Tech Stuff

Waveform diagnostics: Ignition diagnostics you will actually use

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 in Figure 16 (ignition waveforms on the ATS EScope) that had a compression problem.

Figure 16: Ignition waveforms on the ATS EScope that had a compression problem.
<p>Figure 16: Ignition waveforms on the ATS EScope that had a compression problem.</p>

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.

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 (Figures 17 and 18).

Figures 17 and 18: We can diagnose a bad computer by looking at voltage waveforms against amperage waveforms.
<p>Figures 17 and 18: We can diagnose a bad computer by looking at voltage waveforms against amperage waveforms.</p>

Figure 18.
<p>Figure 18.</p>

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 Figures 17 and 18, we can diagnose a bad computer by looking at voltage waveforms against amperage waveforms.)

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. (In Figure 19, can you tell the difference between the secondary and primary waveform? Neither can I.)

Figure 19: Can you tell the difference between the secondary and primary waveform? Most techs, including myself, cannot.
<p>Figure 19: Can you tell the difference between the secondary and primary waveform? Most techs, including myself, cannot.</p>

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.

Real world secondary ignition diagnostic tips

• High voltage at point 3 in Figure 20 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).

Figure 20: Here we illustrate the steps of a secondary ignition waveform.
<p>Figure 20: Here we illustrate the steps of a secondary ignition waveform.</p>

• 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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