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Ignition waveform diagnostics

Scoping out secondary ignition to detect causes of engine misfires

In the normal approach to a misfire code or

Imisfire symptom, most techs initially suspect the ignition system is at fault primarily from a bad coil on a COP type ignition system. While this approach is OK, we must consider that when using a spark tester and removing the coil we have actually disturbed the circuit. In cases where the insulator boot is failing and allowing spark to arc to the plug well, we may never pinpoint that problem with the coil out of the plug well.

On the subject of spark testers, keep in mind that they are not created equally. First and foremost the modern COP, HEI and DIS ignition systems have the ability to crank out 25 KV. Notice the three separate spark testers in Figure 1. The unit on the left is known as a ST125 which requires a true 25 KV.

The unit on the right is known as a ST115 10 By Bill Fulton

which requires 15 KV. The adjustable unit in the rear can be adjusted to create a larger or smaller gap to vary the KV demand. A 3/4-inch gap on this unit creates a KV demand very close to 25 KV. All COP type ignition systems have the ability to deliver 25 KV.

The objective from this article is to explain the diagnostic value of a secondary ignition waveform or a primary ignition waveform to pinpoint lean density misfires, open circuits in secondary, internal coil carbon tracking problems, rich density misfires and cylinders with low compression problems.

In an earlier article we explained that when a single misfire has occurred and set a DTC we should always investigate the freeze frame or fail record information from the scan tool. We went on to state that a single cylinder misfire caused by

Figure 1: Spark testers The unit on the left is known as a ST125 which requires a true 25 KV. The unit on the right is known as a ST115 which requires 15 KV. The adjustable unit in the rear can be adjusted to create a larger or smaller gap to vary the KV demand. A ¾-inch gap on this unit creates a KV demand very close to 25 KV. All COP type ignition systems have the ability to deliver 25 KV.

a lack of spark from say a bad COP coil will not cause a significant shift in long-term and shortterm fuel trim values.

However, a misfire caused by a lean density condition will create double digit positive fuel trim additions. In contrast, a rich condition causing a rich density misfire will create negative double digit fuel trim corrections. You will only see this if the PCM is maintaining closed loop strategies. The modern day PCMs will force the engine back into open loop if the misfire is severe and consistent enough. So if your freeze frame indicated that the engine was forced into open loop, the fuel trim values are worthless.

In the old days, Sun Electric and Allen Test Products made the large analyzers so that technicians could easily pinpoint these problems by reading the waveform. As you have heard before, “a picture is worth a thousand words” and this will apply to the good, the bad and the ugly waveforms we will cover in this article.

First it is important to know the critical points of a secondary ignition waveform. A secondary ignition waveform is indicated in Figure 2. Pont A is the point of primary turn-on and we are starting to saturate the coil. Point B is the point of primary turn-off. So effectively we have charged the coil. At Point B as we turn off primary, the magnetic field collapses and gets mutually inducted into the secondary windings of the coil and the voltage potential is multiplied by 100 to give us the high voltage needed to arc across the pressurized plug gap. Notice that the scope is set at 2 KV per division on the vertical scale. Pont C is known as the firing line and measures a good 12 KV demand. Point D is known as the

Figure 2: Anatomy of a secondary waveform Pont A is the point of primary turn-on and we are starting to saturate the coil. Point B is the point of primary turn-off. So affectively we have charged the coil. At point B as we turn off primary, the magnetic field collapses and gets mutually inducted into the secondary windings of the coil and the voltage potential is multiplied by 100 to give us the high voltage needed to arc across the pressurized plug gap. Notice that the scope is set at 2 KV per division on the vertical scale. Pont C is known as the firing line and measures a good 12 KV demand. Point D is known as the spark KV point and represents the point at which we have established current flow across the spark plug air gap. Notice on the vertical scale the spark KV point comes in at about 1.5 KV. This point is critical to monitor because lean density misfires will cause this point to vertically jump up and down.

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spark KV point and represents the point at which we have established current flow across the spark plug air gap. Notice on the vertical scale the spark KV point comes in at about 1.5 KV. This point is critical to monitor because lean density misfires will cause this point to vertically jump up and down.

A fouled spark plug will elevate this point, followed by a sloping downward spark line. The most critical point occurs at Point E, and I have always referred to this point as our electronic window into the combustion chamber. This is known as the spark line and represents the spark duration period. Notice the time base on the scope is set at 1 millisecond per division.

