When an engine with coil-on-plug (COP) ignition starts to misfire, there are two challenges: finding out which cylinder is misfiring and finding out why. Even if you find a bad coil, simply replacing it is not the whole repair, because like so many other parts of a vehicle, COP ignition coils don’t really die, they’re murdered. We’ll discuss how and why later; first let’s focus on finding the misfire.
Ignition misfire diagnosis can be relatively quick and simple if you have the right tools. Fortunately, there are a lot of different “right tools” available, so it all comes down to the tools you know how to use. Most techs are comfortable using a scan tool, and if the malfunction indicator light (MIL) is on, a scan tool might be enough. But if the diagnostic trouble code (DTC) indicates a random misfire (P0300), the only thing you’ve learned is that the powertrain control module (PCM) “thinks” there’s an ignition-related misfire but it doesn’t have enough information to pinpoint the cylinder.
Remember, the PCM detects misfire with the crankshaft sensor: the crankshaft decelerates as a piston comes up on the compression stroke, then accelerates again on the power stroke after the cylinder fires. If it doesn’t accelerate as expected (per a very complex calculation), the PCM interprets that as a misfire and looks at all the input signals and output devices trying to determine the cause.
Trouble codes with a number lower than P0299 indicate the PCM has found a problem with something in the fuel control or air metering system. The codes from P0300 to P0399 indicate a malfunction in the ignition system, and while these codes can be very specific, any code can be misleading. For instance, P0316 indicates the PCM has detected an ignition misfire within the first 1,000 engine revolutions after start-up. This code doesn’t tell you the real problem... it merely indicates the result of the malfunction. So even though it’s a P03XX code, does the PCM really know enough to be certain it’s an ignition malfunction?
We found a 2006 Dodge Dakota with a 3.7L V-6 engine that consistently misfires on cylinder No. 1, but only for a few seconds immediately after a cold-start. The PCM stored ignition misfire codes for cylinder No. 1, but a detailed and thorough diagnosis shows nothing wrong with any of the COP ignition coils. As it turned out, the misfire is a result of the conditions under which it occurs: only at cold-start and only on that cylinder, plus the position of the engine in the vehicle.
When the engine cools off overnight, the fuel in the rail cools and condenses, leaving a small pocket of vapor at the highest point in the rail, right next to injector No. 1.
Naturally the misfire clears up quickly as the rail fills with fuel, and there’s no reason to chase this problem any further (how could you possibly fix it?). But the question remains: Why did the PCM report this as an ignition misfire?
There is no current in the coil's primary circuit until the dwell period, which is when the coil is earthed and the measured voltage drops to zero. The dwell period is controlled by the ignition amplifier, and the length of the dwell is determined by the time it takes to reach the requisite 5-10 amps, depending on system. Connecting the lab scope With COP, no spark plug cables are present to clamp an ignition pickup on. To measure these signals, the Coil-on-Plug probe TP-COP750 can be used. The Coil-on-Plug probe TP-COP750 is connected to the lab scope using a Measure lead TP-C1812B and the lab scope is set to normal scope mode with the following input setting. Basically, there’s two ways to charge a coil. High current/low inductance and let the current quickly charge it, or low current/high inductance and lean on the inductance over a lengthy dwell. Below is a Lamborghini ignition coil at 2.5ms dwell drawing 14A current, compared to a LS Truck coil at 6.5ms dwell drawing 15A current.
Component monitor
The answer is in the rules that regulate how OBD II works. According to those rules, all the sensors and output devices in the engine management system must be monitored to make sure they operate as expected. A section of software in the PCM called the comprehensive component monitor (CCM) checks most items only after specific conditions are met (coolant temperature, drive time, etc.), but some items are monitored continuously any time the engine is running. This includes fuel injectors and ignition coils.
The PCM monitors the fuel injectors by looking for open or short circuits. It monitors the ignition coils by monitoring the current in each primary circuit to see if it rises to the correct level and then falls again in response to the firing command.
In our misfiring Dodge, the injectors and ignition coils both pass the monitor tests, but the PCM has detected a misfire, and it must be recorded. Since the injectors pass the monitor test, the misfire is reported as a malfunction in the ignition system. This case may be unusual, but it shows why ignition misfire codes don’t always mean there’s something wrong with the ignition system or that there’s nothing wrong with the fuel system.
