Automotive Engine Performance

Ignition Waveform Analysis

16-03 Dissect ignition waveforms to help with ignition diagnostics.

A—battery/system voltage supply to the coil: It is hard to determine the value on this screen due to the scaling for the primary ignition event. If a problem is suspected, rescale to 20 volts and retest. Even though the scope’s connection is to the coil’s negative terminal, battery voltage should be available through the windings of a good coil (which shows continuity) when the primary is not grounded. On breaker-point ignition systems, the available voltage to the coil may be lower, due to a ballast resistor or resistor wire.
B—coil turn-on: The primary winding is grounded by the coil control device when the transistor turns on or when the breaker points close. Current flows into the coil, and the magnetic field begins to build. The turn-on point should be an abrupt 90-degree vertical drop to almost 0 volts. On transistor-controlled ignition systems, the voltage will not reach absolute 0 volts, due to the voltage drop across the transistor.
Depending on the control device, a transistor or MOSFET, the voltage drop will be different. Since technicians have no way of knowing which control device is in use, a rule of thumb is that 0.5 volts ±0.2 volts is a good specification. As the dwell period nears its end, a slight increase in the ground’s voltage drop is normal. The rise is because of the effect of counter-electromotive force (CEMF) from current flowing through the coil’s windings, increasing resistance. If the voltage initially fails to reach almost zero, it points to high resistance in the primary circuit, reducing primary current flow, which affects coil output. A failing transistor can be spotted here.

Furthermore, poor/corroded connections at the coil or high resistance in the power feed wire to the coil or in a conventional ignition system’s pitted points are all potential causes of high resistance. If a concern is suspected, lower the scope’s voltage settings to zoom in on the voltage line. Choose a setting of 1, 2, or 5 volts to see the voltage drop more clearly. The lower the setting, the more apparent any fault will be. If the voltage is seen to be erratic or jumping, suspect a weak transistor in the ICM or bouncing points in a conventional distributor. Remember that the transistor can be found in the ICM/igniter, internal to the PCM or COP coil.
C—coil saturation: Current flows through the primary windings during the saturation period. Once the current in the primary windings rises to the point at which it cannot hold any more, the magnetic field is fully built up. This is the coil saturation point. The voltage should remain relatively flat, near zero, during the coil saturation period.
B to D: The dwell or coil saturation time takes place between points B and D. Recall that dwell is the amount of time that the primary circuit is grounded and that current flows through the primary winding during the saturation period, building the magnetic field. Once the current in the primary windings rises to the point at which it cannot hold any more, the magnetic field is fully built up. This is the coil saturation point. The voltage should remain relatively flat, near zero during the coil saturation period.
A change in dwell does not affect EI systems, since coil on time is controlled electronically by a module, not a mechanical connection. On any ignition system, if there is too little dwell, the coil won’t have enough time to charge fully, reducing its voltage output. If there’s too much dwell, the coil can overheat and short out internally. Misfires, a lack of power, hesitation on acceleration, and pinging are all symptoms of reduced dwell.

Dwell can be measured on a breaker-point system by using a DMM in degrees or a percentage and is adjustable. Dwell affects only ignition timing in a breaker-point system. A 1-degree dwell change results in a 2-degree ignition timing change.
D—the point where the coil is turned off: The ground is turned off (transistor off or points open), and the magnetic field collapses, inducing a current into the coil secondary. The turnoff should be a 90-degree vertical line that rises straight up. The speed of the coil turnoff is primarily responsible for the coil’s output voltage. A sloping or jagged turnoff will reduce coil output. If the coil turnoff is not an abrupt 90-degree angle, suspect a failing transistor in the module or a faulty condenser if used in an EI system. In a conventional ignition system, this points toward damaged points or a weak/bad condenser.
E—the inductive voltage kick created by the collapsing magnetic field as it moves from the coil primary to the secondary: The amplitude of the spike depends on the resistance in the combustion chamber to jump the spark plug’s gap. The resistance varies and is a direct result of the spark plug gap and cylinder compression. The wider the spark plug gap or the higher the compression (e.g., high turbo boost under load), the more voltage will be needed to jump the gap. Use the inductive kick as a comparison between coils on DIS systems and between coils on COP systems. If an individual coil shows a lower amplitude and a shorter firing line compared to the other coils, suspect a failing coil or reduced voltage to the coil from high resistance or poor connections. To verify, use an amp clamp to current ramp the coil before replacing it. Likewise, if the voltage level in a single cylinder is much higher than the other cylinders, suspect high resistance in the secondary, spark plug, plug wire, or high-voltage spring (COP) or suspect a lean cylinder. Note: Remember that the inductive kick can damage the oscilloscope. To prevent damage, use an attenuator between the scope lead and the scope, reducing the voltage to the oscilloscope.
F: After the electrons have bridged the spark plug’s gap, resistance to current flow will drop, and the voltage needed to maintain the arc across the plug’s gap will also fall. When this voltage starts to flow, a “ringing” may be seen in the form of oscillations immediately after the firing line. The ringing is a result of the voltage inducing a magnetic field in the coil windings that opposes the flow of electrons across the gap. Once current begins to flow, the voltage stabilizes and these oscillations gradually diminish into an even voltage, known as the spark line. The current continues to flow across the plug gap until the coil’s energy has fully depleted.
F to G: This horizontal line is the spark line. In this example, it starts at about 40 to 45 volts, gradually dropping until running out of energy when it turns into oscillations. The burn time, or spark duration, is the time during which current is flowing, bridging the plug’s gap. Burn time (spark duration) is a valuable diagnostic tool, not only for ignition purposes but for overall in-cylinder conditions as well. The burn time changes based on the amount of voltage it takes to overcome resistance in the cylinder and start the burning process. Normal, or standard, spark duration is 1.0 to 2.0 ms in a known-good cylinder, regardless of the type of ignition system. Any change in the cylinder conditions or the secondary ignition system will affect this number. If cylinder resistance is low, the burn time will be longer. If cylinder resistance is high, the burn time will be shorter.
A coil can produce only a set amount of voltage. If more energy is needed to start the burning process (kV demand), less is available to maintain the burn (duration) that burns the air-fuel mixture thoroughly. Sources of high resistance include worn spark plugs; high-resistance or open spark plug wires or high-voltage springs; worn distributor cap terminals or rotor tip, which increases the air gap; and a lean or improperly atomized air-fuel mixture. Too much oxygen spreads the hydrocarbon molecules farther apart, making them less conductive, requiring more voltage to start the burning process.

