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Secondary Ignition Waveform Notes
The ignition secondary picture shown in the example waveform is a typical picture from an engine fitted with electronic ignition.
The waveform is an individual secondary High Tension (HT) picture that can be observed one cylinder at a time. This secondary waveform shows the initial voltage to jump the plug gap, know as the ‘Plug Kv’ then shows the length of time that the HT is flowing across the spark plugs electrode plug gap. This time is referred to as either the ‘Burn Time’ or the ‘Spark Duration’.
In the illustration shown it can be seen that the horziontal voltage line (Sparkline) in the centre of the oscilloscope is at fairly constant voltage of approximately 3 Kilo volts (Kv), which then drops sharply into what is referred to as the ‘Coil Oscillations’.
The coil oscillation should display a minimum number of peaks (both upper and lower) and a minimum of 4 - 5 should be seen. A loss of peaks on this oscillation shows that the coil needs substituting.
The period between the coil oscillation and the next ‘drop down’ is when the coil is at rest and there is no voltage in the coils secondary.
The ‘drop down’ is referred to as the ‘Polarity Peak’ and produces a small oscillation in the opposite direction to the plug firing voltage. This is due to the initial switching on of the coil’s primary current.
The voltage within the coil is only released at the correct point of ignition and the HT spark ignites the air/fuel mixture.
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