Look closely at the duration period, which measures 1.3 milliseconds. Normally you should never see this value below 1 millisecond during idle and no load conditions. Lean conditions Figure 3: Secondary KV probes When using a KV probe on a lab scope always use the peak detect mode. The one in the middle is the standard secondary KV probe from Snap-on. The one on the right is a universal KV probe. The unit on the left is a COP wand. Keep in mind that secondary ignition is negatively fired on COP and distributor-equipped engines. You will need to use the invert function of your scope to view negative firing events. On DIS systems half of the cylinders are fired negatively while the other half are fired positively. and high secondary resistances will shorten this duration period. On the contrary, low cylinder compression problems and rich density fuel problems will drastically lengthen this period.

One point that needs to be made here is this waveform example is from a distributorequipped engine. On a COP type ignition system the spark duration period will measure much longer since we only have the one authorized air gap being the spark plug air gap. Normally you should not see a spark duration period below 1.5 milliseconds on the COP systems. It is normal on these COP systems to maintain a spark duration period between 1.5 to 2.0 milliseconds. When monitoring the spark line there are three areas to always focus on: length (duration) at idle, angle and the presence of turbulence during a light power brake condition.

Going back to distributor-equipped engines, we simply clamped our secondary KV probe around the coil wire and clamped our synch probe around the #1 plug wire. We could view the secondary event in a parade pattern, a raster pattern, a super imposed pattern or we could individually look at each cylinder’s event. For those techs who have the modern day lab scopes such as the Mastertech 5200, the Snap-on Vantage Pro, the Snap-on Modis, the Verus and the Zeuis,

Figure 4: Toyota 2.4L (intermittent miss/buck) This was captured during idle no-load conditions. Notice the very short spark duration period of the spark line. It is actually below 1 millisecond. The tech has previously replaced the spark plugs with no improvement. Since this is a cap and rotor equipped engine he proceeded to replace the cap and rotor and plug wires. Remember that on distributor-equipped engines there are two authorized air gaps — the rotor air gap and the spark plug air gap.

Figure 5: Toyota secondary (after new cap and rotor) Do you see a much longer spark duration period? And would you believe the misfire is now gone?

these pattern selections are available. These units have an ignition scope side and a lab scope side.

Now with the advent of COP and DIS ignition systems we can still monitor each cylinder’s secondary event one at a time to not only determine whether we have good spark but can detect a lean cylinder, rich cylinder, low compression cylin14

der and internal coil carbon tracking problems by using a COP wand. There are several COP wands on the market. The one I use the most is the one from AESwave.com. It looks like a small fly swatter. When we lay the probe on the coil we are inductively picking up the collapsing magnetic field from the coils firing. Keep in mind

Figure 6: 1999 Nissan Quest secondary (intermittent misfire, no codes). Symptom system separation logic? This shows a raster pattern from a distributor-equipped engine. Notice the spark duration period of the #3 spark line. In addition, notice the abrupt upward angle of the spark line indicating a lean cylinder. The waveform was captured at a light load 1,100 rpm power brake condition. The resistance specification for the injectors on this engine is 14 to 16 ohms. On this vehicle, the number 3 injector resistance measured 145 ohms, thus causing a lean density misfire.

Figure 7: Secondary raster (lean cylinder) Another example of a lean density condition from a GM V6 4.3L Vortec engine. In this raster pattern notice the spark line duration, angle and presence of turbulence during a light power brake condition. This is another example how lean density conditions caused by bad or restricted injectors can be detected by reading a secondary ignition waveform. 16

that the attenuation factor on these probes is 1,000 to 1. This means that if you set the scope at 1 volt per division, the voltage scale is now 1 KV per division. There are three examples of KV probes in Figure 3. When using a KV probe on a lab scope always use the peak detect mode. The one in the middle is the standard secondary KV probe from Snap-on. The one on the right is a universal KV probe. The unit on the left is a COP wand. Keep in mind that secondary ignition is negatively fired on COP and distributor-equipped engines. You will need to use the invert function of your scope to view negative firing events. On DIS systems half of the cylinders are fired negatively while the other half are fired positively.

