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CHAPTER 8
Circuit Testers and Digital Meters
OBJECTIVES
After studying Chapter 8, the reader will be able to:
1.
Prepare for ASE Electrical/Electronic Systems (A6)
certification test content area “A” (General
Electrical/Electronic System Diagnosis).
2.
Discuss how to safely use a fused jumper wire, a test
light, and a logic probe.
3.
Explain how to set up and use a digital meter to read
voltage, resistance, and current.
4.
Explain meter terms and readings.
5.
Interpret meter readings and compare to factory
specifications.
6.
Discuss how to properly and safely use meters.
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KEY TERMS
•
•
•
•
•
•
•
•
AC/DC clamp-on DMM
Continuity light
DMM
DVOM
High-impedance test meter
IEC
Inductive ammeter
Kilo (k)
•
•
•
•
•
•
•
•
•
•
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LED test light
Logic probe
Mega (M)
Meter accuracy
Meter resolution
Micro (μ)
Milli (m)
OL
RMS
Test light
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FUSED JUMPER WIRE
DEFINITION
•
•
•
A fused jumper wire is used to check a circuit by
bypassing the switch or to provide a power or
ground to a component.
A fused jumper wire, also called a lead, can be
purchased or made by the service technician.
It should include the following features.
•
•
•
Fused
Alligator clip ends
Good-quality insulated wire
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FUSED JUMPER WIRE
DEFINITION
FIGURE 8–1 A technician-made fused
jumper lead, which is equipped with a red
10 ampere fuse. This fused jumper wire
uses terminals for testing circuits at a
connector instead of alligator clips.
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FUSED JUMPER WIRE
USES OF A FUSED JUMPER WIRE
•
A fused jumper wire can be used to help diagnose
a component or circuit by performing the following
procedures.
•
Supply power or ground
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TEST LIGHTS
•
•
•
•
NONPOWERED TEST
LIGHT
USES OF A 12 VOLT
TEST LIGHT
CONTINUITY TEST
LIGHTS
HIGH-IMPEDANCE TEST
LIGHT
FIGURE 8–2 A 12 volt test light is attached to a
good ground while probing for power.
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TEST LIGHTS
FIGURE 8–3 A test light can be used to locate an open in
a circuit. Note that the test light is grounded at a different
location than the circuit itself.
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TEST LIGHTS
FIGURE 8–4 A continuity light should not be used on
computer circuits because the applied voltage can damage
delicate electronic components or circuits.
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TEST LIGHTS
FIGURE 8–5 An LED test light can be easily
made using low cost components and an old
ink pen. With the 470 ohm resistor in series
with the LED, this tester only draws 0.025
ampere (25 milliamperes) from the circuit being
tested. This low current draw helps assure the
technician that the circuit or component being
tested will not be damaged by excessive
current flow.
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LOGIC PROBE
PURPOSE AND FUNCTION
•
A logic probe is an
electronic device that lights
up a red (usually) LED if
the probe is touched to
battery voltage.
•
If the probe is touched to
ground, a green (usually)
LED lights.
FIGURE 8–6 A logic probe connected to the
vehicle battery. When the tip probe is connected to
a circuit, it can check for power, ground, or a pulse.
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LOGIC PROBE
USING A LOGIC PROBE
•
•
A logic probe must first be connected to a power
and ground source such as the vehicle battery.
This connection powers the probe and gives it a
reference low (ground).
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DIGITAL MULTIMETERS
•
•
•
•
TERMINOLOGY
MEASURING VOLTAGE
MEASURING
RESISTANCE
MEASURING AMPERES
FIGURE 8–7 Typical digital multimeter. The black meter
lead always is placed in the COM terminal. The red
meter test lead should be in the volt-ohm terminal
except when measuring current in amperes.
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DIGITAL MULTIMETERS
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DIGITAL MULTIMETERS
FIGURE 8–8 Typical digital multimeter (DMM) set to read DC volts.
