The purpose of this test is to evaluate the correct operation of the secondary ignition circuit within a Coil-On-Plug (COP) style ignition system. The secondary ignition voltage reveals the dwell period, peak firing voltage, burn time, and coil oscillations during engine run conditions.
Connection for diagnostic work will of course vary dependent on application.
Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.
General connection advice;
PicoScope offers a range of options within the test kits.
Dependent on difficulty of access, choose from,
Testing Sensors and Actuators; (to include relevant circuit/connectors).
Connect the end of the TA033 cable with the ground clip to the scope's Channel A input and the other end to the COP Probe as shown in Figure 1. Connect the ground clip to a reliable ground point on the engine block.
When placing the tip of the probe on the coil, ensure that you use the flat edge. Try to be consistent with the position on the coil: find the best signal on the first coil to be tested and then repeat that on the other coils.
With the engine running, an ignition pattern similar to the example below should appear on the screen.
The ignition picture shown in the example waveform is a typical picture from an engine fitted with electronic ignition. The waveform has been taken from the coil on plug.
The secondary waveform shows the length of time for which the HT flows across the spark plug's electrode after the initial peak of voltage required to jump the plug gap. This time is referred to as either the 'burn time' or the 'spark duration'. In the illustration, the horizontal voltage line in the centre of the oscilloscope is at fairly constant voltage, but then drops sharply into what is referred to as the 'coil oscillation' period. The 'burn time' is also illustrated in Figure 3.
The coil oscillation period (as illustrated in Figure 4) should display at least 4 peaks (including upper and lower). A loss of peaks indicates that the coil needs substituting. The period between the coil oscillation and the next 'drop down' is when the coil is at rest and there is no voltage in the coil's secondary circuit. The 'drop down' is referred to as the 'negative polarity peak', (as illustrated in Figure 5) and produces a small oscillation in the opposite direction to the plug firing voltage. This is due to the initial switching on of the coil's primary current. The voltage within the coil is only released at the correct point of ignition when the HT spark ignites the air/fuel mixture.
The operation of the coil on plug is essentially the same as any other ignition system. Each coil has a low primary resistance, and steps up the primary system voltage to as much as 40,000 volts to produce a spark at the plug.
The only real difference between COP and other ignition systems is that each COP coil is mounted directly onto 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 the strongest spark possible and improves the durability of the ignition system.
Using individual coils for each spark plug also means the coils have more time between each firing. Increasing the "coil saturation" time (the time the voltage to the coil is on to build up its magnetic field) increases the coil output voltage at high rpm when misfire is most likely to occur.
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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