Coil-on-plug - secondary voltage (probe)

The purpose of this test is to evaluate the correct operation of the secondary ignition circuit within a Coil-On-Plug (COP) style ignition system. The secondary ignition voltage reveals the dwell period, peak firing voltage, burn time, and coil oscillations  during engine run conditions. 

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should wherever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

  1. Connect the COP signal probe to PicoScope channel A and ensure a good earth connection onto the engine block.
  2. Start and run the engine.
  3. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  4. Select go or press the space bar 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 live waveform on screen select stop or press the space bar to stop your capture.
  7. Turn off the engine.
  8. Use the Waveform Buffer and Zoom tools to examine your 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 'burn time' is also illustrated in Figure 4.

The coil oscillation period (as illustrated in Figure 5) 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 6) 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'. This is shown in Figure 7. 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.

Example COP unit.

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.

GT077-6

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

  • Coil On Plug (COP) and Signal Probe

    £179.00

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Guided test: Coil-on-plug - secondary voltage (probe)