Troubleshooting Ignition Using Secondary Waveforms

APPLICATION NOTES

In the last newsletter, the events of a spark plug secondary voltage event were identified and briefly discussed. In the installment, I would like to give some troubleshooting notes to help identify and pinpoint ignition system problems. The curve below is very nearly ideal with a very low ionization sustaining voltage, good arc duration, normal ionization voltage, and clean ring-down.

Open secondary circuit -- This is by far the most recognizable of abnormal waveforms with an extremely high ionization voltage and almost non-existent arc-duration. The cause is typically a spark plug wire boot that has come off the plug or an open in the wire itself. The problem with an open in the secondary circuit, besides increased HC emissions and poor performance, is that the coil will unleash its maximum voltage reserve under this condition, often causing irreversible damage to the coil.

Excessive secondary resistance -- When the ionization voltage is more than 20% higher than the other cylinders, the arc duration will be brief and coil ring-down will be less than normal. This is because most of the coil energy is being used to ionize the air gap(s). Likely causes include a gap inside a plug wire, loose plug wire, or excessive spark plug gap. If all the cylinders of an ignition system are affected, check the primary waveform.

External secondary resistance -- When the slope of the curve during arc duration is not flat and slopes toward zero, it generally indicates that a break exists in the plug wire or inside the plug. The ionization voltage will be higher than normal and coil voltage will be dissipated in the form of heat energy at the point of the break, instead of being used to maintain the spark.

Excessive gap resistance -- If the arc sustaining voltage is higher than a few tens of volts, then it indicates that more voltage is necessary to sustain the arc after the arc is initiated. This is generally a result of a mixture problem and not an ignition malfunction, for example, poor turbo-charger performance, bad fuel valve, clogged ports, or anything that prevents proper air/fuel ratio. Since this gas will not burn properly, gap resistance will increase after initial ionization and the system will need more voltage to keep the arc alive. If this pattern is common to all cylinders, then look for a universal problem like a turbo-charger, or in a 2-stoke engine, low load.

Fouled plug -- Oil or deposits on spark plugs offer less resistance to current flow than normal. Since these deposits are conductive, a lower ionization voltage will be observed because less voltage is needed to ionize the air gap and more energy is left over to sustain the spark. This will result in an extended arc duration which is typically over 1.5 ms. When the arc duration curve is long and virtually smooth, it's an indication that gas ignition did not occur at all. This would indicate a grounded wire, closed plug gap, or wet plug.

Defective coil windings -- Ring-down being absent and a very short arc duration in the waveform indicates a shorted coil. When the coil windings become shorted, the coil will transfer less energy from the primary winding to the secondary winding, so that after initial ionization of the gas, there is barely any energy remaining to sustain the arc.