Here’s a little-known fact that can help you isolate the problem. You already know that when a cylinder misfires continuously, the PCM will turn off the dead cylinder’s injector to avoid sending gasoline straight into the catalytic converter.
However, it doesn’t remember the misfire, as it discovers the misfire anew each time the engine is started. So the PCM will operate the injector during cranking and for a short period right after start-up until it knows for sure the cylinder isn’t firing. If you’re watching the injectors on a ’scope and they all operate during the first 200 engine revolutions at start-up, the injectors are OK.
Let’s assume the scan tool displays a more useful code, like P0351 (Ignition Coil A Primary/Secondary Circuit Malfunction). This code is usually set because the ignition coil doesn’t pass the CCM. Remember, the CCM monitors the rise and fall of current in the primary circuit, and remember that power is supplied to all of the ignition coils through the same circuit. That means the CCM only needs to monitor one circuit to look for current rise/fall in each ignition coil. (By the way, ignition coil A is the first coil in the firing order, B is the second, etc.).
The monitor will see no change in primary current if the winding inside the coil is open or shorted. However, a broken power or ground wire or a failed coil driver can all cause the same symptom and the same trouble code. Here’s where you need to know if the COP assembly has its own driver (transistor) that controls the ground side of the primary circuit. If there are just two wires connected to the coil, the driver is not in the COP assembly; it’s in a separate ignition control module or (most likely) inside the PCM.
If the coil has more than two wires, you’ll need a wiring diagram to determine how to test it. A coil with three wires has one for power, one for ground and one that carries the command signal from the PCM that operates the coil’s internal switching transistor. If there’s a fourth wire, that one sends a firing confirmation signal to the PCM.
Different tools
Many techs will test a coil primary winding with an ohmmeter, but that only checks the coil itself when it’s cold. If you back-probe the (two-wire) coil’s ground circuit at the PCM connector and look for battery voltage with the engine warmed up but not running (KOEO), that tells you something about the entire circuit. But even these tests have limited value: If the primary and secondary resistance are both correct and the whole circuit is complete, that still doesn’t prove the circuit works properly when the engine is running.
By now you get the point that a scan tool and a digital volt ohm meter (DVOM) don’t always provide enough information for an accurate misfire diagnosis. Any ignition system can be affected by heat and engine load, so testing a coil with the engine running gives you a more complete picture.
There is a new generation of easy-to-use tools that will show without a doubt whether or not the coil is firing. They’re based on magnetic induction, the same thing that makes a coil work in the first place.
By simply touching the inductive pick-up to the top of the coil, the tool picks up the rise and collapse of the magnetic field in the primary circuit. Some tools will simply flash a light to indicate the coil is operating, some display data on a small screen, and some connect directly to an oscilloscope to display a waveform of current flow in that coil’s primary circuit.
Of course the very best way to test any ignition system is with an oscilloscope that can display data from all the cylinders at once. You don’t even have to know that much about what actually appears in the waveform; if one doesn’t look like the others, you’re that much closer to finding the problem. The problem is, if you don’t use a ’scope regularly, you’re likely to forget how to set it up when you really need it.
Remember, the best tools are those you’re comfortable using. It only takes a few minutes to find the fuse that supplies power to all the coils and tap a ’scope into the circuit. Do it often enough to recognize known-good and you’ll quickly recognize something that doesn’t look right on any engine.
Before connecting an oscilloscope directly to the primary voltage circuit, check the ’scope’s maximum allowable input voltage. The primary circuit can create spikes of more than of 400 volts under normal conditions. Most voltmeters can handle this, but most oscilloscopes can’t and you’ll need to connect an attenuator to protect the ’scope. That’s one reason we prefer to look at primary current with an amp probe. An oscilloscope also lets you look at the control signal on three- and four-wire coils. This is typically a square wave signal of about 4 volts that matches the timing of the current ramp in the primary circuit. If you see the command signal but no current ramp, you know the PCM is good but the coil driver is not responding.
What killed the coil?
As noted earlier, even though COP coils are known to fail with some frequency, that failure is usually caused by something outside the coil. The most common causes are worn or incorrect spark plugs, excessively lean air/fuel mixture and liquid getting into the spark plug tubes.