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DIGITAL MULTIMETERS
FIGURE 8–9 A typical autoranging digital
multimeter automatically selects the proper
scale to read the voltage being tested. The
scale selected is usually displayed on the
meter face. (a) Note that the display indicates
“4,” meaning that this range can read up to 4
volts. (b) The range is now set to the 40 volt
scale, meaning that the meter can read up to
40 volts on the scale. Any reading above this
level will cause the meter to reset to a higher
scale. If not set on autoranging, the meter
display would indicate OL if a reading exceeds
the limit of the scale selected. (Courtesy of
Fluke Corporation)
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DIGITAL MULTIMETERS
FIGURE 8–10 Using a digital multimeter set to read ohms (Ω) to test this light bulb.
The meter reads the resistance of the filament.
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DIGITAL MULTIMETERS
FIGURE 8–11 Many digital multimeters can
have the display indicate zero to compensate
for test lead resistance. (1) Connect leads in
the V Ω and COM meter terminals. (2) Select
the Ω scale. (3) Touch the two meter leads
together. (4) Push the “zero” or “relative”
button on the meter. (5) The meter display will
now indicate zero ohms of resistance.
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DIGITAL MULTIMETERS
FIGURE 8–12 Measuring the current
flow required by a horn requires that
the ammeter be connected to the
circuit in series and the horn button be
depressed by an assistant.
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How Much Voltage Does an Ohmmeter
Apply?
•
Most digital meters that are set to measure ohms
(resistance) apply 0.3 to 1 volt to the component
being measured. The voltage comes from the meter
itself to measure the resistance. Two things are
important to remember about an ohmmeter.
•
•
The component or circuit must be disconnected from
any electrical circuit while the resistance is being
measured.
Because the meter itself applies a voltage (even though
it is relatively low), a meter set to measure ohms can
damage electronic circuits. Computer or electronic
chips can be easily damaged if subjected to only a few
milliamperes of current, similar to the amount an
ohmmeter applies when a resistance measurement is
being performed.
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Fuse Your Meter Leads!
•
Most digital meters include an ammeter
capability. When reading amperes, the leads
of the meter must be changed from volts or
ohms (V or Ω) to amperes (A), milliamperes
(mA), or microamperes (μA).
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Fuse Your Meter Leads!
•
A common problem may then occur the next
time voltage is measured. Although the
technician may switch the selector to read
volts, often the leads are not switched back to
the volt or ohm position. Because the
ammeter lead position results in zero ohms of
resistance to current flow through the meter,
the meter or the fuse inside the meter will be
destroyed if the meter is connected to a
battery. Many meter fuses are expensive and
difficult to find.
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Fuse Your Meter Leads!
•
•
To avoid this problem, simply solder an inline 10
ampere blade-fuse holder into one meter lead.
Do not think that this technique is for beginners only.
Experienced technicians often get in a hurry and
forget to switch the lead. A blade fuse is faster, easier,
and less expensive to replace than a meter fuse or
the meter itself. Also, if the soldering is done properly,
the addition of an inline fuse holder and fuse does not
increase the resistance of the meter leads. All meter
leads have some resistance. If the meter is measuring
very low resistance, touch the two leads together and
read the resistance (usually no more than 0.2 ohm).
Simply subtract the resistance of the leads from the
resistance of the component being measured.
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Fuse Your Meter Leads!
FIGURE 8–13 Note the blade-type fuse holder soldered in
series with one of the meter leads. A 10 ampere fuse helps
protect the internal meter fuse (if equipped) and the meter
itself from damage that may result from excessive current
flow if accidentally used incorrectly.
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What Does “CE” Mean on Many Meters?
•
The “CE” means that the meter meets the
newest European Standards and the letters
CE stands for a French term for “Conformite’
Europeenne” meaning European Conformity
in French.
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INDUCTIVE AMMETERS
OPERATION
•
•
•
Inductive ammeters do not make physical contact
with the circuit.
They measure the strength of the magnetic field
surrounding the wire carrying the current, and use
a Hall-effect sensor to measure current.
The Hall-effect sensor detects the strength of the
magnetic field that surrounds the wire carrying an
electrical current.
