4.3 Secondary Ignition Diagnosis
Key Takeaways
- Required (firing) voltage is typically 8–15 kV; available voltage is 25–40 kV — the difference is reserve.
- Required voltage rises with larger gap, higher cylinder pressure, leaner mixtures, and worn electrodes.
- Normal spark duration is 0.8–2.0 ms; firing line and spark line must be read as a pair.
- High firing line + short spark line = open / wide gap / lean cylinder.
- Low firing line + long spark line = fouled plug / shorted path / rich cylinder.
What "Secondary" Means on the L1
The secondary side of an ignition system is everything downstream of the coil's high-voltage output: the coil tower or boot, plug wire (if any), and the spark plug itself. Secondary voltage is the kilovolt (kV) side. The L1 expects you to know not only what numbers are normal, but how the pattern shape changes when something is wrong.
Available Voltage vs. Required (Firing) Voltage
Two related kV numbers come up constantly on the exam.
- Required (Firing) Voltage — the minimum kV needed right now to ionize the gap and start a spark. Typical 8–15 kV under normal load.
- Available Voltage — the maximum kV the coil could deliver before its energy is exhausted. Typical 25–40 kV for modern COP coils.
A healthy ignition system always has available voltage well above required voltage. The difference is called reserve voltage. When required voltage rises (wide gap, lean mixture, high load) and approaches available voltage, the cylinder is on the edge of misfire.
Variables That Raise Required Voltage
| Variable | Effect on Required Voltage |
|---|---|
| Larger spark plug gap | Higher kV (more distance to ionize) |
| Higher cylinder pressure (boost, high load, high CR) | Higher kV (denser mixture, harder to ionize) |
| Lean mixture (high A/F ratio) | Higher kV (lean mixtures need more energy to ignite) |
| Worn / eroded electrodes | Higher kV (rounded edges concentrate field less efficiently) |
Variables That Lower Required Voltage
- Smaller plug gap (less distance to ionize)
- Rich mixture (easier to ignite)
- Fouled / carboned plug (parallel resistive path, sometimes shunts the spark)
- Cracked insulator or shorted plug (spark jumps the easier path)
The Anatomy of a Secondary Waveform
Capture a secondary pattern with a capacitive pickup clamped over the plug wire (DI / DIS), or with a special COP probe / coil adapter (COP). A textbook trace, read left to right, has these regions in order:
- Firing line — a near-vertical kV spike that rises to the peak (the kV needed to ionize the gap).
- Spark line — a roughly horizontal segment that drops to a lower kV and slopes gently downward as current flows across the gap.
- Intermediate / ringing — a damped oscillation that follows the end of the spark line as residual coil energy decays.
- Dwell — a flat baseline while the coil is recharged for the next event.
| Region | What It Represents | Normal Value |
|---|---|---|
| Firing line | Initial kV peak needed to ionize the gap | 8–15 kV (most engines) |
| Spark line | kV maintained while current flows across the gap | 1.5–3 kV, sloped slightly downward |
| Spark duration | Length of the spark line | 0.8–2.0 ms |
| Ringing / intermediate | Residual coil energy decaying as oscillation | 3–5 oscillations of decreasing amplitude |
| Dwell | Coil being recharged for the next event | Varies with RPM |
What Healthy Looks Like
- Firing line 8–15 kV, with all cylinders within ~3 kV of each other.
- Spark line 1.5–3 kV, mostly flat with a small downward slope.
- Spark duration 0.8–2.0 ms.
- 3–5 ringing oscillations after the spark line.
Failure Patterns You Must Recognize
The two classic exam patterns are the high firing / short spark and the low firing / long spark signatures. Burn these into memory.
High Firing Line + Short (or Absent) Spark Line
| What you see | What it means |
|---|---|
| Firing line >20 kV | Required voltage is unusually high |
| Spark line very short (<0.5 ms) or near zero | Spark could not sustain — energy dumped in one quick arc |
| Ringing collapsed | No residual energy left |
Diagnosis: A high-resistance path on the secondary. Most likely causes:
- Worn / wide-gap spark plug
- Open or cracked plug wire / boot
- Lean cylinder (vacuum leak, leaking injector seal)
- Fouled spark plug with a cracked insulator (combination case)
Low Firing Line + Long Spark Line
| What you see | What it means |
|---|---|
| Firing line <8 kV | Required voltage is unusually low |
| Spark line very long (>2.5 ms) | Spark sustained too long — energy dumped slowly through a low-resistance path |
| Ringing present but small | Coil had to release stored energy through the low-impedance path |
Diagnosis: A low-resistance path on the secondary. Most likely causes:
- Fouled spark plug (carbon, oil, fuel deposits) — the most common cause
- Shorted / collapsed plug gap
- Tracking inside the coil boot or distributor cap
- Rich cylinder (stuck-open injector)
Cylinder-to-Cylinder Scatter Pattern
When firing lines vary widely from cylinder to cylinder (e.g., one at 8 kV, another at 22 kV), think fuel delivery before ignition. Ignition components fail individually — they don't produce evenly random kV across all cylinders. Scatter usually means injector imbalance, vacuum leaks, or compression variation that changes each cylinder's required voltage.
L1 exam frame: When the question gives you two numbers — firing line and spark line — read them as a pair, never in isolation. High/short = open; low/long = shorted.
Why Spark Duration Matters
Spark duration (the length of the spark line) is one of the most diagnostic numbers in the entire pattern. Too short means the energy was dumped too quickly (open circuit); too long means the energy is bleeding through a low-resistance path (shorted/fouled). Total available coil energy is fixed — so as firing kV goes up, spark duration goes down (and vice versa). The product of the two is, roughly, the energy delivered to the cylinder.
A secondary ignition scope pattern on one cylinder shows a firing line of 22 kV (specification 10–14 kV) and a spark line of only 0.3 ms (specification 0.8–2.0 ms). The other cylinders look normal. What is the MOST likely cause?
A technician sees a secondary waveform with a firing line of about 6 kV and a spark line that stretches more than 3 ms with very little downward slope. What does this indicate?