Products suited to this guided test*
  • Secondary ignition pickup (capacitive with BNC)

  • HT Extension Lead

  • PicoScope Battery Clip

  • Multimeter Probes

  • *At Pico we are always looking to improve our products. The tools used in this guided test may have been superseded and the products above are our latest versions used to diagnose the fault documented in this case study.

Secondary HT and amplifier digital switching signal

The purpose of this test is to investigate the correlation between the ignition trigger and secondary HT events in a COP type ignition coil.



This test involves measuring a potentially hazardous voltage.

Please ensure you follow manufacturers' safety instructions and working practices and ensure the rated voltage for all accessories you are using meets or exceeds the expected voltage.




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. Use manufacturer data to identify the function of the coil circuits.
  2. Ensure the ignition is turned off.
  3. Connect an HT extension lead between the coil unit and spark plug.
  4. Install an earth fly-lead between the coil unit and an engine earth. (Bolt in unit only).
  5. Connect a secondary pick-up lead to PicoScope Channel A. Ensure the lead is also connected to a good earth and then install around the HT extension lead.
  6. Connect PicoScope Channel B to the ignition coil signal circuit.
  7. Start the engine and allow it to idle.
  8. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  9. Start the scope to see live data.
  10. With your waveforms on screen stop the scope.
  11. Stop the engine and turn the ignition to off.
  12. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.


Preparation for the test will require disconnection, removal and reconnection of the suspect coil unit. 

Example waveform

Waveform notes

The waveform above shows the relationship between the secondary HT on channel A (blue trace) and the digital trigger signal (red trace). When the trigger signal goes high, the coil's primary circuit is closed, causing a current from the battery to flow through it. At the end of the dwell time, the trigger signal returns low, breaking the primary circuit and causing the secondary winding to generate a high tension (HT) voltage.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, COP (Coil on plug ignition) trigger signal

Further guidance

As the illustration is a dual trace picture, the two traces will be explained one at a time.

Digital switching signal (red trace)

The low tension (LT) signal switches between 0 volts and 4 volts. When the trigger signal reaches 4 volts, the coil switches on and the 'saturation' or 'dwell' time begins. As the voltage returns to zero, the current in the coil's primary winding switches off, the magnetic flux in the iron core collapses, induces a voltage in the secondary, and the HT voltage is generated.

The coil switch-on and switch-off times are determined by the vehicle's Electronic Control Module (ECM). The dwell time on an engine with electronic ignition is controlled by the current limiting circuit in the amplifier or ECM.

In a constant energy system, the dwell time is fixed regardless of engine speed. This allows the coil to saturate fully and maximises the strength of the magnetic flux. The dwell angle, measured relative to a complete engine cycle of 360°, increases as the engine speed increases.

Secondary HT waveform (blue trace)

The modern engine management system with Coil-Per-Cylinder (CPC) ignition has all the advantages of a constant energy electronic ignition system, with the added bonus of eliminating the distributor cap, king lead, rotor arm and plug lead. Reliability problems from dampness and tracking are now almost eliminated.

Unlike a conventional Distributorless Ignition System (DIS) which fires the plugs with both a negative and a positive voltage, CPC only fires the plugs with a negative voltage, improving plug life and increasing the service life of the plugs.

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, the central soft iron core serving to intensify the magnetic field between them.

The voltage measured at the spark plug is the voltage required to jump the plug gap under varying conditions. This voltage can be affected by any of the following:

  • The size of the plug gaps: a large gap increases the kV
  • Worn spark plugs: the loss of surface area will increase the plug kV
  • The engine's compression: a low compression reduces the plug kV
  • The engine's fuelling: a rich mixture reduces the plug kV
  • Tracking to earth: reduces the plug kV
  • Fouled plugs: reduces the plug firing voltage 

Pin data

Example pin data for two, three and four pin COP units.


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.

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Guided test: Using the HT Extension Lead