Through the many technical channels and online sources of which I partake, I receive a lot of current probe questions via e-mail, which I thoroughly enjoy. Lately it seems the hot topic is the diagnostic viability of ignition coil primary current oscillations, as seen by a current probe. Some experts seem to have jumped to the foregone conclusion that the oscillations seen by some probes, and not others, are indicative of the quality of the probe, based only on its ability to show them. I've found this true to some extent ... but in the reverse.

The discussion of their diagnostic value has overshadowed the question of their legitimate existence, in the primary current waveform. I can't tell you that the oscillations are not there; they are. These oscillations, which appear at the beginning of the primary current inductive charge angle, and again at the drop of the primary current when the coil fires, are evident in varying degrees, depending on which probe and scope combination you may be using. Although the oscillations that are inherent to the waveform have an intrinsically aesthetic quality, they are a hindrance for diagnostic purposes. I treat them as a nuisance in the current waveform. The interference they create makes it more difficult to determine important characteristics of the waveforms.

The initial inductive charge angle, or “ramp,” is the place in the waveform where the oscillations appear as the coil begins charging. Figure 1 is showing the oscillations in this area, which are inhibiting the view of the waveform. The beginning of the ramp is the area of the waveform where a shorted coil will cause the most change.

The image in Figure 2 is the primary current ramp from a 1987 Blazer 2.8 with a “no start at times” complaint.

Note that the primary current oscillations occur where the current turn-on begins. The images in Figure 2 were taken with a decent current probe, but access to the primary circuit is limited to a section of wire within a few inches of the ignition coil pack (an area known for its high EMI content). This makes the oscillations show up much worse. In Figure 2, you can clearly see the bow to the left in the second waveform. The change in the linear inductive angle that occurs when a coil is shorted becomes obvious when the oscillations are filtered out.

The images in Figure 2 show a comparison of two probes, one with good EMI filtering. With the oscillations filtered out, you can see clearly that the inductive charge angle has a bow to the left at the beginning. A short in the coil's primary windings causes this. The degree of the ramp angle at the 20 mV, 200uS (20 millivolts, 200 microseconds) setting is also too high in Figure 2. With the scope set to 50mV 500uS, the angle should be about 45 degrees during cranking.

This discussion of transient voltage oscillations should begin with the basic operation of an ignition coil. The ignition coil is a step-up transformer. It has two coils of wire wound around a soft iron core. The high voltage or secondary coil is made up of many, many turns of very fine diameter wire, while the primary winding consists of relatively few turns of a heavier gage of wire. Soft iron is used because of its ability to rapidly demagnetize (low coercive field strength). A large amount of energy is needed to produce a high enough voltage to ionize the plug gap under compression pressure. To reach the high voltage required for the ignition spark, the primary coil is used to collect and store energy in the form of a magnetic field. It has to be able to do this at system voltage during cranking (10 volts nominal). Charging this field requires a relatively long period of time, 4 or more mS (milliseconds) that is characterized by the “ramp,” or inductive charge angle in the primary waveform, as seen in Figure 3.

When it's time to fire the plug, the primary current is cut off. The magnetic energy is then converted back to electrical energy as the concentrated magnetic field collapses, rapidly dispersing across the secondary windings. As the magnetic lines of force cut across the many turns of fine wire in the secondary coil, a high voltage potential is created in the circuit, which includes the secondary windings, the spark plugs, and wires on a DIS system. Think of the buildup and subsequent collapse of the magnetic field as the blowing up and popping of a balloon. The sudden discharge of high voltage energy causes stray fields, which are difficult to filter and can interfere with the proper operation of electronic equipment. The below-zero current, “tails,” which occur at the end of the primary current waveform as the current drops off are more evident with high resistance plug wires. They are also difficult to see when the oscillations show up in the primary current waveform. A capacitive probe that covers the ignition coil, such as those used since the early '70s, is capable of picking up these stray fields and displaying them on a scope for viewing. This is an effective tool combination for diagnosing secondary ignition problems. Figure 4 demonstrates the high frequency oscillations that occur during secondary ignition discharge in the capacitive waveform (top), while simultaneously displaying the minimal change in current shown by the current waveform (bottom).

Figure 5 reflects the primary drive (negative side of the coil) voltage signal of one firing event.

When the module switches the primary circuit to ground, and the primary coil begins to charge, the current flow is impeded by its self-induction. This causes a linear current buildup. When a coil has a short in some of its windings, the initial current speeds up at the beginning of the waveform.

Figure 6 shows the '87 Blazer's cranking primary inductive waveform from the new coil. With the same two probes hooked up simultaneously, notice the linearity of the charge angle on the new coil. As I said before, there is some aesthetic value in the interference seen by some probes in the primary circuit current. To me, it's better than some of the abstract art I've seen. The two images in Figure 7 were achieved by setting the scope to record the minimum and maximum current, while operating the vehicle with both probes hooked to the coil primary feed at the same location. On the top image (limited filter probe) you can see the alternator ripple along with the primary circuit ringing. The probe on the bottom shows a clean steady signal on min max.

What do you think: Is it suitable for framing?

Figure 8 is truly a pretty picture. It shows the primary circuit current waveform of one ignition event, with a high degree of capacitive interference. The ringing causes an image of occurrences from turn-on through current limiting, to firing and dissipation. Figure 9 shows the actual primary current amid the coil's EMI (electromagnetic interference).

