Secondary HT and amplifier digital switching signal

The purpose of this test is to verify the signal and the relationship between a secondary ignition event and the digital trigger signal.

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.


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

  1. Use manufacturer data to identify the function of the coil circuits.
  2. Connect an HT extension lead between the removed coil unit and spark plug.
  3. Install the earth fly-lead between the coil unit and an engine earth. (Bolt in unit only).
  4. Connect breakout leads between the coil unit and the coil unit multi-plug.
  5. Connect a secondary pick-up lead into channel A of your PicoScope. 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 breakout lead for the coil signal circuit.
  7. Start the engine and allow it to idle.
  8. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  9. Select Go or press the space bar to see live data.
  10. With your live waveforms on screen select Stop or press the space bar to stop your capture.
  11. Stop the engine and turn off the ignition.
  12. Use the Waveform Buffer and Zoom tools to examine your waveform.

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.

Example 3-wire COP

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.

Suitable accessories

  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • PicoScope Battery Clip


  • Large Dolphin/Gator Clips


  • Premium Test Lead: BNC to 4 mm, 3 m


  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)


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Guided test: Using the HT Extension Lead