Anatomy of a Waveform, Part II

By Bill Fulton. If you remember the three critical points of a secondary waveform discussed in our previous article (March 2002), it’s time to continue what we learned and apply it to Distributor less Ignition Systems, (DIS) now known as EI systems. 

One of the most popular tech line subjects is in fact revolved around EI systems. Always keep in mind that the three critical points on a secondary waveform we talked about on our initial article, “The Anatomy of a Waveform” (DI Systems), will also apply here on EI systems.

Figure 1: DIS Secondary Circuit

In the grand scheme of things, when discussing a secondary waveform on a distributorless system, there are still only two authorized air gaps, just like a DI system. Since we got rid of the rotor air gap you may ask, “where is the other air gap?” Remember we are firing two plugs simultaneously on companion cylinders, such as 1/4 or 3/2. While one cylinder will fire with a negative polarity, the companion cylinder will fire the opposite polarity or positively (see Figures 2 and 3).

While this may seem too much theory, it is important to understand that what affects one side of the circuit electrically will also affect the other side of the circuit, since this is a simple series circuit. In our good example in Figure 2 of a #1 positive firing event and Figure 3 of a #4 negative firing event, notice that the all-important spark duration periods are identical. What you need to keep in mind is that it is impossible for you to see any variance in duration values when comparing companion cylinders. For example, a short spark duration in the #4 cylinder, from let’s say an open plug wire, will look identical in the companion cylinder, in this case the #1 cylinder. It is critical to understand that this is a simple series circuit and that whatever affects the firing event on one side of the circuit will affect the other side, in this case the #1 and #4 firing events.

Keep in mind that both of these firing events are the power or true firing events. You already know that the true firing event occurs on the compression stroke and will require the higher kV due to compression while the companion cylinder is on the exhaust stroke and will require a lot less kV since there is no compression. (Refer to the Law of Ionization discussed in the March issue.)

Figure 2: Positive Firing Event & Figure 3:Negative Firing Event

How did we block out the waste firing event using a standard DSO? By setting the trigger level at about 4 kV or 5 kV so that the scope will only trigger off the higher kV demand from the true or power firing event. You may ask, “Is there any diagnostic value in the waste firing event?” Absolutely! I’ll discuss more on that later. At this point, let’s talk about the true firing kV demands as applied to DIS systems and displayed on your scope screen. In Figure 1, you will see that the direction of current flow is always from a negative potential (an excess of electrons) to a positive potential (a deficiency of electrons). Now notice that the scope kV probe is before the spark plug air gap on the negative firing event, while the scope probe is after the spark plug air gap on the positive firing event in relation to current flow. Because of this, a negative firing event will be displayed as a 2-4 kV higher demand than a positive firing event. Notice in Figure 4 that the #4 and #2 cylinders true firing events are 2-4 kV higher than the #1 and #3. We can tell you that the #4 and #2 cylinders are negatively fired, while cylinders #1 and #3 are positively fired even though our scope sorted the negative events out and stood the firing events up for us.

Some good advice here is don’t look for uniform firing kVs on DIS systems like we did on distributor-equipped cars. You will not see them! While some of this sounds like useless theory to some technicians out there, let’s throw in Murphy’s Law. Have you ever changed plug wires on a DIS system where the coil was buried and you could not find it, feel it or see it? So let’s say “Technician A” put on a set of plug wires, and because of accessibility, did not get the plug wire clipped all the way on the coil pack on the positive side or on the negative side. Remember, on the GM Type II coil pack this is a tight fit and you cannot feel the terminals click in. What would you see on the scope as far as kV values? You now have an unauthorized air gap in the circuit.

Figure 4: DIS Parade

On the negative firing event, the kVs would appear lower than normal because the scope probe will indicate the voltage potential in the circuit after the voltage drop that occurred from the unauthorized air gap. The kVs would appear higher if an air gap existed between the plug wire and the coil pack on the positively fired side because the scope probe would show the voltage potential in the circuit before the voltage drop occurred across the unauthorized air gap. While all of this may sound like “nice to know” information instead of “need to know” information as some techs view it, what if a secondary misfire actually occurred after replacing a set of wires and plugs? Has anyone out there been unfortunate enough to replace a set of secondary leads with a defective set? I have! This is one of the reasons why we say to scope out the system before and after replacing secondary components.

Now if you are one of those technicians who religiously believes in looking at minimum spark duration periods, stay with me. Let’s say an open exists on the positive side of the circuit somewhere between the plug and coil pack. What would happen to the important spark duration period on both positive and negative true firing events? They would both become too short setting the stage for a misfire. Oh, by the way, the leading cause of coil or coil pack failures is extremely high secondary firing kV demands.

