You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.
Note: This helpfile refers to a 10:1 attenuator. If you are using a 20:1 attenuator please adjust the Probe settings for the relevant channel. These settings can be found under the Channel Options button, then: Probe > 20:1 Attenuator.
Plug the 10:1 Attenuator into Channel A on the PicoScope and plug a BNC test lead into the attenuator. Place a large black clip onto the black test plug (negative) and a small red clip onto the colored test plug (positive). Place the black clip onto the battery negative terminal and probe the coil's negative (or number 1) terminal with the small red clip as illustrated in Figure 1.
The example waveform shows a high voltage, so the scaling of the oscilloscope is adjusted to suit. It is important that the 10:1 Attenuator is used in all situations when a voltage exceeding 200 volts is being measured.
With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings.
The ignition primary waveform is measuring the negative side of the ignition coil. The earth path of the coil can produce over 350 volts.
Within the primary picture 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 2.
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 3. 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's primary circuit until the dwell period (Figure 4), 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 reach the requisite 5-10 amps (depending on system). 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'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 (Figure 5). The coil's 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.
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's earth is removed and the magnetic field collapses. As the field inside the coil's 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 Module (ECM).
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.
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