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INDUCTIVE AMMETERS
OPERATION
FIGURE 8–14 An inductive ammeter clamp is used with all
starting and charging testers to measure the current flow
through the battery cables.
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INDUCTIVE AMMETERS
AC/DC CLAMP-ON DIGITAL MULTIMETERS
•
An AC/DC clamp-on
digital multimeter (DMM)
is a useful meter for
automotive diagnostic
work.
FIGURE 8–15 A typical mini clamp-on-type digital
multimeter. This meter is capable of measuring
alternating current (AC) and direct current (DC)
without requiring that the circuit be disconnected to
install the meter in series. The jaws are simply
placed over the wire and current flow through the
circuit is displayed.
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Over Limit Display Does Not Mean the
Meter Is Reading “Nothing”
•
The meaning of the over limit display on a digital
meter often confuses beginning technicians. When
asked what the meter is reading when an over limit
(OL) is displayed on the meter face, the response is
often, “Nothing.” Many meters indicate over limit or
over load, which simply means that the reading is
over the maximum that can be displayed for the
selected range. For example, the meter will display
OL if 12 volts are being measured but the meter has
been set to read a maximum of 4 volts.
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Over Limit Display Does Not Mean the
Meter Is Reading “Nothing”
•
Autoranging meters adjust the range to match what is
being measured. Here OL means a value higher than
the meter can read (unlikely on the voltage scale for
automobile usage), or infinity when measuring
resistance (ohms). Therefore, OL means infinity when
measuring resistance or an open circuit is being
indicated. The meter will read 00.0 if the resistance is
zero, so “nothing” in this case indicates continuity
(zero resistance), whereas OL indicates infinity
resistance. Therefore, when talking with another
technician about a meter reading, make sure you
know exactly what the reading on the face of the
meter means. Also be sure that you are connecting
the meter leads correctly.
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Over Limit Display Does Not Mean the
Meter Is Reading “Nothing”
FIGURE 8–16 Typical digital multimeter
showing OL (over limit) on the readout with
the ohms (Ω) unit selected. This usually
means that the unit being measured is
open (infinity resistance) and has no
continuity.
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DIODE CHECK, PULSE WIDTH, AND
FREQUENCY
•
DIODE CHECK
•
•
PULSE WIDTH
•
•
Diode check is a meter function that can be used to
check diodes including light-emitting diodes (LEDs).
Pulse width is the amount of time in a percentage
that a signal is on compared to being off.
FREQUENCY
•
•
Frequency is a measure of how many times per
second a signal changes.
Frequency is measured in a unit called hertz,
formerly termed “cycles per second.”
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ELECTRICAL UNIT PREFIXES
DEFINITIONS
FIGURE 8–17 Always look at the meter
display when a measurement is being
made, especially if using an autoranging
meter.
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ELECTRICAL UNIT PREFIXES
DEFINITIONS
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ELECTRICAL UNIT PREFIXES
USE OF PREFIXES
•
The prefixes can be confusing because most
digital meters can express values in more than one
unit, especially if the meter is autoranging.
•
•
For example, an ammeter reading may show 36.7
mA on autoranging.
When the scale is changed to amperes (“A” in the
window of the display), the number displayed will be
0.037 A.
•
Note that the resolution of the value is reduced.
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Think of Money
•
Digital meter displays can often be confusing.
The display for a battery measured as 12 1/2
volts would be 12.50 V, just as $12.50 is 12
dollars and 50 cents. A 1/2 volt reading on a
digital meter will be displayed as 0.50 V, just
as $0.50 is half of a dollar.
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Think of Money
•
It is more confusing when low values are
displayed. For example, if a voltage reading is
0.063 volt, an autoranging meter will display
63 millivolts (63 mV), or 63/1,000 of a volt, or
$63 of $1,000. (It takes 1,000 mV to equal 1
volt.) Think of millivolts as one-tenth of a cent,
with 1 volt being $1.00. Therefore, 630
millivolts are equal to $0.63 of $1.00 (630
tenths of a cent, or 63 cents).
