Ignition System Diagnosis and Repair

The short version — The spark comes from a collapsing magnetic field in the coil, not the buildup, and the whole system's timing depends on the crank (and cam) sensor telling the PCM exactly when to fire each cylinder. Most ignition test questions come down to knowing what part causes a single-cylinder miss versus a no-start on all cylinders.

How the system actually makes a spark

The whole point of the ignition system is simple to state: generate a high-voltage spark and deliver it to the combustion chamber at the correct point in the compression stroke, so the air-fuel mixture ignites when it's supposed to. Timing is everything here — a spark at the wrong moment is as bad as no spark at all.

The coil is where the voltage actually gets made. Primary current flows through the coil's primary winding first, building up a magnetic field — that's energy storage, not spark production. The spark itself happens when the PCM or ignition module abruptly interrupts (opens) the primary circuit. That sudden interruption collapses the magnetic field, and it's the collapsing field — not the buildup — that induces the high-voltage pulse in the secondary winding. If you remember only one coil fact for the test, make it this one: buildup stores energy, collapse fires the plug.

Who tells the coil when to fire

None of this happens on its own — it's a signal chain:

• The crankshaft position sensor (and, on distributorless or coil-on-plug systems, the camshaft position sensor) reports engine position to the PCM.
• The PCM uses that signal to time two things together: fuel injection and coil primary switching.
• Coil primary switching itself happens in two stages — dwell/saturation (building the field) then firing (collapsing it) — and this has to happen in the correct cylinder firing order.

This is why a bad crank sensor doesn't just cause a rough idle — it can take out spark and injection timing at the same time, because both depend on the same position signal.

Delivery hardware: distributor vs. coil-on-plug

How the spark actually gets from the coil to the plug depends on the system design:

• Older systems use a distributor that mechanically routes secondary voltage to each spark plug in firing order, using a rotor and cap.
• Coil-on-plug (COP) and coil-near-plug systems give each cylinder its own coil — no distributor and no plug wires are involved at all.

This matters for diagnosis: on a COP system, a coil problem is isolated to one cylinder by design. On a distributor system, one bad component (cap, rotor) can affect multiple cylinders because they all share that one path.

Testing the driver circuit

The ignition module or coil driver is the circuit that actually tells the coil primary to open. To test it, you check for:

• Proper trigger signal — verified with a scope or logic probe.
• Proper ground and power supply to the driver/module.

This matters because a failed driver circuit can disable one cylinder or all of them, depending on whether it's a per-cylinder driver or a shared one. Don't assume "no spark" always means a bad coil — the coil can be perfectly good and still never fire if it never gets triggered or never gets power/ground.

Common failures and what they look like

Match the failure to the symptom — this is where ASE questions like to test recognition:

• Worn or fouled spark plugs → misfire or hard starting.
• Cracked plug wires or boots → arcing, misfire under load, and radio interference (the arcing radiates noise).
• Failed coil → single-cylinder misfire on a COP system, or a total no-start if it's a single-coil system feeding everything.
• Failed crank or cam sensor → no-spark, no-start, or intermittent stalling.

Notice the pattern: single-component failures on COP systems tend to isolate to one cylinder, while shared components (single coil, shared sensor) tend to affect the whole engine.

Safety before you touch anything

Before servicing any ignition component, disconnect the battery or disable the ignition system per the manufacturer's procedure. The reason isn't just abstract safety policy — an intact ignition system can crank and start the engine unexpectedly while your hands are in the bay, and secondary voltage is genuinely capable of a shock. Disable it first, every time.

Easy to mix up

• Coil buildup vs. coil firing — the field building up is just energy storage; it's the field collapsing that produces the spark. Don't reverse this.
• Single-cylinder miss vs. no-start on all cylinders — a failed COP coil or one bad driver channel usually isolates to one cylinder; a failed shared component (single coil, crank sensor, shared driver) tends to kill spark everywhere.
• Crank sensor vs. cam sensor — the crank sensor is used on all systems in this scope; the cam sensor's role here is specifically tied to distributorless/coil-on-plug systems.

Check yourself

Question: What actually produces the high-voltage spark — the buildup of the coil's magnetic field, or its collapse?

The collapse. Primary current builds the magnetic field as stored energy, but the spark is induced in the secondary winding only when the PCM/module abruptly opens the primary circuit and the field collapses.

Question: Technician A says a failed crank position sensor can cause a no-start condition. Technician B says a failed cam position sensor is irrelevant on distributorless/coil-on-plug systems. Who is right?

Technician A is right. A failed crank sensor can cause no-spark, no-start, or intermittent stall. Technician B is wrong — the cam position sensor is specifically used on distributorless/coil-on-plug systems, so a failure there is very relevant, not irrelevant.

Question: On a coil-on-plug engine, one cylinder is misfiring but the others run fine. What does this tell you about likely causes, and what should you check on the driver side?

Because each cylinder has its own coil on a COP system, a single-cylinder misfire points to something isolated to that cylinder — a worn/fouled plug, a failed coil for that cylinder, or a driver channel problem for that specific coil. On the driver side, check for a proper trigger signal (scope or logic probe) and proper power/ground to that driver circuit, since a failed driver can disable just that one cylinder.