Secondary HT and amplifier digital switching signal


How to perform the test

Channel A - High Tension (HT) Secondary

Engines that are fitted with Coil-Per-Cylinder (CPC) ignition systems have the ignition coils bolted directly on top of the spark plugs. This makes it impossible to monitor the performance of the secondary High Tension (HT) circuit.

To overcome this problem, remove the multi-plug connections to the coils and then remove the coils one at a time or, in the case of a cassette coil pack, all together. HT extension leads  can then be coupled between the coils and the spark plugs. Reconnect the coils' multi-plugs and connect additional earth wires between the coil packs and the engine's earth where necessary (following the instructions provided with test lead TA037).

Plug a secondary pick-up lead into Channel A on the scope. Place the lead's clip on a suitable earth and fit the HT clamp on the HT extension lead. Most coil-per-cylinder systems have negative-fired spark plugs.

Channel B - Digital Switching Signal

Plug a BNC test lead into Channel B on the scope. Connect a Back-pinning Probe to the colored (positive) plug on the other end of the lead, and a large clip to the black (earth) plug. Place the clip onto a suitable earth and back-probe the coil's digital switching signal. It may be necessary to refer to the manufacturer's data. The connections are illustrated in Figure 1.

To test each coil, attach the HT pick-up lead and the acupuncture connection to each coil in turn.

Warning: all connections and disconnections should be made with the ignition turned off.

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.

Technical information

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
Coil-on-plug coil connections

Pin data

Our example waveform was taken from a Volkswagen Polo, where the four coil connections are as listed below:

  • Pin 1: Earth
  • Pin 2: Safety earth
  • Pin 3: ECM digital switching signal
  • Pin 4: Voltage supply



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




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Guided test: Using the HT Extension Lead