Some vehicles are equipped with a Distributorless Ignition System where the coils are combined into a coil-on-plug unit that sits directly on top of the spark plugs and houses the coil ignition system. This was commonly fitted to SAAB engines as well as certain Vauxhall, Peugeot and others, as shown in Figure 1 below.
With the engine running, an ignition pattern similar to the example below should appear on the screen.
*Note: the Advanced Kit has four of these leads included as standard. Extra leads can be purchased for working on vehicles such as SAABs with more than 4 cylinders, but you can only test up to 4 at one time if using a 4-channel automotive scope.
Note: if a waveform cannot be seen on any of the channels then this could be because the output is negative- or positive-fired opposite to the setup on the preset. If the HT pickup is moved to another HT extension lead, the waveform should then appear as shown below.
On a 4-cylinder engine with this type of ignition system, it is common to have two negative-fired outputs and two positive-fired outputs .
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.
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.
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.
We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.