If primary current level and dwell time are correct, a coil will generate enough voltage to meet almost any demand. A healthy ignition coil with old worn out spark plugs might develop an initial firing voltage of 80 kV or more.
That’s the coil doing its best to keep up with the demand, and for a while it will. But when a coil works that hard, the secondary winding overheats from generating that much voltage, and eventually the heat will damage the winding or the driver transistor. The coil will either begin to misfire when hot, or it will fail completely, or it will damage the driver.
To understand how hot a coil can get, look up Ford TSB 13-4-17 or 11-8-2. They include pictures of ignition coils that melted because the coil driver inside the PCM shorted to ground and kept the primary current turned on continuously (primary current normally lasts about 5 milliseconds). In case you’re not familiar with this (infamous) issue, the fix is to replace all the ignition coils and the PCM, which requires reprogramming the PCM, which in turn requires two original ignition keys to reprogram/reboot the anti-theft system.
Combine a worn out spark plug with lean air/fuel mixture, and that 80kV is going to find an easier path to ground. Even the best insulator boot can’t contain it indefinitely; at first the spark will leak through the boot to the valve cover only during acceleration, but eventually it will happen all the time. That’s why boots and connector springs are available separately on some models, so they can be replaced when installing new spark plugs. If the old spark plug has a carbon track on the ceramic, that boot should be replaced because there’s a matching track in the boot that offers an easy path to ground.
If there’s liquid in the spark plug well, even a new insulator boot might not be able to contain the secondary voltage. Chrysler issued a recall for the 2004-06 Dodge Durango (18-024-06) to replace ignition coils that were shorting through the (non replaceable) boot to the valve cover. The repair included installing a redesigned windshield cowl to keep rain water out of the engine compartment.
Other vehicles have had similar problems, but not just because of weather. Leaky spark plug tube seals, coolant leaks and even water or mud splashed up from below have all been known to cause this problem.
So now we’ve seen how the PCM detects and reports misfire, and we’ve shown that COP misfire diagnosis can be relatively quick and simple if you have the right tools. We’ve also discussed the importance of finding out what damaged the coil, because COP ignition really is simple and reliable as any other type of ignition system. ■
Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. He began his writing career writing service manuals at Chilton Book Co. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.
Dwell Lab Scope Coil Tester
To read more of Jacques Gordon's articles, click:
Coil-on-plug, or COP, features an individual coil dedicated at each cylinder, with the COP connected directly to the spark plug, eliminating the need for plug wires. Due to variances in COP design among auto makers, spark control, troubleshooting and diagnostics can vary. This article is intended to provide education relative to COP, along with tips and precautions regarding testing and diagnosing engine misfire issues.
At the risk of dating myself, I can remember the good old days of distributor-equipped engines with external coils and a coil wire to which we could conveniently attach our scope’s secondary KV probe. And by using our number 1 trigger, we could sort out each individual cylinder’s firing characteristic (see Figure 1), enabling us to detect lean cylinder conditions, rich cylinder conditions, low cylinder compression problems, high secondary firing KV demands, insufficient spark duration periods or any type of a density misfire.
Now on modern day engines equipped with COP-type ignition systems, access to all the diagnostics of reading and analyzing a secondary ignition waveform creates a challenge and the need to be creative. Creative simply means using a secondary KV wire between the coil and spark plug and attaching our secondary KV probe to the wire and analyzing the single secondary ignition waveform.
The necessity of understanding the secondary ignition waveform still exists. The diagnostics that were yielded to us on distributor-equipped engines are still available using this method on COP-equipped engines by viewing a COP secondary ignition waveform (see Figure 2). The critical part of these secondary ignition waveforms is the spark line in reference to length, angle and the presence of turbulence. Lean conditions will shorten the length of the spark line and bend it upward and increase the turbulence, the same as the distributor-equipped engines.
Remember air molecules are non-conductive, which increases the voltage demand of the spark line to ionize them. Notice, however, the length of the spark line on COP ignition systems is significantly longer in duration simply because we only have the spark plug air gap of the spark to overcome, (no more rotor air gap).
