The purpose of this test is to check the primary ignition event for burn time, the dwell period and the induced voltage.
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 lead (negative) and a small red clip onto the colored test lead (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 that the voltage seen during this test is high and the scaling of the oscilloscope is therefore 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 values.
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 build up 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.
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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