Ignition Coil Inspection, Testing, and Replacement

The short version — the coil stores energy in a magnetic field while the primary is switched ON, then fires the plug when the PCM switches the primary OFF and the field collapses; testing means checking dwell/primary waveform AND secondary output under load, not just for a visible spark.

How the coil actually makes spark

The ignition coil is the energy-transfer device at the heart of the system — it stores energy in a magnetic field through primary winding current, then releases that energy as a high-voltage pulse in the secondary winding to fire the plug. Two separate windings, two separate jobs:

• Primary circuit: low voltage, high current, few turns. Switched on and off by the PCM or an integrated coil driver module.
• Secondary circuit: high voltage, low current, many turns. It's inductively coupled to the primary and delivers the firing voltage to the spark plug.

The sequence matters and it's a favorite test point:

1. PCM commands the primary circuit ON — current builds. This build-up time is called dwell.
2. PCM commands the primary circuit OFF.
3. The collapsing magnetic field induces a much higher voltage in the secondary winding.
4. That induced voltage fires the plug.

Spark happens at primary turn-OFF, not turn-on. If you remember nothing else, remember that — it explains why dwell time (how long the primary was allowed to charge) directly affects how strong the resulting spark will be.

One coil per cylinder, or one coil for many

Design has changed over the years, but the physics above hasn't:

• Coil-on-plug (COP) — one coil sits directly over each spark plug. No plug wires, so there's nothing to crossfire between adjacent wires — this reduces crossfire risk.
• Older systems — one coil, or a coil pack, feeds multiple plugs through secondary ignition wires, sometimes routed through a distributor.

Whichever design you're working on, the same primary-charge / primary-collapse / secondary-fire sequence applies underneath.

Testing: primary side and secondary side, separately

A coil can look fine and still be weak, so you test both circuits:

• Primary side — use a lab scope or DVOM to check dwell time and the primary voltage/current waveform. This confirms the coil is actually charging (building the field) and discharging (collapsing it) with correct timing. A dwell that's too short means the field never fully charges, so the secondary pulse comes out weak even if the coil itself is healthy.
• Secondary side — use an adjustable air-gap spark tester to confirm the coil can produce and sustain adequate voltage under load. This is important: a coil might be able to jump a small, easy gap but fail to sustain voltage across a wider gap that simulates real cylinder pressure. Don't judge a coil by whether it can jump a paperclip gap — that tells you almost nothing about performance under load.

What actually fails, and what it looks like

Know these failure modes and match them to symptoms:

• Open or shorted primary/secondary windings → no spark, or weak spark.
• Cracked coil boot or tower → causes carbon tracking, which shows up as a misfire under load, especially in wet conditions (moisture gives the spark an easier path to ground than the plug gap).
• Overheated or failed coil driver transistor → intermittent or no spark. Remember, on some systems the driver is integrated into the coil assembly, so a "coil failure" complaint may really be a driver failure.
• Internal insulation breakdown → spark leaks or crossfires inside a coil pack, since the high voltage finds a shortcut instead of going out to the correct plug.

Each of these maps to a different customer complaint, so match the symptom pattern (constant vs. wet-weather-only vs. intermittent) to the likely failure before you start swapping parts.

Working safely around secondary voltage

Secondary ignition voltage is high enough to cause injury. Two rules to build into your habits:

• Never touch coil towers, wires, or boots while the engine is running or cranking.
• Disconnect the coil primary connector or disable the ignition before disconnecting any secondary component. This protects you from shock, and it also protects the coil — an open secondary can arc internally and damage the coil if it's still being commanded to fire with nowhere for the voltage to go.

Check yourself

Question 1: At what point in the primary circuit's cycle does the spark plug actually fire — when the primary turns ON, or when it turns OFF? Why?

The spark fires when the primary circuit turns OFF. Turning the primary off causes the magnetic field (built during the ON/dwell period) to collapse, and that collapsing field induces the high voltage in the secondary winding that fires the plug.

Question 2: Technician A says a coil that jumps a small, tight spark-tester gap is proven good. Technician B says the coil should be checked with an adjustable air-gap spark tester to confirm it can sustain voltage under a load similar to real firing conditions. Who is right?

Technician B is right. Jumping a small gap doesn't confirm the coil can sustain adequate voltage under load. The adjustable air-gap tester is used specifically to load the coil more like real cylinder conditions, not just check for any visible spark.

Question 3: A customer complains of a misfire that only happens in wet or rainy weather. Which failure mode fits this pattern, and why does moisture make it worse?

A cracked coil boot or tower causing carbon tracking fits this pattern. Moisture provides an easier path for the high voltage to leak to ground along the crack/track instead of going out through the plug gap, so the misfire shows up (or gets worse) specifically in wet conditions.