Remember when a failure of the coil on the old distributor-equipped engine was rare, when one coil was responsible for firing all the spark plugs of a four-, six- or an eight-cylinder engine? Now that most modern day engines are equipped with COP-type ignition systems, a COP coil failure is becoming common.
This begs the question — how does single coil and distributor-equipped engine reliability compare to common COP coil failures?
If you are a technician who uses and appreciates the diagnostic value of an amp probe coupled with a lab scope, most distributor-equipped engine coils required 4 to 6 amps for full coil saturation. As the rpm increased the point of primary turn on had to occur sooner to ensure enough charge time for sufficient coil saturation. Now with the new COP-type coils the PCM has sufficient time to individually control each individual coil’s dwell period or coil saturation times.
The benefit is that at higher rpm a weak spark from shortened dwell periods (such as those experienced on distributor-equipped single coil engines) has been pretty much eliminated. By comparison purposes, the new COP coils are now saturated with nearly double the amperage values with no limitations with reduced coil charge periods as rpm is increased, a common concern on single coil systems.
There are some distinct differences in the COP units now being used by the automobile manufacturers that can enhance our diagnostic strategies when addressing a misfire. Ford and Chrysler COP units are directly controlled by the PCM, meaning the coil drivers are integrated into the PCM. The concern here is that shorted primary windings or internal coil carbon tracking can take out the PCM.
On the Ford COP-equipped engines, the PCM will multi fire the coils below 1,000 rpm to ensure good combustion during light load lean conditions (see Figure 3). Above 1,000 rpm the PCM will revert back to one firing event
In addition, as we all know a pattern failure misfire on the Ford Triton engines is a loss of insulation on the secondary spark plug boot causing voltage to arc to the plug well. Whenever replacing spark plugs on these engines it is always highly recommended to replace the boots and suppressor springs. These types of misfires usually occur under loaded acceleration conditions when the KV values increase as cylinder pressures increase. Also, I’m sure you have customers like mine who are waiting for the second coming to have their spark plugs replaced.
On some modern Chrysler COP-equipped engines the PCM will monitor the collapse time of each individual firing time (spark duration), however, the actual values are not accurate and should be used for comparison between each individual coil firing times. You can get these values from your scan tool (see Figure 4). Good spark duration periods on COP-type ignition systems will vary between 1.5 and slightly over 2 milliseconds during a park warm idle no load condition.
On Ford and Chrysler systems, using a lab scope and probing the coil negative terminal will yield a primary ignition waveform (see Figure 5). The primary and secondary waveform will mirror each other in the spark line area. The spark line characteristics we discussed earlier still apply.
Notice, however, the secondary ignition waveform voltage per division is at 1 and 2 KV while the primary ignition waveform will vary between 10 and 20 volts per division. The scope time base is at 1 millisecond per division. Also, keep in mind that the scope’s trigger level is best set just above the spark line voltage level.
If the secondary KV demand is too high from, let’s say, worn spark plugs or lean density conditions the primary spark duration periods will be too short and lean cylinder conditions will abruptly bend the spark line voltage up during a power brake condition. On the Chrysler and Ford COP units, a secondary KV wand can be used to pick up a secondary waveform by simply laying the probe on top of the coil. Keep in mind the attenuation factor of the KV probes are 1,000 to 1. This means that if your scope is set to 1 volt per division the attenuation is now 1 KV per division. A time base of 1 ms. per division is usually ideal. The secondary KV wand can also be used on DIS secondary leads, as well.
Half the cylinders are fired at a negative polarity and the other half are fired positively. On the cylinders that are fired with negative polarity, you must use the invert function on your scope while turning it off while viewing the positively fired cylinders. By the way, all distributor-equipped engines and COP-type ignition systems fire secondary at a negative polarity, meaning that you must use the invert function of the scope. The secondary KV wand will not work on most Asian COP units because they are heavily potted causing the magnetic field to be too weak to be sensed by the COP wand. This will require a secondary lead between the coil and spark plug and using the conventional KV probe around the lead. Remember that we refer to the spark line characteristics as our electronic window inside the combustion chamber.
There are several versions of the secondary KV wand available at www.AESWAVE.com
On the GM coil near plug units found on the V-8 Vortec engines, there is a short 8.5-inch secondary lead between the coil and spark plug. The COP wand works very nicely on these systems by laying it next to the plug wire, or you can simply use the conventional secondary KV probe clamped around the plug wire. Again, the invert function must be used.
