Digital Meters and Scopes

1
Chapter 5
Digital Meters and Scopes
Advanced Engine Performance Diagnosis 3rd Edition
2
Chapter 5
Digital Meters and Scopes

• LED Test Light
• Another type of test light uses an LED instead of
  a standard automotive bulb for a visual induction
  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.

Advanced Engine Performance Diagnosis
3
Chapter 5
Digital Meters and Scopes

• 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.

Advanced Engine Performance Diagnosis
4
Chapter 5
Digital Meters and Scopes

• Logic Probe (continued)
• A logic probe can sense the difference between
  high- and low-voltage levels, thus the term
  logic.
• A typical logic probe can also light another
  light (a pulse light) when a change in voltage
  levels occurs.
• This feature is helpful when checking for a
  variable voltage output from a computer or
  ignition sensor.

Advanced Engine Performance Diagnosis
5
Chapter 5
Digital Meters and Scopes

• Digital Multimeters
• Digital multimeter (DMM)
• Electronic high-impedance test meters, which have
  a high internal resistance.
• Most digital meters have 10 megaohms (MO) (10
  million ohms) or more of internal resistance.

Advanced Engine Performance Diagnosis
6
Chapter 5
Digital Meters and Scopes
Digital Multimeters (continued)
Advanced Engine Performance Diagnosis
7
Chapter 5
Digital Meters and Scopes
Digital Multimeters (continued)
Advanced Engine Performance Diagnosis
8
Chapter 5
Digital Meters and Scopes
Digital Multimeters (continued)
Advanced Engine Performance Diagnosis
9
Chapter 5
Digital Meters and Scopes

• 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).
• Many ammeters are the inductive type.

Advanced Engine Performance Diagnosis
10
Chapter 5
Digital Meters and Scopes

• AC/DC Clamp-On Digital Multimeter
• The major advantage of the clamp-on type meter is
  that 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.

Advanced Engine Performance Diagnosis
11
Chapter 5
Digital Meters and Scopes

• Measuring Voltage
• A voltmeter measures the pressure or potential of
  electricity and measures in units of volts.
• A voltmeter 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.

Advanced Engine Performance Diagnosis
12
Chapter 5
Digital Meters and Scopes
Measuring Voltage (continued)
Advanced Engine Performance Diagnosis
13
Chapter 5
Digital Meters and Scopes

• Measuring Resistance
• An ohmmeter measures the resistance in ohms of a
  component or circuit section when no current is
  flowing through the circuit.
• An ohmmeter contains a battery (or other power
  source).
• When the leads are connected to a component,
  current flows through the test leads and actually
  measures the difference in voltage (voltage drop)
  between the leads, which the meter registers as
  resistance on its scale.

Advanced Engine Performance Diagnosis
14
Chapter 5
Digital Meters and Scopes

• Measuring Resistance (continued)
• Zero ohms mean no resistance between the test
  leads, indicating that there is no continuity or
  a continuous path for the current to flow in a
  closed circuit.
• Infinity means no connection, as in an open
  circuit.
• 0.00 O zero resistance
• OL an open circuit (no current flows)

Advanced Engine Performance Diagnosis
15
Chapter 5
Digital Meters and Scopes

• Electrical Unit Prefixes
• The prefix kilo means thousand.
• 1100 volts 1.1 kilovolts (kV)
• 4700 ohms 4.7 kilohms (kO)
• The prefix mega (M)
• 1,100,000 volts 1.1 megavolts (MV)
• 4,700,000 ohms 4.7 megohms (MO)

Advanced Engine Performance Diagnosis
16
Chapter 5
Digital Meters and Scopes

• Electrical Unit Prefixes (continued)
• Small units of measure of 1/1000 are called milli
  (m).
• The micro is represented by the Greek letter mu
  (µ).
• One microampere is one-millionth (1/1,000,000) of
  an ampere.

Advanced Engine Performance Diagnosis
17
Chapter 5
Digital Meters and Scopes

• Electrical Unit Prefixes (continued)
• To summarize
• mega (M) 1,000,000 (decimal point six places to
  the right 1000000.)
• kilo (k) 1000 (decimal point three places to
  the right 1000.)
• milli (m) 1/1000 (decimal point three places to
  the left 0.001)
• micro (µ) 1/1,000,000 (decimal point six places
  to the left 0.000001)

Advanced Engine Performance Diagnosis
18
Chapter 5
Digital Meters and Scopes

• How To Read Digital Meters
• Select the proper unit of electricity for what is
  being measured volts, ohms (resistance), or
  amperes (amount of current flow).
• Place the meter leads into the proper input
  terminals.
• The black lead usually is inserted into the
  common (COM) terminal, and stays in this location
  for all meter functions.
• The red lead is inserted into the volt, ohm, or
  diode check terminal usually labeled V O, when
  voltage, resistance, or diodes are being
  measured.
• When current flow in amperes is being measured,
  most digital meters require that the red test
  lead be inserted in the ammeter terminal, usually
  labeled A or mA.

Advanced Engine Performance Diagnosis
19
Chapter 5
Digital Meters and Scopes

• How To Read Digital Meters (continued)
• Measure the component being tested.
• Interpret the reading.

Advanced Engine Performance Diagnosis
20
Chapter 5
Digital Meters and Scopes

• RMS Versus Average
• Alternating current voltage waveforms can be true
  sinusoidal or nonsinusoidal.
• A true sine wave pattern measurement will be the
  same for both root-mean-square (RMS) and average
  reading meters.
• RMS and averaging are two methods used to measure
  the true effective rating of a signal that is
  constantly changing.

