Distributor king lead secondary voltage (sequential)

The purpose of this test is to evaluate the secondary ignition system, mixture, and combustion process, from the secondary voltage picked up on the distributor king lead.

 

WARNING

Uninsulated HT pickups are designed to clip around double-insulated HT leads only – they are not designed for direct connection to a hazardous live voltage.

To prevent injury or death, when connecting or disconnecting an HT pickup:

  1. switch off the ignition
  2. clean the HT leads
  3. inspect them for damage
  4. clip the HT pickup over the desired undamaged HT lead
  5. ensure all test leads are kept clear of hot or rotating parts before starting the engine

 

How to perform the test

View connection guidance notes.

  1. Turn the ignition off.
  2. Connect a secondary ignition pick up lead to PicoScope Channel A.
  3. Clamp the secondary ignition pick up around the distributor HT king lead and connect its earth fly lead to earth.
  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.
  6. Run the engine.
  7. With your waveform on screen stop the scope.
  8. Turn off the ignition.
  9. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

The secondary circuit voltage is around 0 kV until it steps downwards toward -3 kV when the ignition primary circuit is switched on.

The secondary voltage oscillates for several cycles whilst gradually stabilising back toward 0 kV.

A high but brief secondary voltage spike, reaching around 20 kV, is induced when the primary circuit is switched off.

The secondary voltage drops to around 4 kV, which is maintained for approximately 1 ms, for the duration of the spark.

The secondary voltage increases abruptly by 1 – 2 kV toward the end of the sparking process.

The secondary voltage collapses back towards 0 kV, indicating the end of the spark.

There follow 3 to 5 cycles of diminishing oscillation before the secondary voltage gradually stabilises back toward 0 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

General principles

All (inductive) spark ignition systems use one or more ignition coils. The coils act as both an accumulator, to store energy, and a step-up transformer, to generate the high voltages necessary to produce an electrical spark within a combustion chamber.

An ignition coil consists of a primary coil and a secondary coil, wound around each other in close proximity. The secondary coil has a high ratio of windings to the primary coil. This arrangement creates conditions of high mutual inductance, meaning changes in the magnetic field in the primary coil will produce changes in voltage in the secondary coil.

The primary coil is connected within the primary circuit. When current flows in the primary circuit, energy builds within the coil’s magnetic field. If the current is quickly removed, the magnetic field rapidly collapses and induces a high voltage in the secondary coil. The high voltage is delivered to a spark plug via a secondary circuit.

The period during which the current flows within the primary circuit is known as the dwell period (or the dwell angle, if referenced to the angle of crankshaft rotation). The dwell period determines the charging time of the coil and, therefore, partly controls the energy it stores.

The current and dwell period are Key Performance Indicators (KPIs) for primary circuit control. Please refer to manufacturer technical information to find the specifications for your vehicle.

Distributor ignition

Distributor based ignition systems use a single ignition coil.

The switching of the primary circuit can be controlled using one of two mechanisms:

  • mechanically, by a contact breaker driven by a rotating cam within the distributor
  • using transistorised current switching, triggered by a timing reference signal

Most mechanically triggered primary circuits require a ballast resistor to regulate the current flow, whereas a transistorised system is able to vary the current more freely.

A component rotating internally within the distributor, the rotor, directs the secondary voltages to each of the engine’s spark plugs, in their firing order, as it passes peripheral electrodes connected to the spark plug leads.

Spark generation

A spark plug provides an insulated path for electrical energy to reach the combustion chamber. It consists of two electrodes (a central electrode and an earth electrode) separated by an insulator. A small air gap, the spark plug gap, exists between the electrodes.

Any voltage difference between the electrodes will create an electric field across the spark plug gap. If the voltage is sufficiently high, the electric field will ionize the air/fuel mixture, turning it from an insulator to a conductor. A current is driven through the mixture by the remaining energy within the ignition system, triggering and sustaining the spark.

Spark diagnostics

The potential difference (voltage) at which mixture ionization occurs is known as the plug kV. The following factors increase plug kV:

  • Increased air/fuel mixture density, arising from increased compression pressure or decreased mixture temperature (the greater the density, the lower its electrical permeability).
  • A retarded end of dwell, as it triggers the process later within the compression stroke when the mixture density is higher.
  • Lean mixtures, having excess air or insufficient fuel (fuel is more conductive than air).
  • An excessively large spark plug gap or worn electrodes causing a reduced field strength.

The potential difference during which the spark is sustained is known as the sparkline kV.

The electrical energy within the secondary system is finite so the spark is sustained for a limited period, known as the spark duration.

There is an inversely proportional relationship between sparkline kV and spark duration. If the sparkline kV is too high, the spark duration will be too short, and vice versa.

During a spark event, the sparkline kV may:

  • Slope upwards, indicating a lean mixture.
  • Slope downwards, indicating a problem with the plug or secondary circuit.
  • Have a hash, indicating varying mixture pressure arising from turbulence within the combustion chamber.

A misfire may occur if there is too little energy within the ignition system - for example, if the coil is faulty or there are shorts or high resistance in the primary or secondary circuits.

When the mixture ionization can no longer continue, the current and sparkline kV collapse, leaving the remaining energy to oscillate and dissipate through the coil. These are called coil oscillations.

The plug kV, sparkline kV, spark duration and coil oscillations are ignition system KPIs. Please refer to manufacturer technical information to find the specifications for your vehicle.

GT046-EN

Disclaimer
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)

    £58.00

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Guided test: Distributor king lead secondary voltage (sequential)