Anatomy of a Waveform

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By Bill Fulton. I remember a phrase I heard recently stating that, “necessity is the mother of invention.” As I look at the modern-day digital storage oscilloscopes, I also look back on the history of diagnostics on the coil-fired internal combustion engine and I can’t help but recall all of the great diagnostic information the old Sun scopes gave us in the “good old days.” I mean, after all, we could look at a single cylinder secondary waveform, a parade pattern or a raster pattern under dynamic conditions and make sound diagnostic decisions on air/fuel ratio problems, compression problems and, of course, ignition system electrical problems in both the primary and the secondary ignition system. 

Since the laws of inductively charging the primary circuit and capacitively firing the second circuit have not changed much, I can’t help but wonder why a quick secondary scope check is not part of most modern-day technicians’ diagnostic strategy. I remember many of my own experiences when attacking engine performance problems where I started with scan data or jumped on the pattern failure bandwagon. Many times I simply spent too much futile diagnostic time only to end up chewing on the bitter cud of hindsight, reminding myself of the tests that I “could have, should have or might have done” to make the job easier and quicker. In addition, I may have addressed only the obvious cause of a driveability problem and missed one or more other concerns or problems resulting in the dreaded recheck.

A thorough knowledge of a secondary waveform creates an awareness to most technicians that lends credence to the old adage that “a picture is worth a thousand words.” This can only be true if the picture is viewed with the proper knowledge, meaning the critical points are known and the pattern is monitored under specific dynamic conditions.

The forces that attack the internal combustion engine have not changed all that much in the last 20 years. The truth is that 80% of all engine performance problems are not PCM or sensor related. Of this 80%, as much as 60% are in the air/fuel ratio area. The most common being lean conditions caused by vacuum leaks or restricted injectors. Conversely, some air/fuel ratio problems are caused by shorted or leaking injectors. When these problems affect one or more cylinders and create a total or partial misfire, they can be very tough to find by using OEM-recommended tests or by wasting valuable diagnostic time by following a useless symptom chart.

Unfortunately, we often are led to believe that the secondary scope checks are only good for finding open plug wires, fouled plugs or shorted secondary leads. The root of this problem is, as you will see, that too much attention is given to the firing line and little or no attention is given to the spark KV point and the spark line. In addition, many secondary scope checks are not done under specific dynamic conditions. Since it is no big deal to replace the secondary components on most DI systems, many techs use this “shotgun” approach. After all, if one lead is open or shorted they must certainly need them all. Now, I have no problem in replacing secondary components when the odometer suggests it, but let’s do a 10-minute scope check and be more analytical here. Bad plug leads and fouled plugs are the causes of less than 20% of all misfires. When a secondary waveform is viewed properly, we can diagnose misfires that are caused by air/fuel ratio problems, compression problems or poor performance of the primary and secondary components. This may sound too good to be true, but it’s done every day by technicians who have made the commitment to go through the learning curves of DSO diagnostics on the ignition system.

Too often we base our diagnostic approach to the symptom or a pattern failure and miss some of the most critical information by not scope checking secondary. All of the dynamic information from a secondary waveform is the direct result of the cause and effect, as you will see. As I travel around the country giving seminars on this subject, I see where many techs will make the secondary scope check part of their diagnostic strategy, while others simply don’t, for whatever reason. The latter group reminds me of the old Chinese proverb, “Man stand for long time with mouth open waiting for roasted duck to fly in.”

Let’s take a look at a single cylinder secondary waveform as seen in Figure 1. In this example we are going to focus on the firing section, which begins at point B. The amplitude of point C represents the amount of voltage needed to overcome all the air gaps in the secondary circuit. On a DI system there are only two authorized air gaps – the rotor air gap and the spark plug air gap. Since the plug air gap is subject to cylinder pressures, compression values will directly affect the KV value. This can be applied to the law of ionization, stating that it takes 80 volts of electrical energy to force current flow across a .001 air gap under atmospheric air.

If we use a .045 plug gap exposed to atmospheric air, we’ll see about a 3.5KV demand. As you can probably guess, when we fire a .045 spark plug under peak cylinder compression we can easily see KV values 20KV and beyond. Does this convince you that if we do a WOT cranking KV test that the high KV values are due to good cylinder compression and that a low KV value may point to a low compression cylinder?

Now, no one here is telling you to throw away your compression gauge, but let’s be realistic. If you had a weak cylinder misfire on a Ford 3.0L Aerostar van and a compression test was in order, would you want to manually do a compression test on all six cylinders to find the one weak cylinder? What if we simply did a WOT cranking KV test and noted one cylinder had a lower KV value? We’ll talk about the cranking KV test in a later article.

Typically at idle and no-load conditions, with systems using a .045 to .050 gap plug, an 8 to 12 KV demand is a normal value. This is a good gauge to go by because if you see all cylinders with higher-than-normal KV values it indicates that an open exists somewhere between the coil and the distributor cap. The firing KV demand value should be your first measurement. Notice our example at idle no load in Figure 1 indicates a 10 to 12 KV demand.

The reason that the firing line “floats” as much as 4KV is due to the variable resistances of the plug gap. What do we mean here when all along you thought the spark plug gap was a physically fixed gap? The term variable resistance means that the KV demand will vary because of the varying of non-conducting air molecules and conducting fuel molecules constantly changing inside the spark plug gap. A lower-than-normal firing KV value obviously could be attributed to low cylinder compression or an overly rich condition, especially at idle during no-load conditions.

