Using Oscilloscopes on Vehicles

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Expect spark plugs with plug gaps of .045-.060 inch to require 8 to 12 KV to jump the gap at no-load idle.

Sometimes an issue with weak spark, incorrect spark timing, fuel supply/delivery, low engine compression,

or something else that can cause a misfire can affect firing KV.

In fact, the waveform below is an example of how we can pick out an obvious misfire simply by looking at

ignition voltage.

Just by looking at primary voltage, we can quickly identify the problematic/misfiring cylinder.

Something is definitely making ignition in that cylinder work harder than it needs to!

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2. Spark voltage is the voltage in KV required to maintain the spark across the plug gap for the period of time

required to ignite the gas.

If firing voltage is higher than 2 or 3 KV, this decreases the duration of the spark, known as firing time. Firing

time should be about 1 or 2 mS.

Here we see the spark voltage part of the ignition waveform pattern.

Notes:

*Shorter spark times often indicate a weak ignition coil.

*In times where spark time is short and firing voltage is a typically high, this indicates that something is forcing

the ignition to expend all its energy on the initial spark, leaving nothing left for its duration.

* The slight upward spike in the end of the spark line is normal.

* Lean systems spike up right after the firing spike.

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3. Oscillations occur when the spark has ended because the energy necessary for the spark has been extended.

There is still some energy left in the ignition coil, but not enough to continue the spark event. So, the remaining

power oscillates up and down, effectively ringing itself out.

Oscillations are easily detected in most ignition waveforms.

Notes:

* Sometimes, some ignition systems have one coil oscillation.

* A fouled spark plug has neither a definitive spark line nor oscillations.

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Why look at ignition waveforms?

Spark line voltage, shape and time tell us more about the spark than the firing voltage KV does.
Because of this, try stacking (Raster) the waveforms on your scope. It will make comparing the
spark line much easier to do. Take a look at the spark line on number 6 cylinder that had a
compression problem.


Ignition waveforms on the Scope that had a compression problem.

It’s tough when using labscopes. When are we being too critical and when are we not looking close
enough? Sometimes we only know in retrospect. Here the firing time reflects the cylinder with the
problem, but the firing lines of all the cylinders are hardly identical. That’s why we need many tools
in our boxes to pinpoint vehicle problems, ignition diagnostics is just one of them.

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Comparing voltage and current in primary waveforms

Comparing voltage and current waveforms is a great way to locate the problem and the reason for the problem.

The ICM or PCM is responsible for switching on/off the primary circuit. When the switch opens, it must open quickly

to properly create spark. Slow switching reduces secondary spark intensity. Have trouble visualizing this? Take a look

at how the current drops instantly when the switch opens compared to the opposite in the waveform below.

Why does the spark event look so bad in the below waveform? The current waveform tells us here. The amperage

does not drop straight down during the firing event. It instead steeply slopes down. This means that the switch internal

to the PCM/ICM is opening too slow. We just diagnosed a bad module.

In the pictures we can diagnose a bad computer by looking at voltage waveforms against amperage waveforms.

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Secondary ignition waveform analysis

Now, you won’t be needing to scope too much secondary ignition anymore, but some vehicles still use ignition wires.

And when the mood strikes you to scope secondary ignition, keep in mind that secondary waveforms look a lot like

primary waveforms.

Can you tell the difference between the secondary and primary waveform?

The reason this is so is because primary ignition directly affects secondary. In the below waveform Channel A

is Primary Ignition and Channel B is Secondary Ignition. They both look very similar except for the dwell section.

For this reason if an ignition primary originates on the primary, it will askew your secondary waveforms. Be sure

to scope secondary ignition AFTER confirming primary ignition is good.

This is how secondary waveforms should look.

1. The coil begins charging up full of current. The oscillations are magnetic interference from a working ignition coil.

2. During the dwell period, voltage builds very slowly.

3. The ignition coil unleashes all of its current into the secondary side of that same coil. Voltage skyrockets.

Instantaneously the spark jumps the spark plug gap.

4. Spark line should be relatively high in voltage. Its duration is the firing time.

5. Oscillations happen when there is not enough power to continue the spark and any remaining power is squeezed out.

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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.

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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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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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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.