A capacitive probe that is really no more than a piece of conductive material with some degree of surface area will show EMI when placed in close proximity to its source.

To illustrate the effect of capacitive oscillations, I took a current probe with limited filtering and put it next to the coil on a '97 Jeep. I got the two waveforms in Figure 10 without moving the probe from its position. On the top waveform I had the power feed for the coil running through the probe. I captured the bottom waveform image, in Figure 10, with the wire out of the probe - the waveform you see pictured here is pure interference. If you were to take the top waveform and subtract out the interference (bottom wave), you would see basically the image in Figure 11, which was taken with the probe with good EMI filtering.

The idea of using a current probe is to help you to see what is going on inside the circuit.

The next picture, Figure 12, is showing the primary drive voltage with the poorly shielded probe. The probe is next to the coil and the primary wire is not in the probe's jaw.

In this instance the probe is being used as a capacitive pickup. With no wire in its jaws, it's picking up interference - only interference. This reminds me of a story my brother, Greg, told me about one of his helpers (Greg is a flooring guru.) He said that his helper had asked him if he could borrow a monkey wrench or a pipe wrench. Greg answered that he had both, which one would he like? His helper then said, “Either one, I'm gonna' use it as a hammer.”

Let's see now, where was I? Oh yes, tangents. Now when you're talking about using a current probe picking up interference as a tool to diagnose ignition secondary problems, as my old buddy, Harold, was fond of saying, “If a little is good, more is better.” Harold was one of the best technicians I've worked with. He was working on Cadillacs back in the '50s. He used to talk about a certain engine that had chrome rings that wouldn't seat. The factory recommended a little Bon Ami in the cylinders to help scuff the walls and seat the rings.

Figure 13 shows the result of a probe I designed to pick up just interference in the primary circuit along with the simultaneous primary current waveform.

The capacitive probe that made the waveform image in Figure 13 is pictured here in Figure 14. It is a Dueber special, gold pocket watchcase. It's from around 1900. Although less expensive than a current probe, it makes better oscillations. Aluminum foil works just as well and is cheaper still.

If you marry the two signals (this is a math function of the scope, Channel 1 + Channel 2), you get the waveform in Figure 15.

This interference, in my opinion, raises more questions than it gives answers for.

Just for kicks, I did some testing of various popular probes next to some common EMI sources. Figure 16 was taken next to my cordless shaver.

The DC motor EMI from the shaver was so bad, I had to switch scales on one of the probes to get it to show on the screen.

Mary, a friend of mine, was reading over my article and suggested putting the probe outside the microwave to check for leakage through the door.

I found the thought of microwaving my probes to be slightly humorous - what the heck - testing should include the obvious. So, posted below in Figure 17 are the results of the microwave probe test.

No wonder they give pacemaker warnings.

As you can see, EMI can confuse you if you are not sure whether it's coming from inside the circuit you're trying to diagnose or if it's stray capacitance. Don't let it. These hazards exist largely around the alternator and the secondary ignition systems.

Using a current probe to monitor the operation of an electrical circuit is still the best way I know of to see how the circuit works. It lets you put a physical image to an unseen phenomenon. If in doubt about circuit noise, move the conductor out of your probe's opening, and see if you still have the noise. If so, find a better location in the circuit to take your reading; the current waveform should be the same anywhere in the circuit.

Imagine yourself having to try to understand the workings of something as simple as a differential, if you had never worked on one or seen one operate with the cover off. It would be just another magic box. I've seen a lot of really good mechanics shy away from electrical work over the years because they didn't have a visual grasp on how the electrical system worked. The fact that a technician can admit his limitations, and won't throw a customer's money at a problem by guessing at it, is not a weakness. It's a strength.

It's all about imagery. If a technician can visualize a systems operation, the battle is more than half over. For you technicians who feel that you can fix anything if you can see how it works, and electrical systems haven't previously fit into that category, you may want to recheck your boundaries. The fences have been moved. The current probe takes the cover off of the magic boxes. So, get a probe, blow the dust off that old scope and try probing around a little. You'll be amazed at how fast you can pick this stuff up. It's like swimming - once you get the basic moves down, it doesn't matter how deep the water gets.

Be forewarned, once you get hooked ...

It can change your concept of beauty.

Jeff Bach is the owner of CRT Auto Electronics, an ASA-member shop in Batavia, Ohio. For more information on this topic, contact Bach at (515) 732-3965. His e-mail address is northstarguy@zoomtown.com

RATE THIS ARTICLE What do you think of this article? Your input will help AutoInc. develop additional articles on this subject. Share your thoughts! Your name Your e-mail address

MOST ACCESSED ARTICLES Fuel Injection Service, Not Just Cleaning The Art of Extraction EGR Systems: Operation and Diagnosis Proactive Target Marketing:_Rethinking Your Business Strategy Engine Performance: HO2S Diagnostics MOST E-MAILED ARTICLES Developing Employee Potential How Critical Thinking Can Help Your Business How to Diagnose the Ford Glow Plug What to Look for When Shopping for the Right Shop Management Software Putting a Price Tag on Complaints

AutoInc. Web Site | ASA Web Site | Washington Debuts New Congress, Administration | Today's Meters and Accessories | Paint Trends 2001 | 20 Groups: Can They Help You? | Guest Editorial | Tech to Tech | Tech Tips | Shop Profile | Net Worth | Stat Corner | Chairman's Message