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 type of system was commonly fitted to SAAB engines as well as certain Vauxhalls, Peugeots and others. This type of coil is shown in Figure 1.
With the engine running, a driver pattern similar to the example below should appear on the screen.
The example waveform is from the coil-on-plug unit on the Vectra Z22SE Engine with electronic ignition.
The coil supply is at the battery or charging voltage of 12 volts or more. In this example, the voltage is 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 voltage when the coil is disconnected is of course zero volts, rising to about 0.1 volts when the coil is energized. If the circuit is suffering from a poor earth connection, this voltage will be higher; so the lower the voltage, the better the earth connection.
The operation of the coil-on-plug unit is essentially the same as any other ignition system.
The distributorless ignition systems are fitted only to vehicles that have an even number of cylinders such as 2, 4, 6 or 8. The reason for this is that two cylinders are connected to one coil that produces a spark for both cylinders at the same time. This system is 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 the opposite roles .
On a 4 cylinder engine, there are 2 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 on 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 a stronger spark and makes the ignition system more reliable.
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, so does the voltage drop on any given electrical circuit. 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 primary coil circuit is only closed 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.
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.
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