The early model Ford and Chrysler COP units were not heavily potted, so getting a good stable secondary ignition waveform was easy. The Asian and European COP units and the late model Ford COP units are heavily potted and with the scope setting at 1 V or 1 KV the magnetic field is too weak for the scope to sense. The key here is to drop the voltage per division down to .5 V per division which means with the 1,000 to 1 attenuation factor we now have a setting of 500 volts per division. The lower voltage setting allows the scope to pick up the collapsing magnetic field from the coils’ firing event. In addition, the COP wand can be used on DIS plug wires as well as the 8

Figure 8: Dodge Caravan 3.8L (secondary super-imposed) All cylinder firing events are stacked on top of one another. Notice that two cylinders’ firing events show an elevated spark KV point. This occurs from the carbon fouled spark plug electrodes. In addition, notice the sloping downward spark line voltage from a rich condition. Remember, fuel molecules are conductive, thus drastically reducing the spark line voltage indicated by the sloping down spark line. inch secondary leads found on General Motors’ coil-near-plug equipped engines.

The time base on the scope should be set to 1 millisecond per division. The time base can be reduced to 500 micro-seconds per division to get a better look at the spark line characteristics.

Another option would be to insert a secondary lead between the coil and spark plug and clamp the conventional KV probe around the secondary lead. The Pico scope people make these leads available.

Now let’s begin on a conventional distributor-equipped engine with a misfire under light load. We use a conventional KV probe clamped around the coil tower of the distributor cap. This distributor has a built-in coil so there is no coil wire. Now notice the secondary ignition waveform in Figure 4. This was captured during idle no-load conditions. Notice the very short spark duration period of the spark line. It is actually 18

below 1 millisecond. The tech has previously replaced the spark plugs with no improvement. Since this is a cap and rotor-equipped engine he proceeded to replace the cap and rotor and plug wires. Remember that on distributor-equipped engines there are two authorized air gaps — the rotor air gap and the spark plug air gap. Notice now Figure 5. Do you see a much longer spark duration period? The misfire is now gone.

Let’s look at another example from a distributor-equipped engine from a Nissan with a dead miss with no codes. Figure 6 shows a raster pattern from this distributor-equipped engine. Notice the spark duration period of the #3 spark line. In addition, notice the abrupt upward angle of the spark line indicating a lean cylinder. The waveform was captured at a light load 1,100 rpm power brake condition. The resistance specification for the injectors on this engine is 14 to 16 ohms. On this vehicle, the number 3 injector re

Figure 9: 2001 Dodge Caravan 3.8L (secondary super-imposed less #1 and #5) Here we have taken cylinders 1 and 5 off the screen. Notice that the other 4 cylinders have normal spark KV points and normal spark lines.

sistance measured 145 ohms, thus causing a lean density misfire.

Another example of a lean density condition can be seen in Figure 7 from a GM V6 4.3L Vortec engine. In this raster pattern notice the spark line duration, angle and presence of turbulence during a light power brake condition. This is another example how lean density conditions caused by bad or restricted injectors can be detected by reading a secondary ignition waveform.

We have shown two examples of lean density misfires being detected by scope-checking the secondary ignition and focusing on the spark line during light load power brake conditions. We now want to show how a rich density condition will show up on a secondary ignition waveform. Notice Figure 8. We have used the super imposed screen of our scope.

All cylinder firing events are stacked on top of one another. Notice that two cylinders’ firing events show an elevated spark KV point. This occurs from the carbon fouled spark plug elec20

Figure 10: Chrysler COP (primary/secondary) The Ford PCMs will multi-fire the coils below 1,000 rpms. trodes. In addition, notice the sloping downward spark line voltage from a rich condition. Remember, fuel molecules are conductive, thus drastically reducing the spark line voltage indicated by the sloping down spark line. In Figure 9 we have

Figure 11: 5.4L COP primary multi-spark system Above 1,000 rpm the PCM reverts to one firing event. Notice the three firing events from the primary side of the coil. The scope is set at .5 milliseconds per division. If we combine all three spark line duration periods we come up with about 1.5 milliseconds. taken cylinders 1 and 5 off the screen. Notice that the other 4 cylinders have normal spark KV points and normal spark lines.

On Ford and Chrysler COP-equipped engines, technicians can back probe the COP negative terminal and obtain a primary ignition waveform. The strategy here is that the spark line on a primary ignition waveform will mirror that of a secondary ignition waveform (see Figure 10). The Ford PCMs will multi-fire the coils below 1,000 rpms (see Figure 11).

