—display waveforms as they occur and store for later analysis

– May incorporate their own screen or connect to computer


• Have a very fast sampling rate– Can take many samples per second

• Some take a sample every millionth of a second.

• Advantages over DVOMs:– Signal capture of much higher speeds

– Can display waveforms

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• Usually have two or more channels– Four channels common

– Can compare waveform timings to each other and to manufacturer specifications

• Voltage displayed on vertical axis

• Time displayed on horizontal axis

Waveforms and Scope Testing—Checking Circuit Waveforms (1 of 5)

• Automotive oscilloscope kits with specialized leads and probes:– Ignition

– Fuel injection

– Current probes

– Temperature

– Pressure


• Correct setup is essential to read waveforms without damaging circuit or scope.– Two critical settings:

• Voltage

• Time base

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• Voltage scale – Manual

– Automatic range function

• Time base—amount of time being read per division on screen– Typically 10 divisions, listed horizontally


• Measures very fast signals– 100 nanoseconds to 200 seconds per division

• Capable of reading volts as low as 50 millivolts up to 50 or 100 volts– Higher volts may be measured.

• Example: Primary voltage of ignition coil


• Attenuators to probe leads reduce maximum voltage.– Reduce amount of voltage

• Connect leads according to specs.

• Ground lead is connected directly to negative battery terminal.

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Electrical Circuit Testing

• To diagnose electrical faults:– Understand circuit types and electricity.

– Ability to use meters/oscilloscopes to measure:

• Voltage

• Amperage

• Resistance

– Ability to read wiring diagrams

Electrical Circuit Testing—Using Ohm’s Law (1 of 11)

• Ohm’s law can be used to:– Perform math to predict and verify measurements

– Use relationships demonstrated for diagnosis


• Ohm’s law to calculate electrical quantities– Cross-checking measured results within circuit

• Example: If resistance and voltage are known, can calculate theoretical current.

• Calculated result can be compared to results measured with an ammeter.

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• Quick calculation is often done for an approximate value before actual measurements are taken.– Helps set the measuring tool to correct range

– Calculations yield approximate values only.


• Used to find relationships between:– Volts

– Amps

– Ohms

• Example: If voltage stays the same but resistance decreases, amperage increases.


• Ohm’s law regarding voltage changes– If voltage decreases and resistance stays the same,

amperage will decrease.

– If voltage increases and resistances stays the same, amperage increases.

– If amperage and voltage both increase, electrical power increases.

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• Amperage is a result of voltage and resistance:– Cannot exist without them

– Amperage does what voltage and resistance allow.

• If amperage is low: – Voltage low

– Resistance high

• If amperage is high:– Voltage high

– Resistance low


• Amperage is a product of voltage and resistance.– If circuit fault, amperage will be either high or low.

– In most cases, flow is obvious.

• Fuse blown from too-high current

• Light dimmed from too-low current


• If current is low:– Either voltage is low or resistance is high.

• Ohm’s law

• If current is high:– Either voltage is high or resistance is low.

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• Example: Left front headlight is dim.– Current low, with either low voltage or high resistance

– Voltmeter to measure voltage at battery

– If low, determine why.


• If voltage at battery is good:– Check voltage across both sides of headlight (with

circuit on).

• Should be within 1.0 volts of battery voltage

• If not, look for high resistance.

– Measure voltage drop on each side.

– If excessive on one side, follow back toward battery to identify cause.

– If voltage on both sides are within specs, problem likely headlight.

» Compare to a known good bulb.


• If current flow is too high:– Either too much voltage or too little resistance

– Measure battery voltage.

– Ohmmeter to check load resistance and compare to specs to find any shorts

– If not, ohmmeter to check wire harness for short circuits

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Electrical Circuit Testing—Using Wiring Diagrams to Diagnose Electrical Circuits

(1 of 6)

• Wiring diagrams or schematics available as:– Paper-based manuals

– Computer programs

– Online resources

• Often repair information via Internet by regularly updated subscription services


(2 of 6)

• Must understand symbols, abbreviations, and connector coding

• Like reading a road map:– Interconnect circuits

– Wires

– Components


(3 of 6)

• Circuits usually consist of:– Power source

– Switch

– Load

– Ground

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(4 of 6)

• Jorge Menchu color-crayon approach to understanding circuits in wiring diagrams


(5 of 6)

• On copy of wiring diagram, color:– Wires directly connected to ground—green

– Wires that are “hot” all the time—red

– Wires “switched to power”—orange

– Wires “switched to ground”—yellow

– Wires that reverse polarity—side-by-side orange and yellow lines

– Variable wires—blue


(6 of 6)

• Coloring wires on wire diagram:– Forces determination of what each wire does

– Helps organize and understand how electricity flows through circuit

– Helps prevent losing place or forgetting what a wire does

– Helps in diagnosis

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Electrical Circuit Testing—Using a DVOM to Measure Voltage (1 of 12)

• In most measurements, set to auto range– Select leads and probe ends to match task.

• Example: If needing hands free, use alligator clips.

– Do not exceed maximum allowable voltage or current for DVOM.


