Waveform Pattern

Ignition Waveform Diagnosis

Decode secondary ignition, coil-on-plug and waste-spark waveforms: firing line, spark line, coil ringing and primary current ramps. Learn the physics that separates a worn plug, a weak coil, an open secondary and a lean cylinder.

Why the waveform beats the code

A misfire code says which cylinder. The ignition waveform says why: a worn plug, a coil with shorted turns, a lean mixture, or a secondary leak all set the same code but leave different patterns.

The physics in one paragraph

During dwell, the coil's primary current rises and stores energy: E = ½ L I². When the driver opens, the secondary voltage rises until it's high enough to ionize the plug gap. That voltage is the firing line, and it is set by what the gap demands (gap size, cylinder pressure, mixture, electrode condition), not by what the coil can supply. The remaining stored energy then sustains the arc: the spark line. The more energy it takes to fire, the less is left to burn, so a tall firing line usually comes with a short spark line. When the energy can no longer sustain the arc, what's left rings down through the coil.

Anatomy of a secondary waveform

Firing line: the spike at breakdown. Its height is the voltage the gap required.
Spark line: the arc after breakdown. Its length is burn time (typically about 1–2 ms at idle); its height is the arc voltage.
Ring-down: oscillations after the arc ends, from the coil's inductance and capacitance.

What you'll need

Lab scope, one channel minimum.
Wire-and-plug or distributor systems: a capacitive secondary pickup.
Coil-on-plug: a COP probe held against the coil body, or a low-amp clamp on the coil's primary feed.
Primary voltage: a ÷10 or ÷20 attenuator is required. The primary kick reaches several hundred volts.

COP probes are not calibrated in kV. Their reading depends on position and coil design. Compare cylinders against each other with the probe held the same way; don't read absolute kV from them.

Step-by-step capture

Verify the battery and charging system. Supply voltage sets the primary current ramp.
Warm the engine to operating temperature.
Connect the pickup or probe to the cylinder of interest.
Timebase: 1–2 ms/div for one event; 20 ms/div to see every cylinder at idle.
Trigger on the firing line's rising edge. On waste-spark systems, set the trigger polarity for that cylinder's firing polarity (see below).
Capture at idle, then during a snap throttle. The load raises cylinder pressure, which should raise every firing line together.
Compare cylinder to cylinder.

Reading the secondary pattern

Healthy pattern

Firing lines similar on every cylinder at idle (a relative comparison on COP probes).
Spark lines of similar length, roughly 1–2 ms at idle, fairly flat.
A clean ring-down after the arc ends. The number of oscillations varies by coil design, so compare against the other coils on the same engine.
On snap throttle, every firing line rises together.

Tall firing line, short spark line

High demand used the coil's energy to fire the gap, leaving less to burn. Causes: worn or wide plug gap, a lean cylinder, or a gap in the secondary (a cracked boot or a loose plug wire). Check fuel trims and inspect the plug.

Very tall firing line, little or no spark line

Open secondary: a disconnected or broken lead, or a failed plug. The coil can't discharge through the gap, so voltage rises to the coil's limit. Sustained, this damages coils.

Short firing line, long spark line

Low demand: a small gap, a fouled plug, low cylinder pressure, or a rich mixture. A very short firing line with a long, low spark line can also mean the spark is leaking to ground through a carbon track or a cracked insulator, taking an easier path than the gap.

Weak or missing ring-down, compared with the other coils

Energy is being lost inside the coil: shorted secondary turns or insulation breakdown. Confirm by swapping the coil to another cylinder and seeing whether the pattern moves with it.

Spark line sloping upward or erratic

Turbulence and mixture movement affect the arc. An upward, ragged spark line on one cylinder points to a lean or poorly mixed charge in that cylinder.

Waste-spark systems

One coil fires two cylinders at once: one on compression, one on exhaust. The two plugs fire in opposite polarity.

The compression cylinder's firing line is much taller (high cylinder pressure).
The exhaust cylinder's firing line is small (low pressure).
One cylinder in each pair shows an inverted pattern on a capacitive pickup. Set the trigger polarity to match, or invert the channel.

Coil-on-plug primary current

Clamp the coil's primary feed with a low-amp clamp:

Ramp slope = V / L. Shorted primary turns lower L, so the ramp gets steeper and reaches its current sooner.
Peak current is set by dwell time and supply voltage, or by the driver's current limit (a flat top). A lower peak than the other coils means less stored energy: 10% lower current is about 19% less energy (0.9² = 0.81).
Sharp collapse when the driver opens. No collapse means the driver never opened.

Worked example: misfire on one cylinder (illustrative)

A V6 sets a misfire code on cylinder 3. At idle, cylinder 3's firing line is roughly twice the height of the others, its spark line is well under 1 ms, and its ring-down looks like the other coils.

The coil looks healthy (normal ring-down), and the demand is high.
Swap the cylinder 3 coil with cylinder 5. The tall firing line stays on cylinder 3, so the coil is not the cause.
Inspect the plug: the electrode is worn and the gap is well above spec.
Replace the plug. The firing line drops to match the other cylinders and the misfire stops.

Only replace a coil when its own evidence (weak ring-down, steep primary ramp) moves with it in a swap test.

Common mistakes

Reading absolute kV from a COP probe.
Skipping the snap throttle: some faults only appear under load.
Diagnosing one cylinder without comparing it to the others.
Replacing the coil when the demand problem is the plug.
Measuring primary voltage without an attenuator.
Misreading a waste-spark exhaust event as a weak cylinder.

Frequently asked questions

What should the firing line read?

It's set by what the gap demands, so it varies with gap, cylinder pressure, and mixture. Compare cylinders on the same engine: similar at idle, and rising together on snap throttle. On COP probes, treat readings as relative.

How long should the spark line be?

Roughly 1–2 ms at idle on most systems. It shortens when the firing line is tall, because firing a demanding gap uses more of the coil's energy.

What does the ring-down tell me?

How much energy is being lost inside the coil. A coil that rings down much less than its neighbours on the same engine is losing energy internally, typically through shorted secondary turns.

Can I test COP coils without a secondary probe?

Yes. Capture primary current with a low-amp clamp. Compare the ramp slope and peak across coils: a steeper ramp points to shorted primary turns, a lower peak to less stored energy.