Above 1,000 rpm the PCM reverts to one firing event. Notice the 3 firing events from the primary side of the coil. The scope is set at .5 milliseconds per division. If we combine all three spark line duration periods we come up with about 1.5 milliseconds.

Another unique ignition system can be found on some V6 Toyota engines. Figure 12 shows a combination COP/DIS system. The system uses an external igniter to control the coils. A coil is mounted directly on top of a spark plug, with a secondary lead going to the companion cylinder. This engine had a miss under road load conditions. This is a good example of the diagnostic power of accessing the primary side of the coil. First and foremost, notice the engine decal in Figure 13 indicating that dual ground electrode spark plugs are to be used. Now let’s look at the primary waveform in Figure 14. Do you see a very short spark duration period? We removed a spark plug to find single ground electrode spark plugs were used. After replacing the spark plugs with the proper OE plugs, notice the good spark duration period in Figure 15.

A GM 5.7L Silverado comes in with a type A misfire code (PO301). This is a coil near plug-type ignition system. Using the single cylinder selection on our MT 5200 scope we clamped around #1 plug wire (see Figure 16). Notice the extremely low firing line voltage and the very long spark duration period. These two factors told us the coil was not the cause. A compression test indicated 0 psi. Remember compression has a direct

Figure 12: 3.4L Toyota COP DIS ignition system (misses under load — see schematic) Some V6 Toyota engines feature a combination COP/DIS system. The system uses an external igniter to control the coils. A coil is mounted directly on top of a spark plug, with a secondary lead going to the companion cylinder. This engine had a miss under road load conditions. This is a good example of the diagnostic power of accessing the primary side of the coil.

Figure 13: OEM engine decal An underhood label may indicate that dual ground electrode spark plugs are to be used. affect on the KV demand. On COP-type ignition systems, remember we have only one authorized air gap being the plug gap. In addition, the coils are charged with nearly double the amperage values compared to conventional systems. A good secondary ignition waveform from a GM coil near plug system can be seen in Figure 17. In another example is Figure 18, where we are looking at KV demand during a WOT (wideopen-throttle) cranking clear flood mode on a distributorequipped engine, using the bar graph function. Notice the 24 to 28 KV demand. This is another way of doing a relative compression test.

Another example is from a 3.8L Mustang with an engine misfire from #5 cylinder. Look closely at the secondary ignition waveform from #5 cylinder in Figure 19. Do you see the split firing line and the extremely short spark duration period? There are only two possible causes, one being an open plug wire or, two, an open spark plug resister. The cause in this case was an open plug wire. Notice the good secondary waveform after the plug wire was replaced in Figure 20.

The low inductive current probes used with a lab scope is a powerful diagnostic weapon. The amperage waveform will help us determine if the coil was properly saturated. We simply clamp the current probe around the B+ feed wire to the coils. The most popular attenuation factor is for ev

Figure 14: Toyota 3.4L (bad primary) Do you see a very short spark duration period? We removed a spark plug to discover that single ground electrode spark plugs were installed.

Figure 15: Toyota good primary After replacing the spark plugs with the proper OE plugs, notice the good spark duration period.

Figure 16: 2002 Silverado #1 cylinder secondary ignition Notice the extremely low firing line voltage and the very long spark duration period. These two factors told us the coil was not the cause on the GM 5.7l.

ery 100 millivolts = 1 amp. Some modern COP units have the igniter integrated into the coil so there is no access to the primary negative side of the coil should you consider accessing a primary voltage pattern. The PCM normally controls each individual igniter by forward biasing them with 5 volts which commands the igniter to turn on primary.

When the PCM shifts the 5 volts to 0 volts primary is turned off and the coil fires. You already know that on some modern day COP-equipped engines the coils are buried with no accessibility. We simply need to read the schematic to determine the color code of the B+ feed wire to the coils and clamp the amp probe around this wire at the easy access point. Notice the waveform in Figure 21 from a V6 Honda with a dead miss and no codes. Channel 2 is accessing the igniter con24

Figure 17: 2002 Silverado #8 cylinder secondary ignition This shows a good secondary ignition waveform from a GM coil near plug system.

trol signal from the PCM to the number 4 coil. Channel 1 is the amp probe clamped around the B+ feed wire to the coils and is set at 200 millivolts per division which converts to 2 amps per division. Notice that there is no primary event to the number 4 coil. What we do know is that the control signal from the PCM is there. This coil has the igniter integrated into the coil, which in this case turned out to be a bad coil/igniter. Notice the waveform in Figure 22 after replacing the number 4 coil. In addition, the current probe can help us detect internal coil carbon tracking. Notice the coil amperage waveform in Figure 23.

