Distributorless ignition - primary current

The purpose of this test is to investigate the current value and saturation time within a Distributorless Ignition System (DIS) primary coil.

How to perform the test

View connection guidance notes.

1. Use manufacturer data to identify the coil voltage supply terminal.
2. Connect the low amp clamp to PicoScope Channel A, select the 20 A scale and zero the clamp before attaching to the coil supply voltage circuit.
3. Start the engine and allow it to idle.
4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
5. Start the scope to see live data.
6. With your waveform on screen stop the scope.
7. Stop the engine and turn off the ignition.
8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note

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. 

Example waveform

Waveform notes

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.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, DIS / wasted spark ignition primary current

Further guidance

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.

GT168