Ignition - coil-on-plug secondary (using mV range)

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

Test Notes

This is a method for testing coil-on-plug units that are very well screened and cannot be tested using the standard COP Secondary preset. It uses the sensitive millivolt (mV) ranges of the oscilloscope to obtain the secondary waveform. Using this method, you will lose references to kilovolts (kV), but you will still be able to compare all the coil-on-plug units to try to identify any problems.

Connect the end of the TA033 cable with the ground clip to the scope's Channel A input and the other end to the COP Probe. Connect the ground clip to a reliable ground point on the engine block.

 

When placing the tip of the probe on the coil, ensure that you use the flat edge. Try to be consistent with the position on the coil: find the best signal on the first coil to be tested and then repeat that on the other coils.

With the engine running, an ignition pattern similar to the example below should appear on the screen.

If you still cannot see a clear waveform then reduce the voltage range further to 50 mV or increase it to 200 mV as shown in the waveform below.

Example waveform

Waveform notes

The example waveform show a typical ignition 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 'burn time' is also illustrated in Figure 3.

The coil oscillation period (as illustrated in Figure 4) 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' (as illustrated in Figure 5) 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.

Plug Kv

Figure 6

The plug firing voltage is the voltage required to jump the gap at the plug's electrode, commonly known as the 'plug kV'. This is shown in Figure 6. In the waveform example the plug kV is shown in mV so it cannot be referenced to kV as shown in Figure 7 but it will allow a comparison against other coils.

Coil-on-plug secondary ignition

Figure 7 - Example coil units

Technical information

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 reliability 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.

AT196-3

Disclaimer
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • BNC to BNC Cable with Earth Clamp

    £25.00

  • Coil On Plug (COP) and Signal Probe

    £179.00

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Guided test: COP testing using mV range for Secondary Ignition