Further guidance
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.
Coil technical information
Primary driver - digital switching signal
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.
Live
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.
Earth
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.
