4.2 Primary Ignition Diagnosis
Key Takeaways
- Primary ignition diagnosis focuses on B+ supply, coil ground (low-side driver), dwell time, and saturation current — typically 6–8 A at peak.
- A healthy primary current ramp is a straight line to peak; a slow ramp means high resistance, and a steep/hooked ramp means shorted turns.
- Primary voltage shows three events per cycle: dwell at ~0 V, a 200–400 V inductive kickback at turn-off, and a damped ringing decay.
- An abnormally tall turn-off spike (>500 V) points to an OPEN secondary; an abnormally short spike points to a SHORTED secondary.
- Always confirm coil B+, coil control wire state, and primary/secondary resistance with a DMM before condemning the coil.
Why Scope the Primary Side
The L1 will almost certainly ask you to interpret a primary ignition waveform. The primary side is where the PCM lives — every defect that involves coil drive, dwell, or driver transistor health shows up here first, often before the secondary pattern looks wrong. Primary scoping is also safer (low voltage) and only requires a single test lead clipped to the coil control wire.
What the Primary Circuit Does
A primary ignition circuit has four jobs:
- Supply B+ to the coil positive terminal (battery voltage through the ignition switch and an underhood fuse — usually 10–15 A).
- Switch the coil ground with a power transistor (the driver inside the PCM, ICM, or smart coil).
- Hold the ground long enough to saturate the magnetic core (controlled dwell).
- Open the ground instantly when the spark must occur — the collapsing field induces the secondary pulse.
Primary Current at Saturation
A healthy coil draws about 6–8 A at full saturation. Older coils with internal ballast resistors run lower (~3–4 A); some modern high-energy COP coils run higher (~10 A) for very brief dwell.
| Reading | Healthy | Typical Failure |
|---|---|---|
| Peak primary current | 6–8 A (most COP coils) | < 4 A → high-resistance primary, low B+, weak driver |
| Dwell time | 2–5 ms at idle, decreases at high RPM | > 6 ms → driver stuck on (coil overheats); < 1 ms → too short, weak spark |
| Primary B+ at coil | Battery voltage ± 0.5 V | Voltage drop in supply circuit (fuse, ignition switch) |
| Voltage on coil control wire (key on, engine off) | Near B+ (pulled up through coil) | 0 V → open primary or driver shorted to ground |
Reading a Primary Current Ramp on the Lab Scope
A current probe (low-amp clamp) on the coil ground wire shows current rising from 0 A up to the saturation peak, then dropping abruptly when the driver opens. Three pattern shapes tell the story:
Healthy Ramp — Straight Line
A good coil shows a nearly linear (straight) current ramp from 0 up to ~6–8 A. The slope (di/dt) is determined by the coil's inductance and the supply voltage. The line is straight because, in the early portion of the curve, current rise is essentially linear before saturation.
Slow Ramp / Low Peak — High Primary Resistance
If the primary winding has gone high-resistance (corroded internal joints, partial open), or if there is a voltage drop in the B+ supply, the ramp climbs more slowly and never reaches the normal peak. Spark energy is reduced even though the waveform shape may look "right" if you don't compare to a known-good reference.
Sharp Peak / Hooked Ramp — Shorted Windings
A shorted-turn coil loses inductance. Current rises too fast and peaks too high before falling off in a hook shape; the driver may current-limit before the dwell period ends. Spark energy collapses because the magnetic field stored less flux than the geometry implies.
Reading the Primary Voltage Pattern
Most techs scope voltage on the coil control (negative) wire because it requires no clamp.
Three Events on Every Primary Voltage Trace
- Dwell period (driver ON): The control wire is pulled near 0 V (about 0.2–1.0 V across the driver transistor). Current is building in the coil.
- Turn-OFF spike (start of spark): The driver opens. Collapsing flux induces an inductive kickback of 200–400 V on the primary side (sometimes called the "firing line" on the primary trace). On the secondary, this same event is multiplied to 8–40 kV.
- Spark / ringing / dwell-off: A damped oscillation follows as residual coil energy decays. The control wire then sits at B+ until the next dwell command.
What Each Defect Looks Like on the Primary Voltage Trace
| Symptom | Likely Cause |
|---|---|
| No turn-OFF spike at all | Driver not opening; PCM not commanding spark; open coil primary |
| Spike well below 200 V | Shorted coil primary or shorted secondary loading the primary |
| Spike well above 400 V with no ringing | Open secondary circuit (no path for the energy to dump — the primary spike rises until it arcs) |
| Dwell line not at ~0 V (e.g., 5–6 V) | High-resistance driver / poor ground / bad connection at coil negative |
| Erratic dwell length cycle-to-cycle | Crank/cam signal noise or PCM driver issue |
L1 exam frame: Whenever a question shows a primary trace with an abnormally tall turn-OFF spike, think open secondary (wide plug gap, open plug wire). Whenever it shows a short, weak spike, think shorted secondary (fouled or carboned plug).
Standard Bench Tests Before You Scope
Before condemning a coil, confirm the basics with a DMM:
- Primary winding resistance: Typically 0.5–2 Ω depending on the design. Out of spec — replace.
- Secondary winding resistance: Typically 5–15 kΩ for DI/DIS coils; 5–30 kΩ for COP coils. Out of spec — replace.
- Coil B+ at key-on: Should be battery voltage with the key in RUN.
- Control wire (key on, engine off): Should sit near B+ through the coil; a 0 V reading suggests an open primary or a driver pulled to ground.
A technician scopes the primary current of a COP ignition coil with a low-amp clamp. The current ramp rises very steeply, peaks well above 10 A, and then hooks downward before the driver opens. What does this pattern indicate?
On a primary voltage waveform, the dwell period sits cleanly near 0 V, but when the PCM opens the primary, the turn-off spike measures more than 500 V and the ringing section is very short or absent. What is the most likely cause?