• Use appropriate personal protective equipment for high voltages.– High-voltage safety gloves

– Long-sleeved shirt and pants

– Protective eyewear

• Remove items that may cause short circuits.


• Simple voltage test– Place common lead on good ground.

– Place probing lead on input side.

– Gives reading on amount of voltage at probing lead

– Does not determine voltage started with or how much did not make it through the circuit

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• Voltage drop test– Voltage drop occurs when current flows through

a resistance.

• Higher the resistance, higher the voltage drop

• Measures for excessive resistance

• Always have current turned on.


• Two ways to do a voltage drop test– Direct method—both test leads on same side of

circuit

• Directly reads how much voltage is lost between two points


• Two voltage drop tests (cont’d)– Indirect method—

black lead on negative battery terminal at all times

• Probing lead moved from one point to another

• Second reading subtracted from first reading

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• Placing leads in circuit for direct voltage drop test– If checking positive side of circuit:

• Probe with red lead.

• Place black lead on positive battery post.


• Example: Measure voltage drop on positive side of low beam filament on left headlight.– Voltmeter reads –0.71 on low beam.

• Voltage is 0.71 less than voltage at black lead.

• 0.5-volt drop is maximum allowed.


• Example: Check voltage drop on ground side of circuit.– Place black lead on output terminal of headlight.

– Place red lead on negative battery terminal.

– Turn on low beam light, measure voltage drop.

– Meter reads –0.24 volts.

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• To check a different way:– Place black lead on negative battery terminal.

– Place red lead on output side of headlight.

– Turn headlight on and take reading.

– Measures 0.25 volts


• Indirect method:– Take two voltage readings.

– Subtract them from each other to determine drop.

– Useful when:

• Working far from battery

• Connecting voltmeter leads on battery not possible


• Base reading: Measure voltage at battery with electrical device on.

• Second measurement is taken at load being tested, circuit on.

• Subtract voltage at load from battery voltage– Difference is voltage drop.

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Electrical Circuit Testing—Checking Circuits with a Test Light (1 of 3)

• Nonpowered test lamps determine if electrical power is present in part of circuit.– Test the test light on good power and a ground; if not:

• Circuit missing one or both elements

• Test light faulty


• Good for simple tests

• Test light lead can be grounded quickly and probe end touched to each end of fuse.– If both light, fuse is good.

– If only one end lights, fuse is blown.


• Circuit voltage is not to exceed test light rating.

• Most test lights are rated for 6- to 12-volt system.

• Do not use a test light to test SRS.

• Using test light on computer circuit designed for very small current flows can damage the circuit.

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Electrical Circuit Testing—Checking Circuits with Fused Jumper Leads (1 of 2)

• Can be used to assist in checking circuits

• Can be created or purchased in range of:– Sizes

– Lengths

– Fittings (connectors)

Electrical Circuit Testing—Checking Circuits with Fused Jumper Leads (2 of 2)

• Used to extend connections to allow circuit readings or tests with:– DVOM

– Oscilloscope

– Current clamps on fuses

– Relays or connector plugs

Electrical Circuit Testing—Locating Opens, Shorts, Grounds, and High Resistance (1 of 2)

• Tools most often used to locate opens, shorts, grounds, and high resistance:– DVOMs

– Test lamps

– Simulated loads

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Electrical Circuit Testing—Locating Opens, Shorts, Grounds, and High Resistance (2 of 2)

• Open circuit—break in electrical circuit– Power supply or ground circuit interrupted

– Often can be located by:

• Probing along various points of circuit

• Checking for effective grounding at ground point

Electrical Circuit Testing—Locating Open Circuits (1 of 2)

• Systemic circuit check required– Voltage drop check on each side of circuit

– Will cause voltage drop equal to source voltage

– Once side of drop is found:

• Continue voltage drop testing on that side by working leads closer together in steps.

Electrical Circuit Testing—Locating Open Circuits (2 of 2)

• Consider most likely places for open circuit.– Blown fuse

– Faulty switch

– Open load

• If within specs, check for open load with ohmmeter or manufacturer diagnostics.

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Electrical Circuit Testing—Locating Short Circuits (1 of 3)

• Short circuit—circuit fault in which current travels along an accidental/unintended route– Can occur anywhere in the circuit

– May be intermittent

– May occur within load or in wiring


• Causes lower-than-normal resistance– Abnormally high current flow

– Circuit protection devices open circuit.

– Circuit may remain live after switched off.

• Possible causes: Faulty components or damaged wiring


• Best tested by comparing ohmmeter reading to specifications

• Shorts in wire harnesses best tested by:– Disconnecting each end

– Using ohmmeter to test for unwanted continuity between wires

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Electrical Circuit Testing—Locating Grounds (1 of 2)

• Grounds—reference to short to ground– Conduct initial test by resistance checks or

disconnecting the load.

• Example: To test blower motor, disconnect it.

• If short still in place, wiring between fuse or circuit breaker and load is at fault.

Electrical Circuit Testing—Locating Grounds (2 of 2)

• Connect a test lamp or buzzer in place of a fuse and find a ground through the short.– Disconnect parts of circuit to narrow location.

