Using Oscilloscopes on Vehicles

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Section f - g: the duration of the spark, also known as the burning line, that is, the duration of the electron current after

the breakdown of electrode. The voltage is the spark burning voltage, is the voltage to maintain the spark transfer. Generally,

it’s 1/ 4 of the breakdown voltage. This section also called inductance discharge section. This section should be relatively flat,

clean and non-abnormal waveforms.

Section h - i: winding coil normal attenuation oscillation part: usually there are more than 3 oscillation waveforms, and standard

is 5 to 6 waveforms. That is after the magnetic energy conversion electric energy can not maintain the spark conduction, the

magnetic energy of the primary coil and the secondary coil decay and disappear from each other. If the attenuation wave is

too little, the performance of the coil is degraded.

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Point e: the starting point of spark formation (i.e. the moment when the high voltage breaks down the gap of the

spark plug to form the electron current, the conversion of thermal energy started).

Section c - f: capacitor discharge section (equivalent to a large capacitor discharge, spark formation process,

that is, the formation of electron flow breakdown ionization process).

Point g: the end point of spark (i.e. the lowest energy point of the magnetic energy conversion last until cannot

keep the electron current when breakdown form the electron current ).

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Point d: Breakdown high voltage generated by the secondary coil at the moment of primary power failure

breaks down the highest voltage point of the spark plug gap, immediately dropping once the spark plug

gap has been broken. The breakdown voltage is affected by many factors, such as spark plug gap, spark

plug electrode shape, mixture concentration, cylinder pressure, cylinder compression temperature and

so on.

The larger the spark plug electrode gap, the sharper the electrode shape, the higher the breakdown voltage

and the greater ignition energy required. Mixture is too thick, because HC and air molecules are compressed

and squeeze intensive, high pressure to make it more difficult for positive and negative electron separation,

the need for greater voltage to make it ionized to form an electron flow spark. Mixture is too thin, HC and air

molecules are relatively thin, the molecules can not be ionized at the same time, so that the higher the ionization

voltage can penetrate the slender gap. The higher the temperature, the more active molecules, ionization easier,

the required breakdown voltage is low, on the contrary the required voltage is high. Only moderate gas mixture, oil

and gas molecules are evenly distributed, the electrode generates electron flow is relatively easy, the breakdown

voltage at this time is the standard voltage.

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Section a - c: closing angle (primary coil energization time from primary coil on to the ignition). Generally, 4 cylinders engine

closing angle is 45% - 60%, i.e. calculated according to the camshaft angle is 90 x (45 ~ 60)%; and calculated according to

the crank angle is 180 x (45 ~ 60)%, which means that the closing angle time is one cylinder complete a work cycle, the

camshaft angle is 90 degrees, and the crankshaft rotation angle is 180 degrees, and in the entire working cycle, the ratio

is 45% to 60% of the angle. But 6-cylinder engine closing angle is 60% - 70%.

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Section a - b: The primary coil switch-on, current increases until the ignition controller starts the constant current control

process (build the magnetic field energy).

Point c: The moment of primary coil power-off (ignition controller controls primary coil power-off and starts to ignition).

Section b - c: constant current control section (the holding constant current section when primary coil current rose to the

ignition controller constant current ). Constant current: indicates the ignition primary coil charging current rises to a certain

amperage and stabilizes the charging to coil.

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1 Secondary standard waveform analysis

Secondary coil of the secondary ignition waveform is shown in figure 1, each point analysis is as follows:

Secondary Ignition Standard Waveform

Point a: Primary coil turn on (The current increasing is slow when primary coil turn on, thus the electromotive

force generated from secondary coil is slow as well, and its direction is opposite.)

Point b: Primary coil current reached the point of saturation (i.e. turn on the primary coil, when current rised upto

the point of constant current value, the constant current function of ignition controller start to work.

Section a - b: The primary coil switch-on, current increases until the ignition controller starts the constant current

control process (build the magnetic field energy).

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Application of Secondary Ignition Waveform to Automobile Fault Diagnosis

Ignition waveform reflects the relationship between the variation of primary, secondary electical current and voltage of ignition coil

and the time or crankshaft turn angle of engine during operation of the engine ignition system, which is displayed on an oscilloscope

and can show working mode of the ignition system in real time.

Secondary ignition waveform analysis is a common method of fault diagnosis of modern vehicles.

Let’s analyze the standard waveforms and fault waveform of ignition system and use a typical fault waveform example to illustrate

how to use the secondary ignition waveform for automobile fault diagnosis.

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Sometimes you won’t be able to pick up an issue with the secondary ignition under normal conditions

(such as misfiring at high speeds).

So, a snap throttle test places more demand on the secondary ignition, making the comparison game

easier to play. As you can see below, snapping the throttle can accentuate a misfiring cylinder so you

can catch it.

Snapping the throttle can accentuate a misfiring cylinder so you can catch it.

In cylinder 1 it required more voltage to fire the plug, noticeably shortening the spark duration.

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Secondary waveform comparison game

See below picture. We have our ignition scope hooked up to secondary ignition,

using a parade pattern presentation.

In this example, cylinder number 4 presents an obvious misfiring.

Assume the vehicle has a misfire. Which cylinder is misfiring? Obviously cylinder 4! As you

can see, the comparison game is an effective diagnostic method.

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Real world secondary ignition diagnostic tips

* High voltage at point 3 in below picture is usually the result of a wide plug gap (or some other gap on

the secondary side depending upon the system) or high cylinder pressure. This lessens firing time (point 4).

Here we illustrate the steps of a secondary ignition waveform.

* A voltage drop on the secondary side will not affect firing KV, but will increase the spark line (4) and lessen

firing time.

* With COP you probably won’t be scoping the secondary. This is a tip we covered in part one of this article.

If you really want to, connect ignition wires between the coil and the plugs if possible and measure the old

fashioned way. In the real world just play the comparison game. You have at least four coils to choose from.

Chances are you can just scope the primary and pick out the coil that’s bad. Simply change the coil along

with the plug, or better yet sell the whole tune-up after that.