Conversely, if conditions in the cylinder require less initial voltage (low resistance), there is more energy available to keep the spark burning. Possible causes of low resistance include a rich air-fuel mixture, carbon- or fuel-fouled spark plugs, or spark occurring external to the combustion chamber from a spark plug boot or coil boot that is arcing the cylinder head. Carbon is an electrical conductor and bridges the plug’s gap, so less voltage is required to jump the plug’s gap. Firing to ground under atmospheric pressure is easier than overcoming in-cylinder combustion pressures. This leaves more leftover energy to maintain the spark longer. The excess fuel in a rich air-fuel mixture acts as a conductor, reducing the voltage demands, up to a point. Too much fuel, however, can quench the spark altogether, resulting in a misfire.

To illustrate this concept, think of a piece of rope or wire that simulates a coil’s potential energy with a set length of 6 feet (1.8 meters). Now place the rope on a board that replicates the scope’s screen with a vertical and horizontal run. In the middle of the board, pound a nail into the wood and loop the rope around the nail to act as a pivot point. Begin with an equal amount of rope running in both directions. This represents a “normal” in-cylinder condition. Pull the rope up vertically to simulate high resistance. As the rope moves up, the vertical line becomes longer and the horizontal line shortens. Likewise, if the horizontal line lengthens (less resistance), then at the same time, the vertical line shortens. Let’s take this one step further. To simulate a failing coil, low battery voltage to the coil, or a poor ground, remove two feet from the rope, leaving only 4 feet (1.2 m). Now there is less available rope (voltage), and both the vertical and horizontal planes will be affected equally, a result of less usable energy in the coil.

Normal burn time or spark duration varies slightly by the type of ignition system. Coils used in a breaker-point system do not build the energy of modern coils. Therefore, 0.9 to 1.2 ms is considered normal for spark duration. DIS and COP systems with high-energy coils typically run between 1.2 and 2.0 ms.
G: When there is no longer enough energy left to sustain firing the spark plug, the coil’s remaining energy and heat dissipates into oscillations. The coil still has some energy left, but not enough to keep the spark across, bridging the plug gap. Coil oscillations are present in most ignition system waveforms and are a useful diagnostic tool for in-cylinder conditions.
For diagnostic purposes, compare a coil’s oscillations to other coils on the vehicle or a waveform library of a like vehicle. During analysis, pay close attention to the “nose.” The nose is the first peak at the end of the firing line, and it marks the end of the plug firing event and is enormously informative for diagnosis. No nose and no oscillations signal that the coil’s energy is being sapped to ground instead of firing the spark plug. A missing nose, or missing oscillations, is generally a result of an oil- or fuel-fouled spark plug. Although highly infrequent, a shorted spark plug will also completely deplete the coil’s energy and can also lead to a missing nose and oscillations.

A lean condition will result in a high nose, sloping upward as voltage demand increases due to the lack of hydrocarbons. Conversely, a cylinder running near stoichiometric will have a small number of oscillations and a modest nose. Comparative analysis between cylinders is the best point of reference for finding these issues.

Analyzing a coil’s strength by counting the number of oscillations as once taught is not useful on modern ignition systems that use an ICM or PCM to control dwell electronically. For example, many modern COP systems operate ideally with just one or two coil oscillations, well below the “standard” specification of four or five oscillations for early ignition systems. When diagnosing these non-computer controlled systems, use the number of oscillations to determine a coil’s strength. Anything less than four or five indicates a weak coil in most systems, but high-energy ignition (HEI) systems require a minimum of three oscillations.
H: The coil’s energy has fully depleted, returning to system voltage. The scope shows this available voltage until the next coil firing event, when the sequence repeats.