• Specialized short circuit detection tools– Send signal through wiring harness

– Receiving device moves along wire loom to indicate location.

Electrical Circuit Testing—Short to Power

• Short to power—when power from one circuit leaks into another circuit– Causes strange electrical issues:

• One or more circuits operate when they shouldn’t

• In sensor wires, incorrect signals cause computer to receive faulty data.

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Electrical Circuit Testing—High Resistance (1 of 3)

• High resistance—circuit with unintended resistance– Can be caused by:

• Corroded or loose harness connectors

• Incorrectly sized cable for circuit current flow

• Incorrectly fitted terminals or poorly soldered joints


• Causes unintended voltage drop when current flows– Reduces amount of voltage available to the load

– Reduces current flow in the circuit

– Reduction in voltage and current to the load reduces the amount of electrical power to load.


• Locate by voltage drop testing in power and ground circuits.

• If within the load:– Check resistance with ohmmeter.

– Some devices may need further testing.

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Electrical Circuit Testing—Inspecting and Testing Circuit Protection Devices

(1 of 4)

• Designed to prevent excessive current from flowing in the circuit– Sacrificial—will blow or trip to prevent excessive

current flows

• Fuses

• Fusible links


(2 of 4)

• Circuit breakers can be reset—either automatically or manually.

• Available in various ratings, types, and sizes– Must be replaced with the same rating and type


(3 of 4)

• Usually situated in power or feed side– Fuses are tested with DVOM or test lamp.

• Good fuse—virtually same voltage on both sides

• Blown fuse—battery voltage on one side and 0 volts on the other

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(4 of 4)

• Can sometimes be visually inspected– Fusible element should be intact.

– Should be very low resistance when measured with ohmmeter

– Contacts on fuse and holder should be clean and free of corrosion.

Inspecting and Testing Switches, Connectors, Relays, Solenoid

Solid-State Devices, and Wires (1 of 5)

• Start with visual inspection of electrical circuit, then electrical testing.

• Visually inspect for:– Breakage or corrosion

– Deformity

– Worn or melted insulation



• Electrical inspection necessary for:– Switches

– Solenoid contacts

– Relay contacts

• Example: Switches require voltage drop testing to check for excessive resistance.

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• Some solenoids can be disassembled for visual inspection.– If excessive voltage drop across contacts, will be

pitted and burned

• Measure resistance if shorted relay or solenoid winding is possible.



• DVOMs and test lamps for basic testing– More specialized test equipment if needed

– Test lamps can overpower electronic components—should not be used for them.

– Resistance tests can be conducted on components in or out of the circuit.



• Diagnostic flowcharts from manufacturers show sequence based on test results.– Complex circuits—gather information on circuit

operation and customer concern to formulate a testing sequence.

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Summary (1 of 10)

• The digital volt-ohmmeter (DVOM) or digital multimeter (DMM) is an electrical measurement tool used to diagnose and repair electrical faults.

• To properly use a DVOM requires time and effort to learn the parts and how it works.

• The DVOM can measure volts, ohms, and amps in a circuit.

Summary (2 of 10)

• An advanced DVOM measures frequency and temperature, and has a dedicated diode test capability.

• A DVOM is the first tool used to take electrical measurements.

Summary (3 of 10)

• The DVOM allows the technician to see the movement of electrical impulses that cannot be seen without some type of electrical test equipment.

• The DVOM can measure electrical volts within circuits.

• The DVOM can measure ohms, which is the resistance of a circuit.

• The DVOM can measure amps, which is the current flow of a circuit.

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Summary (4 of 10)

• The main parts of the DVOM are the main body and the two current leads.

• The main body has a function switch, a connection point for the leads, and a digital display to show values.

Summary (5 of 10)

• The leads are red for positive and black for negative connections.

• There is a wide selection of leads for the DVOM to enhance the testing capabilities.

• Before using a DVOM, the technician needs to know the quantity of the measurement (volt, ohm, or amp).

Summary (6 of 10)

• The DVOM can read a wide range of scales depending on the position selected.

• The DVOM can read from low to high values.

• The DVOM in auto range will select the best value for the range being measured.

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Summary (7 of 10)

• The min/max setting gives the technician the ability to measure circuits that are only on momentarily.

• The hold function freezes the value measured.

Summary (8 of 10)

• There are many different ways to probe a circuit depending on the circuit being tested.

• The probes should never be forced as this could damage the circuit being tested and the probes being used.

• If the technician uses the back probe method, the holes probed need to be resealed to keep moisture out.

Summary (9 of 10)

• The most common measurements taken with a DVOM are voltage, current, resistance.

• Depending on measurements taken, the leads need to be in the correct location on the body of the DVOM.

• If the leads are connected in the wrong place on the DVOM, it could cause a fuse to blow.

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Summary (10 of 10)

• When voltage is measured, the leads are placed parallel to the circuit being measured.

• When current is measured, the leads are placed in series with the circuit being measured.

• When resistance is measured, the component should be isolated from the circuit so no power is present.

• The meter is very useful in finding opens, shorts, grounds, and high resistance.

Credits

• Unless otherwise indicated, all photographs and illustrations are under copyright of Jones & Bartlett Learning.

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