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 system was commonly fitted to SAAB engines as well as certain Vauxhalls, Peugeots and others, as shown in Figure 1.
With the engine running, a driver voltage and current pattern similar to the example below should appear on the screen.
The example waveform is a typical picture from an engine fitted with electronic ignition. It was taken from the coil on plug unit on the Vectra Z22SE Engine.
The supply is at the battery or charging voltage of 12 volts or more: in this example, about 14.0 volts. When the coil's primary circuit is switched on, the voltage drops slightly, and as the current in the circuit increases to the target of 10 amps, the voltage drops accordingly. The final voltage is about 13 volts - 1 volt lower than the original voltage.
The low-tension (LT) signal switches between zero volts and about 5 volts. When the trigger signal goes high, it causes the coil to switch on. As the voltage returns to zero, the current in the coil's primary winding switches off, the magnetic flux surrounding the winding collapses, this induces a voltage in the secondary and the coil's HT is fired. The switch-on (zero rising to 5 volts) and switch-off (5 volts to zero) points are determined by the vehicle's Electronic Control Module (ECM). This interval between these events is called either the dwell period or the saturation time. The dwell period on an engine with electronic ignition is controlled by the current-limiting circuit in the amplifier or ECM.
The example four-channel waveform, above, shows the current-limiting circuit in operation. The current switches on as the dwell period starts and rises until about 10 amps is reached in the primary circuit. At this point the current is maintained briefly and then released at the point of ignition. The length of time from the initial switch-on point to the moment the current is released depends on engine speed. The lower the engine speed, the shorter the current ramp; then the ramp lengthens with increasing engine revs.
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, 8. This is because two cylinders are connected to one coil that can produce 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 opposite strokes and the two spark plugs fire again but with opposite roles to before.
On a four-cylinder engine, there are two coils with individual drivers and these tend to 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 delivers the strongest spark possible and improves the durability of the ignition system.
The switch-on (zero rising to 5 volts) and switch-off (5 volts to zero) points of the coil are determined by the vehicle's Electronic Control Module (ECM). The time between these points is called either the dwell period or the coil saturation time. The dwell period on an engine with electronic ignition is controlled by the current-limiting circuit in the amplifier or ECM.
Historically, the supply voltage was present as soon as the ignition switch was turned to the 'on' position. Modern systems, however, do not provide a supply until the key is turned to the 'crank' position and the engine turns. A simple fault such as a non-functioning crank angle sensor may result in a loss of supply voltage, simply because the electronic control circuits do not recognize that the engine is rotating.
The earth connection is essential to the operation of any electrical circuit in an engine. As the current increases in any electrical circuit, so does the voltage drop in the earth. An earth return circuit can only be tested while the circuit is under load, so simple continuity testing to earth with a multimeter is inaccurate. As the coil's primary circuit is only complete during the dwell period, the voltage drop should be monitored during this period. The voltage ramp on the earth signal should not exceed 0.5 volts. The flatter the waveform the better: a waveform with virtually no rise shows that the amplifier or module has a perfect earth. If the ramp is too high, the earth connections need to be investigated to identify the offending connection.
The example waveform shows the current limiting circuit in operation. The current in the primary circuit switches on as the dwell period starts, and then to about 10 amps. This current is maintained until it is released at the moment of ignition.
As the engine speed increases, the dwell angle expands to maintain a constant coil saturation time and therefore constant energy. The coil saturation time can be measured by placing one time ruler at the beginning of the dwell period and the other at the end of the current ramp. The distance between the rulers will remain exactly the same regardless of engine speed.
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.