Ignition Scope Secondary Terminology
16-04 Understand ignition scope secondary terminology.
No matter the scope or ignition system, there is standard terminology, operating procedures, and testing methods that apply to all. Electricity operates the same way in all ignition systems. Looking at different parts of ignition systems waveforms allows the technician to visualize what area of the system may be the culprit of the failure. Utilizing a digital oscilloscope to view these waveforms allows for quick comparison and allows the technician to save any generated information for the customers file.
Firing Line
The firing line (inductive kick) is a vertical line measured in kilovolts and shows the beginning of the spark event. The inductive kick occurs when the primary windings are ungrounded. The voltage it displays represents how much of the coil’s energy is needed to bridge the plug’s gap and ionize the air-fuel mixture. The firing voltage must overcome all secondary resistance, including air-fuel mixture, compression, and ignition components, including the spark plug wires (if present), cap and rotor air gap (if present), the high-voltage spring resistance (COP, if present), and the spark plug. Under normal operating conditions, the spark plug gap should be the highest resistance in the circuit.
Typical firing line voltage should remain relatively equal between cylinders, usually between 5 and 15 kV at idle and cruise, and it should increase to about 20 kV on snap acceleration. The maximum difference between cylinders under any scenario should be under 3 kV (FIGURE 16-8). The normal firing line voltage varies based on system design and its condition. Ignition system components with high mileage will require more voltage than the same system with new components. To determine what is normal, use stored waveform libraries for the system being worked on, or in multi- coil applications, compare cylinders against each other. Always look for the cylinder(s) that stand out from the others.
FIGURE 16-8 Parade pattern showing a normal and an abnormal firing voltage. The parade pattern allows for comparison of all the cylinders at once, making locating the offending cylinder easier. Normal firing voltage at idle and cruise should be about 5 to 15 kV and remain fairly stable at any rpm. The maximum suggested variance between cylinders is 3 kV.
The following are frequent causes of problems with firing line height (kV demand):
- As air-fuel ratio leans out, kV demand increases.
- As in-cylinder pressure (compression) increases, kV demand increases.
- As spark timing retards, kV demand increases.
- An increase in the “air gap” causes kV demand to increase.
The height of the firing line is the voltage required to ignite (ionize) the air-fuel mixture. The amount of energy needed varies depending on the ignition system type and the engine’s operating and mechanical conditions (FIGURE 16-9). Modern ignition systems use a wider spark plug gap, increasing the kV demand. Mechanically, the higher the resistance in the cylinder (higher compression, leaner air-fuel ratio), the more voltage required to bridge the plug’s gap and the higher the firing line (FIGURE 16-10). The voltage output of the coil continues to increase until it is strong enough to jump the gap or the coil runs out of energy. A high firing demand (kV line) will result in a shorter burn time (spark duration) since there is only so much voltage available in any coil (FIGURE 16-11). Conversely, low resistance requires less energy, meaning the firing line will be lower and the spark duration will be longer.
FIGURE 16-9 High even secondary firing voltage indicates concerns that affect all the cylinders equally. Under most operating conditions, the firing kV should be between 5 and 15 kV. Higher voltage requirements indicate high secondary resistance, unless under heavy load or a snap throttle test. Typical causes include worn spark plugs, late timing, a lean air-fuel mixture, excessive rotor air gap (if present), or a high-resistance coil wire (if present).
FIGURE 16-10 A parade pattern display of a V6 engine showing low firing voltage for one cylinder. Apply the firing order of 1, 4, 2, 5, 3, 6, to reveal that Cylinder 4 is low. The parade pattern allows for a quick and easy comparison of all the cylinders firing kV and spark line. Low secondary voltage in a single cylinder is a result of low cylinder resistance. Causes include a fouled spark plug, a cracked plug insulator, too small of a plug gap, a rich air-fuel mixture, a shorted plug wire, or low compression. If all the cylinders are equally low, a rich air-fuel mixture is normally the cause. The excess hydrocarbons act as a conductor, lowering the voltage needed to begin and maintain spark.
FIGURE 16-11 High firing kV spikes point to high secondary resistance. High firing demand can be a result of a lean air-fuel mixture, wide or worn spark plug gap, high-resistance spark plug wire (if present), or COP high-voltage spring.
