Multi-coil-on-plug secondary ignition (4 cylinder test)

The purpose of this test is to check the secondary ignition events where a multi-coil-on-plug unit or cartridge is used.

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever 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

   

WARNING

Uninsulated HT pickups are designed to clip around double-insulated HT cables only – they are not designed for direct connection to a hazardous live voltage. To prevent injury or death, switch off the engine and secure against restart. Clean and inspect the HT lead for damage to insulation and fit only to undamaged fully insulated leads. Ensure that leads are safely clear of hot and / or rotating parts. The engine can now be restarted for the test duration.

  1. Ensure the ignition is switched off.
  2. Remove the multi-coil pack and insert HT extension leads.
  3. Connect HT pick-up leads between the extension leads and the four PicoScope channels. Ensure the leads have a good earth.
  4. Check all connections are secure, start the engine and allow it to idle.
  5. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  6. Start the scope to see live data.
  7. With your live waveforms on screen stop the scope.
  8. Stop the engine and ensure the ignition is off.
  9. Use the Waveform Buffer and Zoom tools to examine your waveform.


    Note; Some multi-coil pack systems employ wasted spark coils, this means there will be both negative and positive fired spark plugs. 

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 unit fitted to the Vectra Z22SE engine.

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.

Example multi-COP unit

Further guidance

The operation of the coil-on-plug unit is essentially the same as any other ignition system.

Distributorless ignition systems are fitted only to vehicles that have an even number of cylinders such as 2, 4, 6 or 8. This is because two cylinders are connected to one coil that produces a spark in both cylinders at the same time. This is commonly known as a wasted spark system. The two spark plugs are arranged so that one is fired on the power stroke of the engine and the other on the exhaust stroke of the opposing cylinder, offset by 360 degrees. After a complete rotation of the engine, the two cylinders are now on the opposing strokes and the two spark plugs fire again but with opposite roles to before.

On a 4-cylinder engine, there are two coils with individual drivers that usually operate cylinders 1 and 4, and 2 and 3. This means there is a dual spark every 180 degrees, with one of those sparks wasted on an exhaust stroke of the opposing cylinder which is firing on the power stroke.

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.

GT185

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

  • HT Extension Lead

    £41.00

  • HT Extension Lead (set of 4 leads)

    £159.00

  • Secondary ignition pickup (capacitive with BNC)

    £58.00

  • Premium 6-way breakout lead set

    £269.00

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Guided test: Secondary 4 Cylinders