Using Oscilloscopes on Vehicles

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Types of Scopes

Oscilloscopes can be classified as analog and digital types.

Analog Oscilloscopes

An analog oscilloscope works by applying the measured signal voltage directly to the vertical axis of an electron beam

that moves from left to right across the oscilloscope screen - usually a cathode-ray tube (CRT). The back side of the

screen is treated with luminous phosphor that glows wherever the electron beam hits it. The signal voltage deflects

the beam up and down proportionally as it moves horizontally across the display, tracing the waveform on the screen.

Analog oscilloscopes are characterised by the large screens used in traditional 'tune-up' machines and the smaller

scopes with the glowing green screens used in electronics. They are excellent tools, however in automotive use they

suffer from major drawbacks - the need for mains power, the greater difficulty in set-up and the absence of a storage

mode that allows the freezing of the on-screen image.

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Digital Oscilloscope

A digital oscilloscope uses an analog-to-digital converter (ADC) to convert the measured voltage into digital information.

It acquires the waveform as a series of samples, and stores these samples until it accumulates enough samples to

describe a waveform. It then re-assembles the waveform for display on the screen.

The digital approach means that the oscilloscope can display any frequency within its range with stability, brightness,

and clarity. It can also easily freeze the waveform, allowing it to be studied at leisure. Digital scopes can usually be

powered by batteries and use an LCD screen. All scope adaptors that are used with laptop PCs are digital.

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Scope Systems and Controls

An oscilloscope has three main controls, labelled Vertical, Horizontal, and Trigger.

You need to adjust these three basic settings to accommodate an incoming signal:

a) The attenuation (reduction) or amplification (increasing) of the signal - use the volts/div

(volts per on-screen division) control to adjust the height of the signal to the desired

measurement range.

b) The time base - use the sec/div (seconds per on-screen division) control to set the

amount of time per division represented horizontally across the screen.

c)The triggering of the oscilloscope - use the trigger level to stabilize a repeating signal,

or to trigger on a single event.

These adjustments sound more complex than they actually are: what you want to see

is a steady waveform that fits on the screen. The first point (a) simply fits the waveform

on the screen vertically, (b) sets the bottom axis so that the waveform repeats sufficiently

that you can recognise it, and (c) makes sure that the waveform is clearly depicted.

And as we said, some digital scopes have an 'auto' button that do all of these things for you!

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Scope Measurement

Voltage Measurements

The oscilloscope is primarily a voltage-measuring device. The most basic method of taking voltage measurements

is to count the number of divisions a waveform spans up the oscilloscope's vertical scale. Adjusting the volts/div control

signal to allow the signal to cover most of the screen vertically makes for the best voltage measurements. The more screen

area you use, the more accurately you can read from the screen. Then it's as simple as reading off how many divisions per

volt the scope is set to, and estimating on-screen how many divisions the waveform covers.

With AC signals (eg a sinewave from a speed sensor), you would normally look at the peak to peak voltage.

With a DC voltage, the whole line will be elevated from the zero point. Some scopes will do these calculations for you.

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Time and Frequency Measurements

You can make time measurements using the horizontal scale of the oscilloscope.

Time measurements include measuring the period and pulse width of pulses.

Remember that frequency is the reciprocal of the period, so once you know the

period, the frequency is one divided by the period. Like voltage measurements,

time measurements are more accurate when you adjust the portion of the signal

to be measured to cover a large area of the screen. Again, some scopes will do

these calculations for you.

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Pulse Width and Rise Time Measurements

In many applications, the details of a pulse's shape are important. Pulses can become distorted and cause a circuit to malfunction,

and the timing of pulses in a pulse train is often significant. Standard pulse measurements are pulse width and pulse rise time.

Rise time is the amount of time a pulse takes to go from a low to high voltage. By convention, the rise time is measured from 10%

to 90% of the full voltage of the pulse. Pulse width is the amount of time the pulse stays high. Some scopes will calculate and display

pulse width (measured in seconds) and also duty cycle (the proportion of time that a pulsetrain is high.)

Conclusion

Using a scope gives you a window into a new world. No longer do you just see a static (or more often, flickering around!) voltages

coming out of a sensor or the ECU. Now you can see the shape of that signal - which is a whole lot more illuminating...

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Waveform diagnostics: Ignition diagnostics you will actually use

We are not going to get into the old stuff here that you’re already well acquainted with. The basic ignition system principles at work in a distributor

are no different from those that make a modern coil-on-plug (COP) ignition work. COP, even without a cap and rotor, still:

* charges and fire a coil with a switched circuit;

* needs good power and ground; and

* needs to know engine speed for ignition timing.

Being that pretty much everything has moved over to COP or waste-spark ignition, we are going to cover the essentials on these systems first.

Ignition coils and primary ignition

The one thing that every ignition system has in common is the ignition coil.

A coil is in effect the “middle” of the ignition system. Every component in the ignition system leading up into the coil is primary ignition. If you want to

get real technical, there is a metal coil with a carbon bar in the middle in the first part of the coil. Secondary ignition is every part after the coil. Again,

technically speaking there is a second set of winding, in the second half of the coil.

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Swapping coils and just looking at misfire counter on Mode 6 or checking for misfire DTCs is a common,

but less precise, practice. Personally, we sell the customer on all new ignition coils or threaten them with

diagnostic costs to pick out the bad ones. That seems to do the trick.

Every coil, no matter the vehicle, needs power. So anything that inhibits the coil getting power will

compromise its performance. Be sure to check for the following:

? Correct voltage — A voltage drop to the coil can make it ineffective.

? Good switching — If the “switch” wherever it’s location (points, ICM, D.I.S, PCM these days, etc.) contact

has high resistance, or if the ground connection is bad, power to the coil is reduced, weakening the spark.

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Secondary ignition

On old vehicles, after the ignition coil was an ignition rotor, distributor cap, spark plug wires and spark plugs. The ignition coil provided the high voltage power,

and this power was distributed and transferred to the plugs through the rotor, cap and wires. All of this is secondary ignition.

Modern ignition systems still have what we call “secondary,” but they use fewer parts.

*Waste-spark ignition gets rid of the distributor cap and just has spark plug wires and spark plugs.

This is what the internal construction of an ignition coil looks like.

*COP gets rid of everything but the plugs! As we can see above picture, the secondary boot and spring on these coils essentially connects right to the plugs,

making everything else unnecessary. The secondary has more windings than the primary. The windings increase the voltage, but decrease current. This voltage

increase enables power to jump the spark plug gap.

*NOTE: Replace ignition coil boots when doing tune-ups whenever possible, especially if they are on GMs or you see any evidence of arcing (white crust).

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Coil-on-plug ignition in detail

COP is here to stay until vehicles become all like diesels and ignite using compression. Because COPs do not use plug wires,

this reduces the amount of ignition parts and makes them better suited for high kilovolts (KV) demand during high engine loads.

With one coil per plug, a dead coil affects only one cylinder. Each coil has a two-pin connector. One pin receives system voltage

and the other is grounded to the PCM, allowing the coil to charge and discharge. This makes them very easy to diagnose via coil

swapping or the “comparison game” with the scope. As a side note, leaking coil boots should be replaced, not only to stop misfires

but also to protect the PCM.