If the firing voltage is too high, there is a breakdown in the secondary, and a misfire will occur (FIGURE 16-12). Typically, the misfire begins under heavy loads; as the condition worsens, the miss may become noticeable at all engine speeds, including idle. Excessive misfires can damage oxygen (O2) sensors and catalytic converters if not repaired promptly.
FIGURE 16-12 The firing line of a DIS system. The typical modern EI system firing voltage should be between 5 and 22 kV, depending on engine load. The higher the load, the higher the firing demand. Spark kV is affected by secondary circuit resistance. In a DIS or a distributor ignition system, an open or high-resistance spark plug wire or a spark plug or an excessive plug gap will increase kV demand. In a COP system, the secondary ignition system includes the coil, high-voltage spring, and the spark plug and gap. Additional causes of high kV in a distributor ignition system include the distributor cap and rotor air gap.
Increasing the firing voltage too high—over 30,000 volts—too often can cause ignition system damage. Depending on the system, the spark may arc out the side of the spark plug wire or boot, the COP boot, or the coil itself internally or externally as the voltage looks for a path to ground. Since the voltage level is so high, all of these alternate paths for current are normally visible, except internal arcing of the coil.
Spark Line
Once the spark has been created, the air-fuel mixture is ionized, resistance breaks down (decreases), and the voltage level drops as current flows across the plug gap. This lower level of voltage is the spark voltage. The scope shows the spark voltage as a horizontal line that connects to the firing line. The height of the spark line indicates how much energy is required to sustain the spark across the electrodes. The length of time the spark line lasts is called the burn time, or spark duration. The voltage level of the spark line is measured vertically in kV, and the burn time (duration) is the length of the line end, measured in ms (FIGURE 16-13). The duration of the burn time is a direct result of the coil’s reserve and secondary resistance. Secondary circuit resistance includes electrical, fuel, and mechanical systems and components.
FIGURE 16-13 A COP secondary waveform showing the amount of voltage and the duration of the spark across the spark plug’s electrodes. The length of the spark line is known as “spark duration” or “burn time.” The spark line should be relatively flat, but it may show some turbulence, will start at about one-quarter of the firing voltage, and should last about 1.0 to 2.0 ms. Most ignition or mechanical faults will show up as a shorter than normal burn time when compared to a known-good cylinder.
In addition to igniting the spark plug at the precise time, the ignition system must maintain the firing of the spark plug as long as there are sufficient hydrocarbons in the cylinder. If the coil cannot sustain the arc across the spark plug’s gap, the cylinder will misfire, especially under load. The misfire will cause engine efficiency to drop, and emissions will increase.
TECHNICIAN TIPSome “floating” of the firing line voltage, up to 4 kV is normal at idle and steady throttle. The floating is due to the variance in resistance from the air-fuel ratio. The air-fuel mixture in the combustion chamber is not totally homogenous, containing some pockets of mostly air and other pockets of more fuel than air. This causes the firing demand of the spark plug to change. When more air than fuel is in the spark plug gap, there is a lack of conductors (hydrocarbons), so the kV demand is increased. When there is more fuel than air, the kV demand is lower. A fault is present if the firing line is always higher (increased resistance), or lower (decreased resistance), than the other cylinders or a known-good waveform. Additionally, firing kV is difficult for the scope to measure accurately. An oscilloscope randomly samples voltage, and it is possible that the scope is not measuring voltage at the highest spike and therefore cannot display it resulting in a lower reading. Therefore, firing voltage typically varies from peak to peak.
TECHNICIAN TIPA firing line does not guarantee spark; there must be a spark line to show that spark is occurring.
TECHNICIAN TIPThe voltage (kV) requirement for older distributor engines tends to be lower than that for modern designs. The firing kV on most conventional ignition systems falls between 5 and 12 kV. Higher compression ratios, leaner air-fuel mixtures, and wider spark plug gaps all contribute to increased kV demands in late-model applications. Firing voltages in these systems range from 5 to 20 kV, depending on engine load.
Any voltage that is not used to ionize the air-fuel mixture, as seen in the firing line, is then consumed by the spark line. Every coil has a fixed amount of energy. If the coil uses more energy to cross a large gap to fire the spark plug, there will be less voltage available for the burn time. Therefore, anything that affects the firing line has an equal but opposite effect on the spark line. If the firing line is too high, the spark line will be short; if the firing line is too short, the spark line will be too long.