Advanced Engine Performance Diagnosis
21
Chapter 5
Digital Meters and Scopes
RMS Versus Average (continued)
Advanced Engine Performance Diagnosis
22
Chapter 5
Digital Meters and Scopes

• Resolution, Digits, And Counts
• Resolution refers to how small or fine a
  measurement the meter can make.
• By knowing the resolution of a DMM, you can
  determine if the meter could measure down to 1
  volt or to 1 millivolt (1/1000 of a volt).
• The terms digits and counts are used to describe
  a meters resolution.
• DMMs are grouped by the number of counts or
  digits they display.

Advanced Engine Performance Diagnosis
23
Chapter 5
Digital Meters and Scopes

• Resolution, Digits, And Counts (continued)
• A 3.5-digit meter can display three full digits
  ranging from 0 to 9, and one half digit which
  displays only a 1 or is left blank.
• A 3.5-digit meter will display up to 1999 counts
  of resolution.
• A 4.5-digit meter can display up to 19,000 counts
  of resolution.

Advanced Engine Performance Diagnosis
24
Chapter 5
Digital Meters and Scopes

• Accuracy
• Accuracy is the largest allowable error that will
  occur under specific operating conditions.
• How close the DMMs displayed measurement is to
  the actual value of the signal being measured.
• Thus, a lower accuracy percentage is better.
• Unacceptable 1.00
• Okay 0.50 (1/2)
• Good 0.25 (1/4)
• Excellent 0.10 (1/10)

Advanced Engine Performance Diagnosis
25
Chapter 5
Digital Meters and Scopes

• Accuracy (continued)
• For example, if a battery had 12.6 volts, a meter
  could read between the following, based on its
  accuracy.
• 0.1 high 12.61
• low 12.59
• 0.25 high 12.63
• low 12.57
• 0.50 high 12.66
• low 12.54
• 1.00 high 12.73
• low 12.47

Advanced Engine Performance Diagnosis
26
Chapter 5
Digital Meters and Scopes

• Analog Versus Digital Storage Oscilloscope
• An oscilloscope (usually called a scope) is a
  visual voltmeter with a timer (clock) that shows
  when a voltage changes.
• An analog scope uses a cathode ray tube (CRT)
  similar to a television screen to display voltage
  patterns.
• The scope screen displays the electrical signal
  constantly.

Advanced Engine Performance Diagnosis
27
Chapter 5
Digital Meters and Scopes

• Analog Versus Digital Storage Oscilloscope
  (continued)
• A digital scope commonly uses a LCD, but a CRT
  may also be used on some digital scopes.
• A digital scope takes samples of the signals that
  can be stopped or stored hence the term digital
  storage oscilloscope (DSO).

Advanced Engine Performance Diagnosis
28
Chapter 5
Digital Meters and Scopes
Analog Versus Digital Storage Oscilloscope
(continued)
Advanced Engine Performance Diagnosis
29
Chapter 5
Digital Meters and Scopes

• Oscilloscope Display Grid
• A typical scope face has 8 grids vertically (up
  and down) and 10 grids horizontally.
• The grid lines on the scope screen are used as a
  reference scale, which is called a graticule.
• This arrangement is commonly called an 8 x 10
  display.

Advanced Engine Performance Diagnosis
30
Chapter 5
Digital Meters and Scopes

• Setting The Time Base
• Time base indicates how much time will be
  displayed in each division.
• The time base should be set to an amount of time
  that allows two or four events to be displayed.

Advanced Engine Performance Diagnosis
31
Chapter 5
Digital Meters and Scopes

• Setting The Time Base (continued)
• Increasing the time base reduces the number of
  samples per second.
• Time per division can vary greatly in automotive
  use, as follows
• Fuel injector 2 ms per division (20 ms total)
• Throttle position (TP) sensor 100 ms per
  division (1 sec total)
• Oxygen sensor 1 sec per division (10 sec total)

Advanced Engine Performance Diagnosis
32
Chapter 5
Digital Meters and Scopes

• Volts Per Division
• The vertical scale has eight divisions.
• If each division is set to equal 1 volt, the
  display will show 0 to 8 volts.
• The volts per division (V/div) should be set so
  that the entire anticipated waveform can be
  viewed.
• TP sensor 1 V/div (8 V total)
• Battery, starting and charging 2 V/div (16 V
  total)
• Oxygen sensor 200 mV/div (1.6 V total)

Advanced Engine Performance Diagnosis
33
Chapter 5
Digital Meters and Scopes

• Pulse Trains
• A DC voltage that turns on and off in a series of
  pulses is called a pulse train.

Advanced Engine Performance Diagnosis
34
Chapter 5
Digital Meters and Scopes

• Pulse Trains (continued)
• Frequency is the number of cycles per second,
  measured in hertz.
• Duty cycle refers to the percentage of on-time of
  the signal during one complete cycle.

Advanced Engine Performance Diagnosis
35
Chapter 5
Digital Meters and Scopes

• Pulse Trains (continued)
• The pulse width is a measure of the actual
  on-time measured in milliseconds.

Advanced Engine Performance Diagnosis
36
Chapter 5
Digital Meters and Scopes

• Pulse Trains (continued)
• An external trigger is when the trace starts when
  a signal is received from another (external)
  source.
• A scope will not start displaying a voltage
  signal until it is triggered or told to start.
• The trigger level must be set to start the
  display.

Advanced Engine Performance Diagnosis
37
Chapter 5
Digital Meters and Scopes

• Pulse Trains (continued)
• The trigger slope is the voltage direction that a
  waveform requires to start display.
• When the voltage rises past the trigger-level
  voltage, called a positive slope.
• When the trigger is activated by the voltage
  falling past the higher level, it is called a
  negative slope.

Advanced Engine Performance Diagnosis