There are three different vendors that supply the coils for GM. They are Delphi, Melco and Denso. The coils are not interchangeable, but the secondary leads look identical and are not interchangeable because the resistance values vary greatly.
As we stated earlier, access to the primary side of a Ford or Chrysler can be done by back probing the negative terminal of these coils and thus viewing the primary side of the coil.
That test is not possible on the GM coil near plug units because the ignitor is integrated into each individual coil. The PCM uses low current drivers to bias (turn on) the coil current. The signal from the PCM is a 5 volt / 0 volt toggle. The rising edge to 5 volts turns primary on while the falling edge to 0 volts turns primary off causing the primary magnetic field to collapse, which is mutually inducted into secondary and multiplied to create the high secondary firing voltages needed.
A tech tip here may be needed. During KOEO we can bias these coils with a standard 12 volt test light. By picking up 12 volts with the alligator end of the test light and by piercing the ignitor control wire and momentarily touching the ignitor control wire, we will fire a 25 KV spark tester.
While we are on the subject of using a spark tester, keep in mind that all good COP units can easily fire an ST125 spark tester. For those of you who use the adjustable spark testers, a 3/4-inch gap is very close to a 25 KV demand.
One important note here on the GM coil near plug units: The coils are powered up from a single main ignition relay and all the coils and ignitors units get their ground at one location, so don’t forget about power and grounds (see Figure 6). Your scan tool may have the ability to turn on this relay during KOEO. In addition, the PCM monitors this voltage from the ignition relay so you will have a scan tool parameter to monitor the ignition feed values from this relay.
I recently had a 4.8L come in with an intermittent miss and a P0300 MIL. The owner had previously paid a shop $1,500 to replace the plugs, coils and secondary leads — to no avail. The freeze frame data indicated a 45% addition to the short- and long-term fuel trim values.
Initially my first suspicions were from a lean density misfire, since all the cylinders on bank 1 indicated multiple live and history misfires. On my diagnostics using my secondary KV probe, I found no secondary events happening on the bank 1 cylinders. By probing the pink power supply wire to the bank 1 coils I found no voltage.
An open circuit at the connector on top of the valve cover was the fault. A current probe clamped around this power supply pink wire would have also pinpointed the problem.
On most all Asian COP units the coils also have the ignitors integrated into them and are forward biased (turned on) by a 5 volts square waveform. As with the GM coil near plug units, the falling 5 volt to 0 volt toggle turns primary off and fires the coils. Using a COP wand in the Asian COP units does not yield a good secondary ignition waveform because the coils are so heavily potted the magnetic field is too weak to pick up. The website www.AESwave.com offers the secondary leads to marry the coils to the spark plugs, meaning you can use a secondary KV probe to view secondary in the conventional way.
Again, access to the primary side is not possible due to the ignitor integrated inside the coil. However, an amp probe clamped around the coil positive feed wire can verify good or insufficient coil saturation values.
The point here is most critical. A single cylinder misfire can easily be a bad coil, but looking at the design of the system, a control circuit issue or a bad PCM driver could also cause a loss of spark.
In the real world of diagnostics, whenever a misfire problem occurs we as technicians usually rely on scan data initially. Having said that, the cardinal rule is that a single cylinder misfire from no spark will create very minor and very brief single digit fuel trim corrections while a lean density misfire from, say, a bad injector, low fuel pressure, vacuum leak or a bad MAF sensor will create double digit addition to fuel trim. Conversely speaking, a rich density misfire will create double digit negative fuel trim corrections.
Also keep in mind that in most modern day systems, whenever the misfire is severe enough the PCM will force the engine back into open loop and cut off the injector from the misfiring cylinder, which means the fuel trim values cannot be used as we explained earlier. It would be necessary to view the scan tool fuel trim parameters before the PCM forces the engine back into open loop.
A recent case study involved a 2001 Honda Odyssey that came into my shop with a misfire symptom and no MIL with no codes, not even a pending code. Since this was a non-CAN-compliant system there were no misfire test results in the Mode 6 menu. You can’t say enough about the good old “feel through the seat of your pants,” the engine was running on five cylinders!