The spark line should be horizontal but not smooth, and it should begin at about one-quarter of the voltage of the firing line. The spark line “jumps out” from the firing line at a right angle, typically between 1.0 and 4.0 kV. On some DISs and COP systems, the spark line can reach 6 kV. The spark line should bend down slightly at first, then at about the halfway point, it should start bending up. The burn time generally falls between 1.0 and 2.0 ms; most modern systems are between 0.8 and 1.5 ms. The voltage level and burn time should be even on all cylinders and all should start at the same point.
The spark line is much easier for a lab scope to read compared to the firing kV, offering more reliable data. The spark line provides much more information about the condition of the spark and other engine operating conditions than the firing kV. When analyzing a secondary ignition pattern, focus on the spark line voltage and shape. Most problems that cause a high firing voltage will cause a short burn time. When both the firing line and spark line are low, it indicates low coil energy. Check the coil and the voltage supply to its primary wiring.
Spark duration or coil burn time is the time in milliseconds during which the coil continues to supply energy to keep the spark moving across the plug gap (FIGURE 16-14). The duration is a direct result of the secondary circuit resistance. On most current ignition systems, the duration should last between 1.0 and 1.5 ms. The burn time of a conventional ignition system is generally between 0.8 and 1.5 ms. When any cylinder is under 0.8 ms, it usually points to a misfire—a result of either low resistance due to a fouled plug or excessive firing voltage from high secondary resistance. A fouled plug provides an easier path to ground for the high voltage, instead of having to bridge the gap between the electrodes. Because the coil does not need to maintain energy across the gap, the burn time is short (FIGURE 16-15). Conversely, an increase in resistance uses more of the coil’s energy for the initial firing voltage, leaving less reserve energy to maintain current flow, which means a shorter burn time.
TECHNICIAN TIPFiring kV and spark duration (burn time) are inversely proportional. In other words, an increase in firing kV will always result in a decrease in burn time. Likewise, a decrease in firing kV extends the length of the spark line. High internal resistance due to an ignition, fuel, or mechanical issue will result in a higher firing kV demand with a shorter burn time. The high resistance will cause the spark line to slope upward from the firing line. Low resistance will result in a lower firing kV and an extended spark line.
FIGURE 16-14 One of the tricks many scope experts use is to divide the spark line in half. Any fault that occurs between the firing line and the halfway point of the spark line points toward a problem outside the cylinder. If the spark line shows a concern from the midpoint toward the coil oscillations, there is a problem inside the cylinder.
FIGURE 16-15 The most important part of any ignition pattern is the spark line. The spark line shows what is happening inside the cylinder. Diagnosing misfires can be challenging because not all misfires are ignition related. By analyzing the height, slope, turbulence, and duration of the spark line, a well-read scope user can determine the cause of a misfire. The slope of the line will show ignition system, fuel system, and some mechanical concerns.
On the left is a normally operating cylinder. The spark line starts at less than one-quarter of the firing voltage and runs almost horizontal to the firing line. Slight turbulence is seen on the spark line, with an upward pointing nose before the coil oscillations. On the right is an open spark plug wire. The firing line is almost as high as the firing kV. There is virtually no burn time or coil oscillations after the spark line, since the coil used all its energy to jump the undesired gap in the open spark plug wire.
Typical Spark Line Firing Voltage and Duration, by Ignition System
Distributor ignition systems have two gaps, requiring more coil energy to keep the spark burning: the first at the rotor to distributor cap terminal and the second at the spark plug. Expect a spark line voltage between 2 and 4 kV, with a minimum duration of between 1.3 and 1.5 ms.
Waste spark systems/DISs also have two gaps, one at each spark plug of the paired cylinders. Waste spark engines also run leaner than most distributor ignition systems, adding even more secondary resistance. Expect a spark line voltage similar to a distributor ignition system: between 2 and 4 kV on average, with a minimum duration of 1.0 ms.
COP systems have one designed gap at the spark plug, lowering the spark line voltage to between 1.0 and 3.0 kV on most systems, with an average spark duration between 1.0 and 1.5 ms. Gasoline direct-injected (GDI) and newer engine systems tend to have a shorter duration and slightly higher spark line voltage due to leaner air-fuel mixtures.
