To minimize this possible interference with the oscilloscope, keep these tips and suggestions in mind:
Most interference will be picked up by the oscilloscope test leads.
• Route the test leads away from all ignition wires and components whenever possible.
• Use the shortest test leads possible, since other test leads may act as an antenna and increase the potential for
interference, especially at higher frequency levels t hat are found when probing near the vehicle s on-board
computer.
• With the potential for RF interference in the engine compartment, if possible, use the vehicle chassis as ground
when connecting the oscilloscope test leads. In some cases the engine block can actually act as an antenna for
the RF signals.
• The t est leads are a very important part of any oscilloscope. Substituting other leads in both length and
capability may alter the signals on your display.
The oscilloscope can also pick up interference like the test leads.
• Because the oscilloscope circuits are so sensitive, and therefore powerful, do not place the oscilloscope directly
on ignition wires or near high energy ignition components, like coil packs.
• If you are using the AC or DC charger/adaptor to power the oscilloscope, keep the external power leads far
away from the engine and ignition if possible.
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3.2 CRITICAL CHARACTERISTICS OF AUTOMOTIVE ELECTRONIC SIGNALS
Only 5 critical characteristics (or information types) given from the Automotive electronic signals are import ant
because the vehicle’s PCM considers them important.
• Amplitude - The voltage of the electronic signal at a certain point in time.
• Frequency - The time between events, or cycles, of the electronic signal, usually given in cycles per second
(Hertz).
• Shape - The signature of the electronic signal, with its unique curves, contours, and corners.
• Duty Cycle - The on-time, or relative pulse width of the electronic signal.
• Pattern - The repeated patterns within the signal that make up specific messages, like synchronous pulses that
tell the PCM that cylinder #1 is at TDC (Top Dead Cent er), or a repeated pattern in the serial data
stream that tells the scan tool the coolant temperature is 212 F (or 100 C), etc.
3.3 THE GOLDEN RULE OF ELECTRONIC SYSTEM DIAGNOSIS
For t he vehic le’s comput er sy stem to function properly, it mus t send and receiv e s ignals wi th the critical
characteristics it was designed to communicate with.
Each of the primary types of electronic signals use the critical characteristics to establish electronic communication.
They each use different combinations of the critical characteristics to communicat e. Here’s a list of which critical
characteristics each of the primary signal types uses to communicate:
• Direct Current signals use Amplitude only.
• Alternating Current signals use Amplitude, Frequency, and Shape.
• Frequency Modulated signals use Amplitude, Frequency, and Shape.
• Pulse Width Modulated signals use Amplitude, Frequency, Shape, and Duty Cycle.
• Serial Data signals use Amplitude, Frequency, Shape, Duty Cycle, and Pattern.
The list will help to give you a better understanding of which signal types use which critical characteristics to do their
electronic communication. The above rules work very well and hold up in most cases, but there are exceptions to its
rules. Not many, but a few.
It may come no surprise to some that serial data signals are the most complex signals in the vehicle. They use all 5
critical characteristics to communicate with. Thus, they take a special analyzer to decode them - one very familiar to
most technicians - the scan tool.
3.4 SIGNAL PROBING WITH AN OSCILLOSCOPE
The engine compartment of a running vehicle is a very unfriendly environment for automotive signals t o live.
Temperature extremes, dirt and corrosion, and electrical leaks, or noises from the high voltage pulses generated
from a t ypical ignition system can produce interf erence that can contribute significantly to the cause of many
driveability problems.
When you are probing components, sensors and circuits, be aware that the electrical noises from today’s high output
ignition systems can produce an RF energy that is similar to a radio station. Since oscilloscopes are so sensitive,
this interference can actually override the signals you are trying t o capt ure and give you a false reading on the
display.
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