The purpose of this test is to investigate the current value and saturation time within a Distributorless Ignition System (DIS) primary coil.
This test involves measuring a potentially hazardous voltage.
Please ensure you follow manufacturers' safety instructions and working practices and ensure the rated voltage for all accessories you are using meets or exceeds the expected voltage.
The orientation of the current clamp relative to the wire will determine whether it has a positive or negative output. If a live waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.
In the example waveform above, the current limiting circuit can be seen in operation. The current switches on as the dwell period starts and rises until the requisite 5-10 amps (depending on system) 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 one time ruler is placed at the beginning of the dwell period and the other on the induced voltage line, the coil saturation time can be measured. This will remain exactly the same regardless of engine speed.
The waveform will show a curving line that indicates the speed at which the coil is being 'saturated'. The flatter the line, the longer it is taking to 'fill' the coil with voltage. The waveform flattens out for a time, where the current is being maintained by the amplifier once it has reached its requisite current. The current is held until the amplifier releases the earth path, when the waveform drops vertically. This vertical line is also important, as a 'sloping' line indicates that the amplifier is not switching fast enough and the induced voltage will suffer as a result.
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