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Think of Money
•
To avoid confusion, try to manually range the
meter to read base units (whole volts). If the
meter is ranged to base unit volts, 63 millivolts
would be displayed as 0.063 or maybe just
0.06, depending on the display capabilities of
the meter.
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HOW TO READ DIGITAL METERS
STEPS TO FOLLOW
•
•
•
•
STEP 1 Select the proper unit of electricity for
what is being measured.
STEP 2 Place the meter leads into the proper
input terminals.
STEP 3 Measure the component being tested.
STEP 4 Interpret the reading
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HOW TO READ DIGITAL METERS
STEPS TO FOLLOW
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HOW TO READ DIGITAL METERS
•
•
•
RMS VERSUS AVERAGE
RESOLUTION, DIGITS, AND COUNTS
ACCURACY
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Purchase a Digital Meter That Will Work
for Automotive Use
•
Try to purchase a digital meter that is capable of
reading the following:
•
•
•
•
•
•
DC volts
AC volts
DC amperes (up to 10 A or more is helpful)
Ohms (Ω) up to 40 MΩ (40 million ohms)
Diode check
Additional features for advanced automotive diagnosis
include:
•
•
•
•
Frequency (hertz, abbreviated Hz)
Temperature probe (°F and/or °C)
Pulse width (millisecond, abbreviated ms)
Duty cycle (%)
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HOW TO READ DIGITAL METERS
FIGURE 8–18 When reading AC voltage signals, a true RMS meter (such as a Fluke
87) provides a different reading than an average responding meter (such as a Fluke
88). The only place this difference is important is when a reading is to be compared
with a specification.
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HOW TO READ DIGITAL METERS
FIGURE 8–19 This meter display shows
052.2 AC volts. Notice that the zero beside
the 5 indicates that the meter can read over
100 volts AC with a resolution of 0.1 volt.
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Meter Usage on Hybrid Electric Vehicles
•
Many hybrid electric vehicles use system voltage as
high as 650 volts DC. Be sure to follow all vehicle
manufacturer’s testing procedures; and if a voltage
measurement is needed, be sure to use a meter and
test leads that are designed to insulate against high
voltages. The International Electrotechnical
Commission (IEC) has several categories of voltage
standards for meter and meter leads. These
categories are ratings for overvoltage protection and
are rated CAT I, CAT II, CAT III, and CAT IV. The
higher the category, the greater the protection against
voltage spikes caused by high-energy circuits. Under
each category there are various energy and voltage
ratings.
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Meter Usage on Hybrid Electric Vehicles
•
•
CAT I Typically a CAT I meter is used for
lowenergy voltage measurements such as at
wall outlets in the home. Meters with a CAT I
rating are usually rated at 300 to 800 volts.
CAT II This higher rated meter would be
typically used for checking higher energy level
voltages at the fuse panel in the home. Meters
with a CAT II rating are usually rated at 300 to
600 volts.
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Meter Usage on Hybrid Electric Vehicles
•
•
CAT III This minimum rated meter should be
used for hybrid vehicles. The CAT III category
is designed for high-energy levels and voltage
measurements at the service pole at the
transformer. Meters with this rating are usually
rated at 600 to 1,000 volts.
CAT IV CAT IV meters are for clamp-on
meters only. If a clamp-on meter also has
meter leads for voltage measurements, that
part of the meter will be rated as CAT III.
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Meter Usage on Hybrid Electric Vehicles
FIGURE 8–21 Always use meter leads that are
CAT III rated on a meter that is also CAT III rated,
to maintain the protection needed when working
on hybrid vehicles.
FIGURE 8–20 Be sure to only use a meter that is
CAT III rated when taking electrical voltage
measurements on a hybrid vehicle.
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SUMMARY
1.
2.
3.
4.
5.
6.
Digital multimeter (DMM) and digital volt-ohmmilliammeter (DVOM) are terms commonly used for
electronic highimpedance test meters.