Now let’s talk about the kV needed to conduct the waste firing event. The waste firing kV demand is a good way to check the electrical integrity of the secondary circuit. Since we are firing a plug on the exhaust stroke with no compression, the kV demand will be minimum. Typically, on .045 to .050 gap plugs, the kV requirement for the waste firing events will fall into the 2-4 kV range (see Figure 5). Do you see the 2-4 kV demand for the waste events? Now look at Figure 6. Do you see something wrong with the kV demand for the #4 waste-firing event? Notice that it is over 5 kV. Could this be caused by an unauthorized air gap? Yes, and because the kV demand is so high, we know that the scope’s kV probe is before the unauthorized open in relationship to the direction of current flow. If the kV value indicated on the scope was below 2 kV, than we would know that the scope kV probe would be downstream of the unauthorized air gap. The critical point here is that any unauthorized open in a DIS secondary circuit will raise the kV demand or lower the kV demand as displayed on the scope screen, and is best seen while viewing the waste firing events. This is always important to view since this could drastically affect the spark duration’s periods on the true firing event. Note: In our Enhanced Ignition Systems Testing manual and video set, we show the typical good spark duration periods for most domestic EI and DI systems, as well as COP systems.

Figure 5: Measuring Waste Spark KV's & Figure 6: High Waste Firing KV

Now let’s jump over to Figure 9. This is known as the Circle Ohm Check. Notice that we have disconnected the two plug wires from the plugs from a DIS systems companion cylinder, in this case #1 and #4. Typically the ohmic value per foot of secondary suppression leads range in the neighborhood of 3,000 to 5,000 ohms per foot. Let’s say that each plug wire is 3’ long or roughly 9,000 ohms per lead. This gives us a ballpark value of 18,000 ohms for the secondary leads. Add this to the ohmic values of the secondary coil pack windings (in this case 5,000 to 7,000 ohms from a GM Type II). Now notice the ohmmeter in Figure 9 indicates 22.975K ohms. Do we have a secondary open? No. Are we in a normal ballpark value? Yes. Is this test conclusive? No. Stay tuned. At least this tells us that an open does not exist and that this initially is a good test where again some coil packs are buried where you cannot find them, feel them or see them.

It is no secret that if a coil pack “ohms out” normal, it is no guarantee that it may still be defective. What usually occurs is what is known as “internal coil carbon tracking,” created from an air pocket left inside the coil pack during the potting process that became ionized (usually from neglected tune-ups and high firing kVs). Many times the coil stress test can find this problem by using one or two ST125 spark testers and noting the spark.

Often, internal coil carbon tracking is temperature related and typically shows up more often after initial startup. It goes away after the coil warms up. This problem can be easily detected by doing an EKG on the coil pack. Do you remember your last medical checkup when you had an EKG? What were you doing? Laying down, I bet. Let’s apply the same principle to the automotive coil.

Do you recall the effects of compression of the spark line from our previous article? It creates jaggies and turbulence on the spark line. Now let’s take the effects of compression out of the picture. Figure 7 was obtained by clamping two .045 gap spark plugs together using two known-good 3’ long secondary leads. We allowed the two plugs to hang over the radiator hose or just hang in the air without touching any metal. The scope can be hooked to either lead. There are three issues we need to focus on:

Figure 7: GM Type II, EI System - "Good"

#1 – The smooth transfer of energy in and out the coil pack;
#2 – How long was the energy transfer sustained, known as the spark duration period; and
#3 – What reserve energy was left over indicated by the coil oscillations.

Figure 8: GM Type II, EI System - "Ugly"

Notice in our good example, we have a nice smooth transfer of energy that lasted for over 1 ms with two good coil oscillations. Now refer to Figure 8. Do you notice the erratic transfer of energy as indicated on the spark line? Do you also notice the shorter than normal spark duration period and the reduction of coil oscillations? Of course you do. We just did an EKG on a coil pack by creating a controlled environment.

Oh, by the way, the coil pack in Figure 8 ohmed-out fine and would fire two ST125 spark testers when the coil pack warmed up. Our critics would say that to run an engine by doing this test would dump raw fuel into the combustion chamber. The test was done by disabling the fuel pump or injectors and biasing the ignition control wire at the module using a popular and inexpensive hand-held sensor simulator that can simulate an rpm range from idle to 6,000 rpm!

Figure 9: Circle Ohm Test

Always keep in mind the three critical points of the secondary waveform we talked about in the Part I article and apply them equally as well on EI (DIS) systems. This is where you will win the most battles. Remember that 60% of all misfires are density-type misfires, in most cases “lean” density-misfires.

In our next article, we’ll feature the five types of misfires and the five critical secondary tests. Until next time, SCOPE IT OUT. You may be surprised at what you may find or what you would have missed!