Distributorless ignition (negative-fired) - secondary voltage

The purpose of this test is to investigate a negative fired secondary ignition waveform from a Distributorless Ignition System (DIS).

How to perform the test

View connection guidance notes.

​

1. Connect a secondary ignition pick-up lead to PicoScope Channel A.
2. Ensure the ignition is off.
3. Connect the secondary ignition pick-up lead around the suspect plug lead and to a good earth.
4. Start the engine and allow it to idle.
5. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
6. Start the scope to see live data.
7. With your waveform on screen stop the scope.
8. Stop the engine and ensure the ignition is off.
9. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note

If the HT waveform is missing from the live readings, or appears inverted, the suspect lead may be positive fired.

Select the guided test for a positive fired HT lead from the automotive drop-down menu and repeat the test.

Example waveform

Waveform notes

When the plug kV's are viewed on a DIS or coil-per-cylinder ignition system, the waveform should be in the orientation shown and not inverted, as this would suggest that either the wrong polarity has been selected from the menu or, in the case of DIS, the wrong plug lead chosen. The plug voltage continuously fluctuates while the engine is running, causing the waveform to move up and down. To find the maximum voltage at the spark plug, use the 'Ch A: Maximum (kV)' reading at the bottom of the screen.

Snap the throttle and observe the voltage change when the engine is under load. This is the only time that the plugs are placed under any strain and is a fair assessment of how they will perform on the road.

The part of the waveform immediately following the trigger point, at approximately -1.3 kV, is known as the sparkline voltage. This is the voltage required to keep the plug firing after the spark has initially jumped the gap. This voltage is proportional to the resistance in the secondary circuit. The sparkline in the example lasts for about 1.4 ms. This is the spark duration, the length of time for which the spark is active in the plug gap.

Further information on secondary waveforms can be found in the 'Secondary - distributor system king (or plug) lead' information pages, selected from the main menu.

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 secondary voltage (negative)

Further guidance

The coil consists of a primary winding that surrounds the secondary winding. The secondary winding is coiled around a multi-laminated iron core and has about 20,000 to 30,000 turns. One end is connected to the primary terminal and the other to the coil tower.

The High Tension (HT) voltage is produced by mutual induction between the primary winding and the secondary winding, the central soft iron core intensifying the magnetic field between them.

The voltage measured at the spark plug is the voltage required to jump the plug gap in varying conditions, and is determined by any or all of the following:

The plug kilovolt (kV) requirements of older engines tend to be lower than those of modern engines, as later designs run higher compression ratios, leaner air/fuel ratios and larger spark plug gaps.

The modern engine with a Distributorless Ignition System (DIS) has all the advantages of a constant energy electronic ignition system, but with the added bonus of eliminating the distributor cap, king lead and rotor arm from the system. Reliability problems from dampness and tracking are now almost eliminated.

DIS has its own drawbacks by having half of the plugs firing with an acceptable negative voltage, while the other half are fired by the less acceptable positive polarity. This causes pronounced plug wear on the positive-fired plugs.

This system fires the plugs each revolution instead of every other, and is known as a wasted spark system. This does not mean that the plugs will wear at twice the normal rate, as the wasted spark is on the exhaust stroke and is therefore under no compression. If the spark plugs are removed after several thousand miles and examined, two of the plugs will show almost square electrodes, while the plugs that have been fired positive will have pronounced plug wear.

AT081