Secondary - Distributor system (checked at the king lead)

The purpose of this test is to evaluate the correct operation of the secondary ignition circuit within a distributor-style ignition system via the King Lead. The secondary ignition voltage reveals the dwell period, peak firing voltage, burn time, and coil oscillations during engine run conditions. 

How to perform the test

Plug a high-tension secondary ignition pick-up lead into Channel A on the PicoScope, clip the lead's fly lead on a suitable earth and clip the HT clip onto to the coil lead (king lead). For previous engine analyser users there may be a noticeable difference in the waveform pattern and plug kV readings when checking the coil lead and the individual plug leads.

Figure 1 shows the HT pick-up lead connected to the distributor coil lead.

Warning: When attaching or removing secondary ignition pickups from damaged HT leads, there is a danger of electric shock. To avoid this risk, attach and remove the secondary ignition pickup with the ignition turned off.

Although it is possible to use two HT pick-ups and produce a secondary parade waveform, triggered by cylinder number 1 (using Channel B), without the aid of a 720° trigger the timebase will need to be constantly altered to view the correct number of cylinders. It is far easier to look at the individual plug lead HT waveforms and identify any discrepancies from this information.

Example waveform

Waveform notes

The ignition secondary picture shown in the example waveform is a typical picture from an engine fitted with electronic ignition. The waveform has been taken from the coil lead (king lead).

The secondary waveform shows the length of time that the HT is flowing across the spark plug's electrode after the initial peak of voltage required to jump the plug gap. This time is referred to as either the 'burn time' or the 'spark duration'. The illustration shows that the horizontal voltage line in the centre of the oscilloscope is at fairly constant voltage of approximately 4 kilovolts (kV), which then drops sharply into what is referred to as the 'coil oscillation' period. The 'burn time' is also illustrated in Figure 3.

The coil oscillation period (as illustrated in Figure 3) should display a minimum of 4 to 5 peaks (both upper and lower). A loss of peaks indicates that the coil needs substituting. The period between the coil oscillation and the next 'drop down' is when the coil is at rest and there is no voltage in the coil's secondary circuit. The 'drop down' is referred to as the 'negative polarity peak' (as illustrated in Figure 2), and produces a small oscillation in the opposite direction to the plug firing voltage. This is due to the initial switching on of the coil's primary current. The voltage within the coil is only released at the correct point of ignition, and the HT spark ignites the air/fuel mixture.

The plug firing voltage, or 'plug kV', is the voltage required to jump the gap at the plug's electrode. This is shown in Figure 4. In this example the plug kV is 13.5 kV.

Technical information

Situated within the coil's primary winding is the secondary winding. This winding 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. The central soft iron core intensifies the magnetic field between them.

On a distributor system, the secondary HT voltage produced by the coil is allocated to the appropriate spark plug via the contacts inside the distributor cap.

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

The plug kV's will be increased by: The plug kV's will be decreased by:
Large plug gaps Small plug gaps
A large rotor air gap Low compression
A break in a plug lead Rich mixture
A break in the king lead Incorrect ignition timing
Worn spark plugs Tracking to earth
A lean mixture Fouled plugs
Rotor to reluctor misalignment  

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 that the distributor cap, king lead and rotor arm are eliminated 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 a normal negative voltage, while the other half are fired by the less acceptable positive polarity. This has the effect of pronounced plug wear on the positive fired plugs.

This system, because of its nature, 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, two of the plugs will be found to have relatively square electrodes, while the plugs that have been fired positive will have pronounced plug wear.

Figure 6 shows an example coil.


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

  • Secondary ignition pickup (capacitive with BNC)


Help us improve our tests

We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.

Add comment

Your email address will not be published. Required fields are marked *

Guided test: Secondary Distributor (King Lead)