Use of a high-impedance digital meter is required on any
computer-related circuit or component.
Ammeters measure current and must be connected in
series in the circuit.
Voltmeters measure voltage and are connected in
parallel.
Ohmmeters measure resistance of a component and must
be connected in parallel, with the circuit or component
disconnected from power.
Logic probes can indicate the presence of power, ground,
or pulsed signals.
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REVIEW QUESTIONS
1.
2.
3.
Why should high-impedance meters be used
when measuring voltage on computer-controlled
circuits?
How is an ammeter connected to an electrical
circuit?
Why must an ohmmeter be connected to a
disconnected circuit or component?
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CHAPTER QUIZ
1. Inductive ammeters work because of what
principle?
a.
b.
c.
d.
Magic
Electrostatic electricity
A magnetic field surrounds any wire carrying a
current
Voltage drop as it flows through a conductor
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CHAPTER QUIZ
2. A meter used to measure amperes is called a(n)
________ .
a.
b.
c.
d.
Amp meter
Ampmeter
Ammeter
Coulomb meter
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CHAPTER QUIZ
3. A voltmeter should be connected to the circuit
being tested ________ .
a.
b.
c.
d.
In series
In parallel
Only when no power is flowing
Both a and c
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CHAPTER QUIZ
4. An ohmmeter should be connected to the circuit
or component being tested ________ .
a.
b.
c.
d.
With current flowing in the circuit or through the
component
When connected to the battery of the vehicle to
power the meter
Only when no power is flowing (electrically open
circuit)
Both b and c
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CHAPTER QUIZ
5. A high-impedance meter ________ .
a.
b.
c.
d.
Measures a high amount of current flow
Measures a high amount of resistance
Can measure a high voltage
Has a high internal resistance
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CHAPTER QUIZ
6. A meter is set to read DC volts on the 4 volt
scale. The meter leads are connected at a 12
volt battery. The display will read ________ .
a.
b.
c.
d.
0.00
OL
12 V
0.012 V
Automotive Electricity and Electronics, 3/e
By James D. Halderman
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Copyright © 2011, 2009, 2005 Pearson Education, Inc.,
Upper Saddle River, NJ 07458 • All rights reserved.
CHAPTER QUIZ
7. What could happen if the meter leads were
connected to the positive and negative terminals
of the battery while the meter and leads were set
to read amperes?
a.
b.
c.
d.
Could blow an internal fuse or damage the meter
Would read volts instead of amperes
Would display OL
Would display 0.00
Automotive Electricity and Electronics, 3/e
By James D. Halderman
57
Copyright © 2011, 2009, 2005 Pearson Education, Inc.,
Upper Saddle River, NJ 07458 • All rights reserved.
CHAPTER QUIZ
8. The highest amount of resistance that can be
read by the meter set to the 2 kΩ scale is
________ .
a.
b.
c.
d.
2,000 ohms
200 ohms
200 kΩ (200,000 ohms)
20,000,000 ohms
Automotive Electricity and Electronics, 3/e
By James D. Halderman
58
Copyright © 2011, 2009, 2005 Pearson Education, Inc.,
Upper Saddle River, NJ 07458 • All rights reserved.
CHAPTER QUIZ
9. If a digital meter face shows 0.93 when set to
read kΩ, the reading means ________ .
a.
b.
c.
d.
93 ohms
930 ohms
9,300 ohms
93,000 ohms
Automotive Electricity and Electronics, 3/e
By James D. Halderman
59
Copyright © 2011, 2009, 2005 Pearson Education, Inc.,
Upper Saddle River, NJ 07458 • All rights reserved.
CHAPTER QUIZ
10. A reading of 432 shows on the face of the meter
set to the millivolt scale. The reading means
________ .
a.
b.
c.
d.
0.432 volt
4.32 volts
43.2 volts
4,320 volts
Automotive Electricity and Electronics, 3/e
By James D. Halderman
60
Copyright © 2011, 2009, 2005 Pearson Education, Inc.,
Upper Saddle River, NJ 07458 • All rights reserved.