DIS primary ignition (current) (using a low-amp current clamp)

The purpose of this test is to monitor the current and and therefore the coil saturation time and see the current-limiting circuit in operation. 

How to perform the test

Connect the low-amp current clamp to Channel A of the PicoScope. Select the 20 A range if available and switch the current clamp on. Remeber to zero the clamp before placing it on the vehicle.

The current clamp should be placed directly onto the coil's supply cable and not around the loom that also contains the negative (or negatives depending on the ignition system). The connection is illustrated in Figure 1.

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.

With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings.

Waveform notes

In one of the example waveforms 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.


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.

Suitable accessories

  • 30 A (low amps) current clamp


  • 20 A / 60 A DC (low amps) current clamp


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Guided test: Primary Current