The spark line should slightly rise, or arc up, at the end, before it flows into coil oscillations. This bump is called the nose and shows that there is energy left in the coil. A missing nose suggests that spark did not occur in the cylinder and that the coil’s energy was bridged directly to ground usually, through a shorted or fouled spark plug. A plug wire with a dead short to ground (no air gap) can also cause a no-nose condition. The shape and height of the nose are directly related to the resistance of the gases remaining in the cylinder. For proper combustion and burning of the air-fuel mixture, the hydrocarbons and length of the spark should match. In a properly operating cylinder, the flame front should last longer than the spark, and the nose will show a slight upward bump. If the arc of the nose is high compared to the other cylinders and if there’s a high firing kV, suspect a lean cylinder. Without hydrocarbons remaining in the combustion chamber, the resistance increases and the nose will rise upward. An erratic spark line, with a significant increase (bump) in the rise of the nose, indicates that the spark has outlasted the flame front, a result of a lean condition. If all the cylinders show a sharp increase in the bump of the nose, there is a lean condition common to all cylinders. Potential causes include low fuel pressure or volume or a faulty mass airflow (MAF) or manifold absolute pressure (MAP) input.
If the spark line starts higher, over 6 kV, look for high resistance in the secondary circuit. The most frequent causes are an open or worn/wide gapped spark plug, high resistance, or an open in the spark plug wire or the COP high-voltage spring. Additional ignition system faults include a worn or corroded distributor cap or rotor. Lean air-fuel mixtures will also cause a high beginning-spark voltage.
Low spark line voltage is most frequently a result of arcing outside of the cylinder. External carbon tracking, a coil boot or spark plug boot arcing to ground, and a leaking spark wire are all possibilities for a low spark line. Internal carbon tracking of the coil’s insulation is another possible cause. Excessive external carbon tracking will result in a misfire and may show almost 0 kV and no turbulence on the spark line.
Most ignition system or mechanical faults will stick out in comparison to a good cylinder. The spark line’s duration will be shorter than known-good cylinders. Anything under 0.8 ms indicates an ignition misfire. As with the firing or kV demand, spark kV is affected by circuit resistance.
The length and shape of the spark line also provide diagnostic information. If spark is occurring, any problem in the cylinder will be revealed by a reduced burn time (spark duration), change of slope, or hash of the spark line. Hash is a result of normal cylinder turbulence and combustion; their presence indicates that the ignition event took place in the cylinder not arcing to ground. An absence of turbulence means nothing is happening in the cylinder. In any functioning cylinder, the mix of air and fuel is not homogenous (consistent), leaving pockets of air only or fuel only. These independent pockets of air and pockets of fuel cause the air-fuel ratio in that part of the cylinder to change. As the hydrocarbons and oxygen molecules change, the flame front fluctuates, appearing as hash in the spark line. Therefore, some turbulence is considered normal and is an indication that the spark is burning in the cylinder.
Additionally, turbulence is a design characteristic of an engine and a result of valve overlap, the shape of the top of the piston, and exhaust gas recirculation (EGR). The amount of turbulence in a cylinder increases as load and engine rpm increase. As with all ignition waveform analysis, compare all cylinders against each other and look for the one that stands out. For example, if only one cylinder is showing excessive turbulence, suspect excessive carbon buildup on top of the piston. A borescope inserted into the spark plug hole may help reveal excessive carbon.
The combination of a short spark line with normal firing kV shows a lack of electrical energy. Check for low primary current flow, a weak coil, or a worn spark plug. To verify, check primary current flow by using an amp clamp. If current flow is normal, suspect a weak coil.
A spark line that begins in the normal range with a downward slope indicates high resistance in the spark plug or spark plug wire or a high-voltage spring in a COP coil. In a conventional ignition system, corrosion at the cylinders terminals or a loose spark plug cable at the distributor cap will also cause high resistance.
TECHNICIAN TIPA high, fast upsweep of the spark line indicates a very lean cylinder. Suspect a failing injector, injector driver, or cylinder-specific vacuum leak. A slow upsweep of the spark line also shows a slightly lean cylinder. To verify whether the lean condition is a result of a vacuum leak or the fuel delivery problem, raise the engine speed to about 2,500 rpm. If the cause of the lean condition is due to a vacuum leak, at higher rpm the spark line slope should return to a relatively flat horizontal line. If the upsweep of the spark line remains, investigate for a fuel system concern. Check the fuel injector for being inoperative or partially restricted. If the injector is not operating correctly, verify power and control before condemning an injector. A downward sloping spark line is an indication of a rich condition. If it’s on only one cylinder, suspect a dripping fuel injector or sticking fuel injector driver.
