Distributorless ignition (positive-fired) - secondary voltage

The purpose of this test is to investigate a positive 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 negative fired.

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

Example waveform

Waveform notes

When the positive fired plug kV's are recorded on Distributorless Ignition Systems (DIS), the voltage should be as shown and not inverted, as this would suggest that the wrong lead has been chosen.

While the engine is running, the plug voltage continuously fluctuates and the display moves up and down. To record 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 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 after the trigger point is known as the sparkline voltage, and is at about 0.75 kV. This is the voltage required to keep the spark alive after it has jumped the gap. This voltage is proportional to the resistance in the secondary circuit. The length of the line is about approximately 1.4 ms. This is the spark duration, the length of time the spark exists in the plug gap.

It is worth remembering that a positive fired spark plug will require a greater voltage to fire it than a negative fired plug. This is because, when the plug gets hot, negative electrons 'boil away' from the metal surface in a process called thermionic emission. This can be seen as greater wear on the plug's electrode than for a negative-fired plug.

Further information on secondary waveforms can be found on 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 (positive)

Further guidance

Inside the coil's primary winding is the secondary winding. This is coiled around a multi-laminated iron core and has approximately 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, with the central soft iron core intensifying the magnetic field between them.

The secondary HT voltage produced by the coil is allocated to the appropriate spark plug by the ignition system.

The voltage measured at the spark plug is the voltage required to jump the plug gap in varying conditions. This voltage is determined by the following:

The plug kilovolt (kV) requirement of older engines tends to be lower than that of the modern engine, as the later designs run higher compression ratios, leaner air/fuel ratios and larger spark plug gaps.

The modern engine with Distributorless Ignition System (DIS) has all the advantages of a constant energy electronic ignition system, but with the added bonus of the distributor cap, king lead and rotor arm being removed 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 has the effect of increased 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 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, they will be found to have relatively square electrodes, while the plugs that have been fired positive will have pronounced plug wear.

GT082