chap34 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 34, the reader should
be able to:

•
•
•

Prepare for ASE Engine Performance (A8) certification test

content area “E” (Computerized Engine Controls Diagnosis
and Repair).
Explain how to set a digital meter to read volts, amperes, and
ohms.
List the precautions necessary when working with test
equipment.

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OBJECTIVES:

After studying Chapter 34, the reader should
be able to:

•
•

Explain how a digital storage scope differs from an analog
scope.
Describe how to safely connect test equipment or a meter to a
circuit or component without doing any harm.

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KEY TERMS:
8 ×10 display • AC coupling • AC/DC clamp-on DMM •
alternating current (AC)
BNC connector
cathode ray tube (CRT) • CAT ratings • continuity light • counts
• current analog
DC coupling • digits • digital meter • digital multimeter (DMM)
• digital storage oscilloscope (DSO) • digital volt-ohmmilliammeter (DVOM) • direct current (DC) • duty cycle

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KEY TERMS:
external trigger • frequency
graphing multimeter (GMM) • graticule
high-impedance meter
inductive ammeter • International Electrotechnical Commission
(IEC)
kilo • LED test light • logic probe
mega • micro • milli
oscilloscope • over limit (OL)
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KEY TERMS:
pulse trains • pulse width • pulse width modulation (PWM)
resolution • root-mean-square (RMS)
scope • self-powered test light
test light • time base • trigger level • trigger slope
volts per division

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Test equipment must be used by the service tech because
electrical signals cannot be seen or felt
and therefore must be measured.
This chapter reviews basics of electrical measurement and
provides background for measuring components and circuits
in the remaining chapters.

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TEST LIGHTS

A test light is simply a light bulb with two wires attached. It is used
to test for low-voltage level (6 to 12 volts). Battery voltage can’t be
seen or felt and can be detected only with test equipment.

A test light can be purchased
or homemade.
A purchased test light could be
labeled as a 6- to 12-volt test
light.

Figure 34–1 A 12-volt test light is attached to
a good ground while probing for power.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Do not purchase a test light
designed for household current
(110 or 220 volts). It will not
light with 12 volts.
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Always Test Your Equipment
More than one tech has misdiagnosed a problem because the test equipment did not work
correctly. Examples include the following:
Test light bulb burned out. A test light bulb can be easily checked by regularly testing the test
light bulb across a battery. A burned out test light bulb will not be able to detect voltage.
Meter fuse blown. If an ammeter fuse is blown (open), the meter will simply read 0.0 amperes
(A) and not conduct current in the circuit. If an inline fuse is used in the meter leads, all functions
of the meter will not work. Volts will read 0.0 all the time and often will read OL (over limit)
on all ohmmeter scales.

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Figure 34–2 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.

CAUTION: The use of a continuity test light is not recommended on any
electronic circuit. Because a continuity light contains a battery and applies
voltage, it may harm delicate electronic components.

Continued
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Self-Powered Test Lights A self-powered test light, also called a
continuity light, is similar to a test light but includes a battery. A selfpowered light will light when connected to both ends of a wire that has
continuity or that is not broken.
Figure 34–3 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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Led Test Light Another type of test light uses an LED instead of a
standard automotive bulb for a visual indication of voltage.

An LED test light requires
only about 25 mA (0.025
A) to light, so it can be
used on electronic circuits
and on standard circuits.

Figure 34–4 High-impedance test light. 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 amperes (25 milliamperes) from the circuit being tested. This low current draw
helps assure the tech that the circuit or component being tested will not be damaged by
excessive current flow.
Continued
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LOGIC PROBE
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.
A logic probe can “sense” the
difference between high- and
low- voltage levels, thus the
term logic.
Figure 34–5 A logic probe connected to the
vehicle battery and relay used to check for
power, ground, or a pulse.

A typical logic probe can also

light another light (a “pulse”
light) when a change in voltage
levels occurs.
Continued
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A logic probe must be first connected to a power source (vehicle
battery). This connection powers the probe and gives it a reference
low (ground).
Most logic probes also make a distinctive sound for each high- and
low-voltage level, which makes troubleshooting easier when

probing connectors or component terminals. A sound (usually a
beep) is heard when the probe tip is touched to a voltage source that
is changing.
The changing voltage also usually lights the pulse light on the logic
probe. Therefore, the probe can be used to check components such
as pickup coils, Hall-effect sensors, magnetic sensors, and many
other circuits.