A high initial spark line with a short downward slope indicates a plug gap that is too wide or a plug wire/high-voltage spring that is open or has very high internal resistance. The difference here is the short spark line. Excessively high resistance in the cylinder’s secondary circuit requires greater than average voltage to begin ionizing the air-fuel mixture. The result is a high initial firing kV and a short, downward sloping spark line (FIGURE 16-16). The downward slope indicates high resistance, and the short spark line is due to the coil running out of energy due to the high initial demand. Do not forget that high resistance in a cylinder can also be a non-electrical fault like a lean air-fuel mixture due to a vacuum leak, a clogged or restricted injector, improper injector spray pattern, or carbon on the back of the intake valve that is absorbing the hydrocarbons from the fuel. If all the cylinders are showing high resistance, check fuel pump pressure and volume. Late valve or ignition timing will also affect all cylinders equally. On a V-type engine with variable valve timing (VVT), a mechanical fault in only one bank can cause high resistance in only half the cylinders.
FIGURE 16-16 The raster pattern stacks the cylinders from the bottom to the top following the firing order. In most applications, this means Cylinder 1 is on the bottom. The last cylinder in the firing order is on the top. The cursor (dashed line) indicates the average of all the cylinders. The dashed line provides a reference point showing that Cylinder 6 has a long burn time and that Cylinder 5 has a short burn time.
A long burn time (spark duration) can be due to a fouled or tightly gapped plug, rich mixture, or low compression. These faults reduce internal cylinder resistance, requiring less voltage to initially jump the plug gap, leaving more energy to maintain the spark. A rich mixture is more conductive than a stoichiometric or lean air-fuel mixture. Low compression reduces cylinder pressures.
A short burn time can indicate that the coil is weak, having used all its available energy to fire the spark plug, leaving little energy to continue the spark across the plug gap. Other potential causes are worn plugs, high-resistance plug wires or COP springs, a lean air-fuel ratio, or a high compression. These faults all increase resistance in the combustion chamber making the coil work harder to start and maintain the spark.
Hydrocarbons in the air-fuel mixture cause the mixture to be conductive. Conversely, a lack of hydrocarbons (lean air-fuel mixture) increases resistance. In a lean cylinder, the available hydrocarbons are consumed before the spark extinguishes, causing an increase in resistance that is noticeable in the spark line (FIGURE 16-17).
FIGURE 16-17 A cylinder that is running lean will have a high firing kV and a spark line that curves up disproportionately to the other cylinders. The increase in voltage required is due to the extra air that acts as an insulator, increasing the energy needed to keep the mixture burning. The hydrocarbons in the fuel serve as a conductor in a normally operating cylinder. In this example, Cylinder 2 shows an extremely lean condition noted by the sharp increase in spark line kV.
A long low initial spark duration line that slopes down, coupled with a low firing kV, points toward a fouled or shorted spark plug or a low-resistance spark plug lead. No spark line means infinite resistance, due to an open in the spark plug or an open or disconnected plug wire or high-voltage COP spring.
Different Spark Lines in Different Situations
FIGURE 16-18 The spark line juts out at a right angle from the firing line and remains relatively horizontal. There is a slight increase in the spark line’s voltage after the firing line due to changing in-cylinder resistance. The hash and turbulence on the spark line show that the spark took place in the cylinder. The slight upward tip of the nose indicates that the coil has reserve energy left. When the air-fuel mixture begins to burn out, resistance increases. As a result, the spark line slopes upward at the nose, showing an increase in the required voltage, and the plug no longer fires. The coil oscillations are then the gradual dissipation of reserve energy and heat from the coil since it can no longer sustain spark.
FIGURE 16-19 The spark line is short and curves up from the firing line. The spark line starts out normally, but due to a changing resistance inside the cylinder, the spark line is very erratic, moving upward from the corner, where it starts indicating a lean condition. A lack of hydrocarbons, which serves as the conductor, adds extra resistance to the spark line, increasing the in-cylinder resistance. Notice the absence of a nose and increased turbulence on the spark line due to the lean air-fuel mixture. To better find a lean condition, perform a snap throttle test. At idle or at low rpm, the PCM can compensate for a lean condition by adding fuel, using the fuel trims. A lean misfire can result from a clogged/restricted fuel injector, excessive/unwanted EGR, or a leaking valve(s).
FIGURE 16-20 In this example, the lack of normal turbulence on the spark line and relatively normal spark duration indicate that the spark found its way to ground outside of the cylinder. The length of the spark line aids in determining whether a spark actually occurred.
