The purpose of this test is to check primary ignition voltage and ignition coil current on an external coil based ignition system.
Note; Adjustment to the voltage scale may be needed to clarify the waveform.
In the Non-current limiting waveform example Channel B shows a rise in primary current taking just over 5 ms and reaching around 5.5 amps maximum.
Channel A shows a primary voltage with good characteristics reaching a peak voltage which is over 300 volts.
Your waveform may show a Current limiting system. In this waveform example on Channel B you can clearly see the peak current is limited by the ignition system. You will also see the current build time is much shorter.
Channel A once again shows a good primary voltage. In this waveform you will also see the characteristic jump in voltage at the point the system limits the current rise.
Note; values are system specific.
The primary ignition is so called as it forms the first part of the ignition circuit. Through the ignition coil, it drives the secondary High Tension (HT) output. The primary circuit has evolved from the basic contact breaker points and condenser to the distributorless and coil-per-cylinder systems in common use today. All of these ignition systems rely on the magnetic induction principle.
This principle starts with a magnetic field 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 becomes magnetically saturated. At the predetermined point of ignition the coil earth is removed and the magnetic field collapses. As the field inside the coil 250 to 350 primary windings collapses it induces a voltage of 150 to 350 volts.
The induced voltage is determined by,
Dwell is measured as an angle, with contact ignition, this is determined by the points gap. 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 has a dwell of approximately 45 degrees, which is 50% of one cylinder's 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 Unit (ECU).
The dwell angle on a constant energy system expands as the engine speed increases, to compensate for a shorter period of rotation and maximise the strength of the magnetic field. The term constant energy refers to the available voltage produced by the coil. This remains constant regardless of engine speed, unlike contact ignition where an increase in engine speed means the contacts are closed for a shorter time and gives the coil less time to saturate .
The induced voltage on a variable dwell system remains constant regardless of engine speed, while it reduces on contact systems. This induced voltage can be seen on a primary waveform.
Both dwell periods will expand as the engine revs are increased. This is to maintain a constant coil saturation time, hence the term constant energy. If time rulers are placed at the beginning of the dwell period and on the induced voltage line the coil saturation time can be measured. This will remain exactly the same regardless of engine speed.
Within the primary voltage waveform there are several sections that need closer examination. In the waveform shown the horizontal voltage line at the centre of the oscilloscope begins fairly constant at about 40 volts, but then drops sharply into what is referred to as the coil oscillation. This can also be seen in Figure 3.
The length of the aforementioned horizontal voltage line is the spark duration or burn time, which in this case is 1.036 ms. This can again be seen in Figure 4. The coil oscillation period should display at least 4 peaks counting both upper and lower. A loss of peaks indicates that the coil needs substituting for another of comparable specifications.
There is no current in the coil primary circuit until the dwell period Figure 5, which is when the coil is earthed and the measured voltage 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 maintains it until the earth is removed from the coil at the precise moment of ignition.
The vertical line at the centre of the trace, called the induced voltage, is above 200 volts. The induced voltage is produced by a process called magnetic induction. At the point of ignition, the coil earth circuit is removed and the magnetic field or flux collapses across the windings. This in turn induces an average voltage between 150 to 350 volts Figure 6. The coil High Tension (HT) output is proportional to the induced voltage. The height of the induced voltage is sometimes referred to as the primary peak volts.
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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