The front coils were easy to get to so we manually disconnected one at a time and monitored the rpm drop. While disconnecting the number 4 coil we never got the rpm drop. Putting a spark tester on the number 4 coil showed no spark. Could it be a common COP failure? What about the ignitor drive signal from the PCM (see Figure 7)? You will see a 5 volt/0 volt toggle, meaning that the PCM is sending the control signal and just like the GM systems the ignitor is forward biased by the PCM. Now let’s say the ignitor signal was flat lined at 0 volts. Could the ignitor have shorted the PCM driver?
We could easily find out by unplugging the coil. The 5V ignitor control voltage is sourced at the PCM. If we get our 5 volts back then we know the ignitor has shorted the PCM control voltage. However, in this case the ignitor control signal was present and the problem was simply a bad coil. Once the coil was replaced we should still analyze the waveform (see Figure 8). Is there a noticeable lower coil saturation value on the number 4 coil?
If your answer is yes, you are correct. If the primary feed voltage is good to this coil, then the coil should be returned to the parts supplier because the coil’s amperage saturation values are significantly lower.
On the Toyota COP systems the ignitors are also integrated into the coils the same as the GM and Honda systems, so access to a primary ignition voltage waveform is not possible. However, the amp probe becomes a valuable tool to ensure good coil saturation values.
These systems are unique in the fact that the PCM uses a separate IGT (ignitor trigger control circuit) for each individual coil. The PCM also monitors each coil’s firing event by monitoring the IGF (ignition confirmation circuit). In the event of a loss of the IGF signal to the PCM, the PCM will shut down the cylinders injector. A loss of all of the IGF signals will cause the PCM to shut down all of the injectors.
The scenario you could have here is that on a no start you could have no spark and no injector pulses at the same time. We have had a case where a Toyota came in as a no start indicating no spark and no injector pulses. Monitoring the IGF circuit with a lab scope showed no IGF pulses at all. As we unplugged number 3 coil the IGF pulses came back and the engine started — on 5 cylinders.
The point here is that a bad coil can pull down the IGF circuit and cause the PCM to shut down all of the injectors and disable primary on the other cylinders. Figure 9 indicates a known good representation of the Toyota COP systems showing the comparison of coil amperage saturation, IGT (trigger) and IGF (ignition confirmation) from a single cylinder firing event. The bottom trace is the IGT signal while the middle trace is the IGF signal and the upper trace is coil current using the amp probe.
Let’s look at another example of an amp probe, not only looking at good coil saturation but also how well the energy is transferred between the primary side of the coil into the secondary side.
Modern day ignition systems are known as “divorced,” meaning there is no hard-wired circuit between the primary coil windings and the secondary coil windings. There is simply an air gap. As the primary field collapses it is mutually inducted into the secondary windings across this air gap and multiplied several hundred times. Internal coil carbon tracking has a major effect on the smooth and complete transfer of this energy.
Take a look at Figure 10. At the amperage waveform’s point of primary turn off, note the erratic oscillations caused by internal coil carbon tracking. This problem can easily take out the primary coil driver in the PCM as in the Ford and Chrysler systems.
We talked earlier about the use of a secondary lead between the coil and spark plug and then using our secondary KV probe around the plug wire to analyze more completely the firing and combustion event. Figure 11 shows a good secondary ignition waveform from a Toyota engine. Notice the good spark duration period of over 1.5 milliseconds.
In the real world we all certainly use spark testers to check for spark. As I stated earlier, all COP coils have the ability to crank out a good and consistent 25 KV demand.
Dwell Lab Scope Coil Kit
While that is the real world initial test, we explained the diagnostic value of using the amp probe and by looking at secondary with the secondary lead between the coil and spark plug and then by using the secondary KV probe from your scope to monitor not only spark but the combustion event as well. In addition, as you will recall there are some very distinct differences between the COP systems we covered in this article. ■
Dwell Lab Scope Coil Mounts
Bill Fulton is the author of Mitchell’s Advanced Engine Performance Diagnostics and Advanced Engine Diagnostics manuals. He is also the author of several lab scope and drivability manuals. He is a certified Master Technician with over 30 years of training and R&D experience. He currently owns and operates Ohio Automotive Technology in the Columbus, Ohio, area, which is an automotive repair and research development center.