Coil-on-plug - secondary voltage (probe)

The purpose of this test is to investigate and examine the secondary voltage waveform from a Coil on Plug (COP) ignition unit using a COP probe.

How to perform the test

View connection guidance notes.

1. Connect the COP signal probe to PicoScope Channel A. and ensure a good earth connection onto the engine block.
2. Start and allow the engine to idle.
3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
4. Start the scope to see live data.
5. Place the foot of the probe squarely onto the top of the COP unit to be tested.
6. With your waveform on screen stop the scope.
7. Turn off the engine.
8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

The ignition picture shown in the example waveform is a typical picture from an engine fitted with electronic ignition. The waveform has been taken from the coil on plug.

The secondary waveform shows the length of time for which the HT flows across the spark plug's electrode after the initial peak of voltage required to jump the plug gap. This time is referred to as either the 'burn time' or the 'spark duration'. In the illustration, the horizontal voltage line in the centre of the oscilloscope is at fairly constant voltage, but then drops sharply into what is referred to as the 'coil oscillation' period.

The coil oscillation period should display at least 4 peaks (including upper and lower). A loss of peaks indicates 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 coil's secondary circuit. The 'drop down' is referred to as the 'negative 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 when the HT spark ignites the air/fuel mixture.

The plug firing voltage is the voltage required to jump the gap at the plug's electrode, commonly known as the 'plug kV'. In this example the plug kV is 13.5 kV.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select COP (Coil on plug ignition) secondary voltage.

Further guidance

The operation of the coil on plug is essentially the same as any other ignition system. Each coil has a low primary resistance, and steps up the primary system voltage to as much as 40,000 volts to produce a spark at the plug.

The only real difference between COP and other ignition systems is that each COP coil is mounted directly onto the spark plug, so the voltage goes directly to the plug electrodes without having to pass through a distributor or plug leads. This direct connection method delivers the strongest spark possible and improves the durability of the ignition system.

Using individual coils for each spark plug also means the coils have more time between each firing. Increasing the "coil saturation" time (the time the voltage to the coil is on to build up its magnetic field) increases the coil output voltage at high rpm when misfire is most likely to occur.

GT077-EN