If all cylinder KV values appear normal at idle, but one cylinder goes low on a WOT snap test, the cause most likely would be a secondary insulation problem causing an arc to ground. This may occur only when the throttle plates are opened, which directly increases the amount of air inside the combustion chamber, thus increasing compression resulting in a higher KV demand and stressing the secondary insulation. Since our KV demand is greatly affected by compression, it makes sense to view this demand. In addition, keep in mind that 64% of the cars on the road today have more than 75,000 miles on the clock and that most techs see some cars with more than 200,000 on the odometer! Do we have to be concerned with compression in today’s arena?

Point D on the secondary waveform in Figure 1 represents the spark KV point. It is actually at this point that current flow is established across the plug’s gap. Obviously, nothing will happen inside the combustion chamber until the spark KV point has been established. The spark KV point actually represents the voltage drop across the plug gap. If you remember your Ohm’s Law training, the formula to calculate V/D was current times resistance. Remember, you do not have any current flow until the spark KV point has been established. Many technicians still believe that a high ohmic value spark plug lead will result in a higher-than-normal firing KV demand. It will not, unless the plug wire is actually open. Normally, good spark KV points will be between 1 and 2 KV. When the spark KV point is much above 2 KV, it represents that the fixed resistance in that circuit is high and it will result in shorter spark duration periods of the important spark line. The second measurement of the secondary waveform should be the spark KV point values.

This brings us to the most critical portion of the secondary waveform known as the spark line. There are three critical points here to always remember:

1. Length
2. Angle
3. The Presence of Turbulence

Stay with me here because this is where your learning curves will pay off big dividends. The length of the spark line or what some call spark duration is critical because a shorter-than-normal spark line can and will result in a total misfire or a partial misfire. The spark line duration should be your third measurement.

Typically, we have taught that this duration should not drop before 1.3 ms during idle and no-load conditions on systems that use a .045 to .050 plug gap. Most DI systems will hang around 1.5 ms. If the firing KV demand is too high or the spark KV point is too high, the available energy to sustain the current flow across the plug gap will be reduced, resulting in a shorter-than-normal spark line. In reality, this spark line varies by as much as 30% due to the changing variable resistances inside the combustion chamber we talked about earlier.

Some of the most common occurrences are lean conditions from restricted injectors or vacuum leaks. Since the cylinder is lean or void of the conducting fuel molecules, the voltage required to sustain current flow increases, directly resulting in a decrease of the spark duration period (more about that in a later article). The best time base to use on your DSO to make this measurement would be 1 ms or 500 microseconds (.5 ms) time base. That function was simply not available in the “good old days.”

The second consideration of the spark line would be the angle. If the spark line bends upward it indicates that the voltage demand has increased, thus causing the spark line to bend upward. Since our example was captured at idle with no load, note how the spark line is relatively level as indicated in Figure 1. Why? Have you ever done a compression test of a particular cylinder at idle to find out that you have only about half the compression you obtained during the WOT cranking? Remember, the volumetric efficiency of the engine is the lowest with the throttle plates closed, resulting in lower cylinder pressures. Because compression values are lowest at idle, the spark line angle will be relatively smooth and level. A lack of compression or a fouled plug will cause the spark line to bend downward.

Notice that at 1 ms per division, our spark duration period measures about 1.4 ms in Figure 1. Now comes the critical point. As we open up the throttle plates during a power brake condition, the spark line will, in fact, end upward (see Figure 2). This is due to the increase in cylinder pressure from opening up the throttle plates.

Now let’s set the stage for finding out if the lean cylinders form restricted injectors or vacuum leaks. We shift our time base to .5 ms and conduct an off idle, loaded power brake condition. This effectively causes the ignition system to retard the firing event causing the plug to fire very close to base timing during near-peak cylinder pressure conditions. This is what causes the turbulence effect on the spark line. It is perfectly normal for the spark line to bend upward at about the midway point, however lean cylinders cause the spark line to bend consistently upward before the midway point with a high blow out point as indicated in Figure 3 on page 28. In addition, lean cylinders will create spark lines with excessive turbulence since there is no help from conductive fuel molecules inside the combustion chamber.

Point F on our secondary waveform in Figure 1 indicates the residual energy left over the coil after the firing event. Do you remember what they used to teach you in the “good old days”? They taught that weak coils will have less than three coil oscillations and if you saw less than three, you simply replaced the coil. Do you see three oscillations in any of our examples? No. Why? There are two considerations here. First, modern-day coils were redesigned back in the mid-1980s to conduct a higher energy transfer rate from primary into secondary and, of course, out of the secondary circuit. You simply do not see three any longer.

Second, there are some systems out there, like a GM type 1 EI system that have no coil oscillations! Most other modern-day systems will show you 1-1/2 to 2. What do you think would happen to a perfectly good coil if our secondary KV demand was extremely high from an unauthorized open somewhere in the circuit creating a secondary firing KV demand of 20 KV? Since the coil has released too much energy too quickly and too soon, will it exhibit good coil oscillations? No. Did we diagnose the problem by noting the excessively high secondary KV demand? Yes. By the way, most coils don’t die a natural death. Excessively high secondary firing demands are the leading cause of early coil failures.

A friend of mine recently attended a training seminar and told me the instructor opened up the seminar by asking the question, “If you had to verify the mechanical integrity of an engine before spending a lot of diagnostic time, what piece of test equipment would you prefer?” Most techs in the class responded by saying a vacuum gauge or a compression gauge. The battlegrounds were set early on by the instructor picking a scan tool as the tool of choice! Suppose that we have convinced you at this point that a 10-minute scope check should be in order, would you have answered, “a secondary scope check”? We hope so! If not, stay tuned for more articles on primary and secondary scope checks. Until then, scope it out! You may be surprised what you find or what you may have missed!