Using Oscilloscopes on Vehicles

WisdomAugust

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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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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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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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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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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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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To understand the voltage waveform, you need to isolate each part of it to know what’s going on.

1. Firing voltage is the voltage in KV required to jump the largest single gap in the secondary (most

likely the spark plug gap). The gap between the rotor and distributor cap sometimes may be larger,

and this will affect what you see on your waveform.

The firing voltage section of the waveform.

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The voltage waveform

First, let’s begin with a good overview of what makes up an ignition waveform.

Different parts of an ignition waveform.

1. The switch internal to the PCM (or ICM/points) closes. Current rushes into the coil

and begins to build, which is why voltage drops close to ground and essentially remains

there until the firing spark.

2. The coil is now saturated with electricity, as indicated by the jump in voltage. The

coil is no longer charging up thanks to the ICM/PCM.

3. The PCM switch opens, unleashing all the built-up current. Amps drop like a rock

and voltage skyrockets.

4. The spark line indicates the length of the spark event at the plug.

5. When not enough power is left for the spark, remaining power is rung out and the event

begins all over again.

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The current waveform

1. The PCM (or points, or ICM, or etcetera) closes the ignition circuit and the coil begins to charge up.

That steady increase in amperage indicates that the coil is charging up.

2. The PCM opens the circuit just when amperage reaches its peak, causing current to plummet.

Instantly, voltage skyrockets allowing spark at a low current to jump the gap.

This current waveform shows a lower than normal rise and less of an angle. The coil is obviously defective,

notice the burn in the circle?

Following picture shows a real-world example that helps us understand the difference between a good and

bad ignition waveform.

This real-world example helps us understand the difference between a good and bad ignition waveform.

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Interpreting ignition waveforms

There are two different ways to measure the spark firing event: hooking up an amp clamp on the primary side

of the ignition coil for current and backprobing the voltage primary side of the coil with a labscope lead to see

the waveform. In below picture, we can see how the amperage (above) and voltage (below) ignition waveforms

differ.

Here we test an insulation leak by making it worse by wrapping tin foil around the boot.