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DIGITAL MULTIMETERS

Digital multimeter (DMM) and digital volt-ohm-milliammeter
(DVOM) are terms commonly used for electronic high-impedance
test meters, which have a high internal resistance.
Most digital meters have 10 megohms (MΩ) (10 million ohms) or
more of internal -resistance.
Analog (needle-type) meters are almost always lower than 10
megohms and should not be used to measure any computer circuit.
A high-impedance meter can be used to measure any automotive
circuit within the ranges of the meter.
See Figures 34–6 through 34–8
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 34–6 Typical digital multimeter. The black meter lead always is placed in the COM
terminal. Except when measuring the current in amperes, the red meter test lead remaIns in the
VW terminal.

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Figure 34–7 Common abbreviations used on the display face of many digital mutimeters.

See the chart
on Page 342
of your textbook.

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Figure 34–8 A summary chart indicating what measurement type may be used to test which
vehicle system.

See the chart on Page 343 of your textbook.
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MEASURING AMPERES
An ammeter measures the flow of current through a complete
circuit in units of amperes. The ammeter has to be installed in the
circuit (in series) so that it can measure all the current flow in that
circuit, just as a water flow meter would measure the amount of
water flow (cubic feet per minute, for example).
CAUTION: An ammeter must be installed in the circuit to measure the
current flow in the circuit. If a meter set to read amperes is connected in

parallel, such as across a battery, the meter fuse will blow or the meter
itself may be destroyed by the current available across the battery.

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Digital meters require that the meter leads be moved to the
ammeter terminals. Most digital meters have an ampere scale that
can accommodate a maximum of 10 A.
Many ammeters are the inductive type, meaning the meter probe
surrounds the wire(s) carrying current and measures the strength of

the magnetic field that surrounds any conductor carrying a current.

Figure 34–9 An inductive ammeter uses a
clamp that measures the current through
the wire by using the strength of the
magnetic field surrounding the wire.

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Fuse Your Meter Leads - Part 1

Most digital meters include an ammeter capability. When reading amperes, the leads of the meter must be moved from volts or ohms (V or Ω) to amperes (A) or milliamperes (mA) or microamperes (µA).
A common problem may occur the next time voltage is measured.
Although the tech 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 0 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. To solve this problem, simply solder an inline blade fuse holder into one meter lead.

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Fuse Your Meter Leads - Part 2
Do not think this fuse is necessary only for beginners. Experienced techs 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.

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
Subtract the resistance of the leads
from the resistance of the
component being measured.
Figure 34–10 Note the blade-type fuse holder soldered in series with one of the meter leads. A 10amp fuse helps protect the internal meter fuse (if equipped) and the meter itself from damage that
might result from excessive current flow if accidentally used incorrectly.
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AC/DC CLAMP-ON DIGITAL MULTIMETER
An AC/DC clamp-on DMM is a useful meter for automotive diagnostic work. See Figure 34–11.
The major advantage of the clamp-on-type meter is there is no need
to break the circuit to measure current (amperes). Simply clamp the
jaws of the meter around the power lead(s) or ground lead(s) of the
component being measured and read the display.
Most clamp-on meters can also measure AC, which is helpful in the
diagnosis of a generator (alternator) problem.
Volts, ohms, frequency, and temperature can also be measured with
the typical clamp-on DMM.
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Figure 34–11 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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MEASURING VOLTAGE
A voltmeter measures the pressure or potential of electricity in units
of volts, and is connected to a circuit in parallel. All voltmeters have a
large, built-in resistance so that the current flow through the meter
will not affect either the circuit being tested or the meter.
Most digital meters have an internal resistance of 10 M Ω (10,000,000
ohms) or more on the voltmeter scale only. This is called the
impedance of the meter and represents the total internal resistance of
the meter circuit due to internal coils, capacitors, and resistors.
A typical analog voltmeter has only about 12,000 ohms of internal
resistance. Although this may sound like a lot of resistance, it is too
low for electronic and computer circuit measurement.
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