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Primary - Ignition Diagnostics
(scroll for Primary - Ignition Current)

The ignition primary is measured on the negative side of the ignition coil. The earth path of the coil can produce over 350 volts.

Within the primary the horizontal voltage line is at fairly constant voltage of approximately 40 volts, which then drops sharply into what is referred to as the coil oscillation, these can also be seen here
The length of the aforementioned horizontal voltage line is the 'spark duration' or 'burn time', which in this particular case is 1.036 ms, this can again be seen here

The coil oscillation period should display a minimum number of 4 to 5 peaks (both upper and lower). A loss of peaks would indicate that the coil needs substituting for another of comparable values.
There is no current in the coil's primary circuit until the dwell period, this is when the coil is earthed and the voltage seen drops to zero.

The dwell period is controlled by the ignition amplifier and the length of the dwell is determined by the time it takes to build up approximately 8 amps. When this predetermined current has been reached, the amplifier stops increasing the primary current and it is maintained until the earth is removed from the coil, at the precise moment of ignition.

The vertical line at the centre of the trace is in excess of 200 volts, this is called the 'induced voltage'. The induced voltage is produced by a process called magnetic inductance. At the point of ignition, the coil's earth circuit is removed and the magnetic field or flux collapses across the coil's windings, this in turn induces an average voltage between 150 to 350 volts.
The coil's High Tension (HT) output will be proportional to the induced voltage. The height of the induced voltage is sometimes referred to as the primary peak volts.

Technical Information

The primary ignition is so called as it forms the first part of the ignition circuit. This circuit is used to provide the initial stage towards the secondary High Tension (HT) output. The primary circuit has, over the past few years, evolved from the basic contact breaker points and condenser to the distributorless and coil per cylinder systems in common use today. The basic origin of all of these ignition systems evolves around the magnetic inductive principle.

Magnetic Inductance

This principle is based around a magnetic field (or flux) being produced, as the coil's earth circuit is completed by either the contacts or the amplifier providing the coil negative terminal with a path to earth. When this circuit is complete, a magnetic field is produced and builds until the coil's magnetic field becomes maximised or saturated. At the predetermined point of ignition, the coil's earth is removed and the magnetic field or flux collapses across the coils 250 to 350 primary windings, which in turn induces a voltage of 150 to 350 volts.

This induced voltage will be determined by :

The number of turns in the primary winding.

The strength of the magnetic flux, which is proportionate to the current in the primary circuit.

The rate of collapse, which is determined by the speed of the switching of the earth path.

Dwell period

Dwell is measured as an angle: with contact ignition, the points gap determines the dwell angle. The definition of contact ignition dwell is: 'the number of degrees of distributor rotation with the contacts closed'.

As an example, a 4 cylinder engine will have a dwell of approximately 45 degrees, which is 50% of one cylinders complete primary cycle. The dwell period on an engine with electronic ignition is controlled by the current limiting circuit within the amplifier or Electronic Control Module (ECM).

The dwell on a constant energy system will be seen to expand as the engine speed increases, to compensate for a shorter time period. The term 'constant energy' refers to the available voltage produced by the coil. This, regardless of engine speed, will remain constant, as opposed to contact ignition where an increase in engine speed means the contacts are closed for a shorter time period. This reduces the effective time that the coil has to fully saturate and maximise the strength of the magnetic flux.

The induced voltage on a variable dwell system will remain constant regardless of engine speed, while this voltage will reduce on contact systems. This induced voltage can be seen on a primary waveform.

Primary - Ignition (Current) Diagnostics

The current switches on as the dwell period starts and rises until the requisite 8 amps is achieved within the primary circuit, at which point the current is maintained until it is released at the point of ignition.

The dwell will expand as the engine revs are increased, this is to maintain a constant coil saturation time, hence the term 'constant energy'. If the 'X' cursor was placed at the beginning of the dwell period and the 'O' cursor placed on the induced voltage line, the coil saturation time can be measured. This will remain exactly the same regardless of engine speed.

Technical Information

The purpose of the ignition amplifier is to switch the relatively high primary current of approximately 8 to 10 amps to earth when the component receives a signal from either the pick-up or Electronic Control Module (ECM).

The output from the coil is determined by the speed of the switching: the faster this occurs the greater the induced voltage into the coil's primary winding.

The amplifier can best be described as a solid state relay, allowing a small current to switch a much higher current. Due to this high amperage, the component will get hot, and a liberal application of heat transfer grease should be applied to the mating surface to dissipate the heat: without this precaution the component will overheat and fail. The earth connection is crucial to the correct operation of the amplifier, an earth path waveform and topic can be found in the main menu.

The amplifier can also be referred to as a 'module' or an 'igniter'.