FIGURE 16-21 A long low spark line reveals low resistance. The gradual drop in voltage without the normal slight increase after the firing line shows that the coil’s energy is pulled directly to ground instead of creating a spark across the intended air gap. The missing hash and turbulence indicates that energy is going directly to ground instead of creating a spark. A carbon-bridged spark plug gap is the most probable cause. The coil’s voltage follows the carbon to ground instead of jumping the air gap. A leaking secondary component will also cause a low, short spark line as the spark travels directly to ground instead of jumping an air gap. The lack of a nose and coil oscillations at the end of the spark line shows a lack of reserve energy as the coil’s energy is drained off directly to ground. A spark plug wire shorted directly to ground will also divert the coil’s energy away from the spark plug gap, bearing similar results.
FIGURE 16-22 Scope capture showing multiple cylinders with faults. Cylinders 3, 4, 5, and 6 are all showing high firing voltage above 25 kV. The spark line on all four cylinders slopes up at the nose of the spark line. The combination of the high spark line and high nose indicates a lean condition. Suspect clogged injectors or low fuel pressure. Cylinder 6 has another issue, in addition to the lean condition. The spark line kV starts very high at about 5 kV, double that of the other five cylinders. A high initial spark kV is due to resistance outside the cylinder, due to either a high-resistance spark plug wire or spark plug.
FIGURE 16-23 Screen capture after cleaning the fuel injectors and replacing the spark plug wires and spark plugs. Cleaning the injectors repaired the lean condition. Replacing the spark plug wires and spark plugs reduced the high resistance outside of the cylinder. Notice the reduction in the firing kV for all the cylinders. The firing demand is also reactively equal on all cylinders, well within the 3 kV allowance. The spark line is relatively level at about 2 to 3 kV and equal on all cylinders—within accepted levels.
Coil Oscillations
The spark line lasts only as long as the coil has enough energy to keep the plug gap ionized. The oscillations at the end of the spark line show that there is still energy left in the coil, but it is not enough to maintain spark. The coil oscillations have no effect on the ignition event. They are useful, however, for determining the condition of the coil. The nose is also important for troubleshooting. A missing nose shows the coil has no residual energy.
A properly operating ignition system should show coil oscillations. These oscillations are leftover energy from after the spark has extinguished (FIGURE 16-24). Most modern ignition systems will typically show two to four peaks (upper and lower oscillations). A lack of oscillations points toward a weak coil. The coil has enough voltage to produce and maintain spark for the current load and rpm, but there may be insufficient reserve energy for a heavy load condition. If there are no oscillations, suspect a failing coil.
FIGURE 16-24 Coil oscillations appear at the end of the spark line. The oscillations indicate that the coil has residual energy left after producing and maintaining spark for the complete cycle. A good coil should show an upward bump. known as the nose. followed by one or two additional swings (two to four upper and lower peaks). The lack of a nose indicates that the plug did not fire and that the coil’s energy went directly to ground, so suspect a fouled or shorted spark plug. A loss of oscillations, or no peaks at all, generally means a failing coil. Replace the coil and retest.
Dwell Section of the Spark Line
Dwell is the amount of time that the coil is charging. The vertical cursors in the screenshot show the dwell period. Dwell changes with rpm. Idle and low rpm operation will have a short dwell time. High enigne speeds require a longer dwell time. Since each cylinder in a COP system has its own coil and coil driver, the dwell section (coil saturation time) of a waveform can be used as a valuable diagnostic tool (FIGURE 16-25). To use dwell as a diagnostic tool, the entire pattern needs to be displayed on the scope. As with most scope diagnoses, the best test is to compare all the cylinders against each other. To identify a concern, look for a cylinder(s) with a variaton in dwell time that is not common to all cylinders. A misfire or low-performing cylinder will have a shorter dwell compared to the other cylinders. Substantiate the diagnosis with a power balance or cylinder deactivation test. The offending cylinder should have a reduced rpm drop.
FIGURE 16-25 The horizontal cursor across the bottom of the waveform shows the gradual increase in ground control. When first activated, the voltage should be pulled to almost 0 volts. As the transistor heats up, a slight increase in the voltage drop is considered normal. Some systems will also show a current-limiting hump in the dwell time before the firing line. If the rise in voltage drop is excessive when compared to the other cylinders, suspect shorted coil windings or a weak transistor driver.