Distributor (king lead) - secondary voltage

The purpose of this test is to examine secondary ignition voltage characteristics via the distributor king lead.

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test



Uninsulated HT pickups are designed to clip around double-insulated HT cables only – they are not designed for direct connection to a hazardous live voltage. To prevent injury or death, switch off the engine and secure against restart. Clean and inspect the HT lead for damage to insulation and fit only to undamaged fully insulated leads. Ensure that leads are safely clear of hot and / or rotating parts. The engine can now be restarted for the test duration.

  1. Connect a secondary ignition pick up lead to channel A of your PicoScope.
  2. Make sure the ignition is turned off.
  3. Clamp the secondary ignition pick up lead around the distributor HT king lead and to a good earth.
  4. Start the engine and run at idle.
  5. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  6. Start the scope to see live data.
  7. With your live waveform on screen stop the scope.
  8. Turn off the engine and ensure the ignition stays off.
  9. Use the Waveform Buffer, Zoom and Measuring tools to examine your waveform.

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 electrode after the initial peak of voltage required to jump the plug gap. This time is referred to as either the burn time or spark duration. The illustration shows that the horizontal voltage line in the centre of the oscilloscope is at fairly constant voltage of approximately 4 KV which then drops sharply into what is referred to as the coil oscillation period.

The coil oscillation period should display a minimum of 4 to 5 peaks counting both upper and lower. A loss of peaks indicates that the coil needs replacing. The next event in the pattern is referred to as the negative polarity peak 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 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 is the voltage required to jump the gap at the plug electrode and is indicated as plug KV.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select Ignition coil secondary voltage.

Further guidance

Situated within the coil 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 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 will be increased by: The plug KV 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 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.


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)


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Guided test: Distributor (king lead) - secondary voltage