Ignition System Diagnosis
Coil types and how they fail
Distributor, waste-spark, and coil-on-plug systems all need adequate primary current, a good ground, and a secondary path with correct resistance. Waste-spark coils fire two cylinders at once, one with reversed polarity, so a scope pattern on that pair will show one inverted trace. That is normal, not a fault.
Reading the scope
Watch firing line kV, burn time, and dwell. Firing voltage rises with a wider gap, a lean mixture, high compression, and high secondary resistance; it falls with a fouled plug, a flooded cylinder, or a shorted wire. Burn time of about 1.5-2.0 ms is typical and lengthens with a lean mixture or wide gap.
Testing
Use an adjustable spark tester to confirm the system can jump a load. Measure coil primary and secondary resistance against spec, then current-ramp the primary with a low-amp clamp to spot a shorted turn. Verify plug type, gap, and torque; a loose plug runs hot and can misfire or eject.
Traps
Judging a coil by resistance alone; missing a carbon-tracked cap or burned plug boot; replacing plugs to cure an oil-fouled cylinder; and forgetting that a lean condition often shows up first as an ignition misfire.
Rule of thumb
If one cylinder shows both a high firing line and a high misfire count, look for high resistance in the plug gap, wire, or boot. If it shows a low firing line, look for a fouled plug or a shorted secondary.