After the point of primary turn-off, notice the erratic transfer of energy from primary into secondary. This problem is very common in taking out the primary drivers inside the PCM as in Ford and Chrysler systems.

Some engine misfires are not caused by a problem in the secondary side of the coil or the

Figure 18: Secondary cranking KV test Here we’re looking at KV demand during a WOT (wide-open-throttle) cranking clear flood mode on a distributor-equipped engine, using the bar graph function. Notice the 24 to 28 KV demand. This is another way of doing a relative compression test.

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Figure 19: 1998 Mustang 3.8L #5 secondary Another example from a 3.8L Mustang with an engine misfire from #5 cylinder. Look closely at the secondary ignition waveform from #5 cylinder. Do you see the split firing line and the extremely short spark duration period? There are only two possible causes, one being an open plug wire or two, or an open spark plug resister. The cause in this case was an open plug wire.

Figure 20: 1998 Mustang #5 secondary after fix Notice the good secondary waveform after the plug wire was replaced. primary side of the coil or an air fuel ratio problem. A Chevy Express van came in with an intermittent stall and intermittent no-start with a PO316 CKP code. The crank sensor had been replaced twice by another shop. The crank sensor on these engines is mounted on the front timing case cover and triggered by the reluctor on the end of the crank.

A common problem on these Vortec engines is created by main bearing wear which causes the loss of the air gap between the reluctor and the tip of the sensor causing mechanical interference. GM will sell you a shim to restore the air Figure 21: 2003 Honda Odyssey Notice the waveform from a V6 Honda with a dead miss and no codes. Channel 2 is accessing the igniter control signal from the PCM to the number 4 coil. Channel 1 is the amp probe clamped around the B+ feed wire to the coils and is set at 200 millivolts per division which converts to 2 amps per division. Notice that there is no primary event to the number 4 coil.

gap. On my inspection of the tip of the sensor I found no interference. Let’s look at a secondary ignition waveform during the misfire symptom in Figure 24. Notice the loss of the point of primary turn-on. We know that the whole ignition process begins with the CKP input. Notice the waveform in Figure 25 of the CKP signal. You can see the intermittent dropouts of the CKP signal. We simply ran a redundant circuit between

Figure 22: 2003 Honda Odyssey (after fix) Notice the waveform after replacing the number 4 coil.

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Figure 24: 2001 Chevy Express van (loss of secondary) On my inspection of the tip of the Chevy Express crank sensor, I found no interference. Shown here is a secondary ignition waveform during the misfire symptom. Notice the loss of the point of primary turn-on.

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Figure 25: 2001 Express van (loss of CKP) Notice the waveform of the CKP signal. You can see the intermittent dropouts of the CKP signal. 26

COURTESY OF ROBERT BOSCH LLC Figure 23: Coil current (carbon tracking) Notice the coil amperage waveform. After the point of primary turn-off, notice the erratic transfer of energy from primary into secondary. This problem is very common in taking out the primary drivers inside the PCM as in Ford and Chrysler systems.

the CKP sensor and the PCM. The problem went away. The obvious cause was a wiring problem in the sensor circuit from the CKP sensor and the PCM.

That sums up this article this article on ignition waveform diagnostics. The more disciplined you are in consistently scoping out secondary ignition, the quicker and easier it will be to detect causes of engine misfires.

Bill Fulton is the author of Mitchell 1’s Advanced Engine Performance Diagnostics and Advanced Engine Diagnostics manuals. He is also the author of several lab scope and drivability manuals such as Ford, Toyota, GM, and Chrysler OBD I and OBD II systems, Fuel System Testing, many other training manuals in addition to his own 101 Lab Scope Testing Tips. He is a certified Master Technician with over 30 years of training and R&D experience. He was rated in the top three nationally in Motor Service Magazine’s Top Technical Trainer Award and has instructed for Mitchell 1, Precision Tune, OTC, O’Reilly Auto Parts, BWD, JD Byrider, Snap-on Vetronix and Standard Ignition programs. You may have also seen Fulton in many Lightning Bolt Training videos and DVDs and read his articles in many auto service magazines. He currently owns and operates Ohio Automotive Technology, which is an automotive repair and research development center.