Testing and Measuring Equipment

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Testing and Measuring Equipment

• Test lights
• Logic probes
• DMMs
• Oscilloscopes

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Test lights

• Always test your test equipment before attempting
  to diagnose electrical problems
• Standard test lights have limited use on late
  model vehicles
• Test lights can draw enough current to
• Damage electronic components
• Deploy air bags
• Test lights cannot differentiate positive and
  negative sides of a circuit
• Test lights cannot measure resistance

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Test lights

• Self powered test lights battery operated and
  light up when connected to both sides of a
  circuit
• LED test lights should be used in place of
  standard test lights on computer controlled
  vehicles
• LEDs draw very small amounts of current and
  cannot damage electronic components

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Test lights

• Logic probes sense difference between high and
  low voltage and can test a pulsing voltage signal
• Illuminates red LED when touched to battery
  voltage
• Illuminates green LED when touched to ground or
  return
• Illuminates yellow LED when checking a pulsing
  voltage signal

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Logic probe
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Digital multimeters

• High impedance high internal resistance that
  limits current output
• Can measure amps, volts and resistance
• Automotive requirements for a DMM
• Diode checking
• Frequency
• Temperature
• Duty cycle
• RPM
• Dwell

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DMMs

• Voltmeter measures difference in voltage between
  leads
• Must be hooked up in parallel
• Internal resistance keeps meter from affecting
  circuit being tested

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DMMs

• Ammeter measures amount of current flowing
  through circuit
• Most meters cannot measure more than 10 amps
• Must be hooked up in series or installed inside
  of the circuit to measure amount of current
  flowing

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DMMs

• Inductive ammeters
• Clamps around wires to measure current flow
• Measures strength of magnetic field
• More practical for ease of hookup
• Circuit does not need to be opened
• Inductive ammeters can measure much more amperage
  than series ammeters

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DMMs

• Ohmmeters
• Requires open circuit
• Ohmmeter is self powered and may be damaged if
  connected to a live circuit
• Zero ohms means no resistance in circuit or
  continuity

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DMMs

• OL
• Displayed on most meters
• What it means
• Open leads
• Infinite resistance
• Over limit
• Over maximum meter can read

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DMMs

• Use of DMMs
• Select proper unit for what is being measured
• Place leads in proper input sockets
• Make measurement
• Interpret reading

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Oscilloscopes

• DSOs most popular for automotive use
• A visual voltmeter with a clock
• Shows an image of a signal over time
• Because it can show an image over time, it is
  much more useful for finding intermittent problems

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Oscilloscopes

• Voltage is displayed as a line vertically from
  the bottom
• Vertical scale usually has 8 to 10 divisions
• Volts should be set so that entire wavelength can
  be viewed

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Volts per division
Voltage side
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Oscilloscopes

• The scope illustrates time from left to right
• Example if scope is set for 100ms per division,
  total time is 1 second (100msx10)
• Increasing the time base reduces the number of
  samples per minute

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200ms per division
Total time displayed is 2 seconds
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Oscilloscopes

• Trigger slope voltage direction that a waveform
  requires to start the display
• Example for a TPS the trigger should be to
  positive slope
• This will start the display when the voltage
  rises above a set level
• Attach scope leads
• Example for a TPS the positive lead would be set
  to the signal circuit and other to the sensor
  ground

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An example of a 12 volt square wave over 10ms
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An example of a alternating voltage signal