Products suited to this guided test*
  • 20 A / 60 A DC (low amps) current clamp

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 The tool used in this guided test may have been superseded and the product above is our latest version used to diagnose the fault documented in this case study.

Coil primary current

The purpose of this test is to examine the primary current characteristics on an external coil type ignition system.

How to perform the test

View connection guidance notes.

  1. Connect a low amp clamp to PicoScope channel A.
  2. Select the 20 A scale and zero the clamp before clamping around the ignition coil negative lead.
  3. Start the engine and allow it to idle.
  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 to see live data.
  6. With your waveform on screen stop the scope.
  7. Stop the engine and turn off the ignition.
  8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.


The orientation of the clamp relative to the wire will determine whether it has a positive or negative output. If a live waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

  • There is no current, 0 A, until the circuit is on
  • When switched on current begins to flow and build up to around 7 A.
  • When the primary circuit is switched off after 1.2 ms, current immediately ceases to flow, 0 A.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, Distributor ignition primary current

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 time the coil takes to reach its maximum magnetic field strength (its saturation time) depends on the peak primary circuit current, which, in turn, depends on the total primary circuit resistance and the primary coil's tendency to resist the build-up of current (its inductance).

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 must be sufficiently long (at all engine speeds) to allow the primary coil to reach maximum magnetic field strength (i.e. to saturate).

The peak current and dwell period are Key Performance Indicators (KPIs) for primary circuit control. Please refer to manufacturer’s 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.

Typical coil faults

A coil is susceptible to typical circuit related issues in both its primary and secondary windings, such as open or short circuits or high resistances. These may be caused by the breakdown of its windings (due to excessive thermal or vibration stress) or corroded connections.

Typical symptoms

Symptoms of a faulty coil can be:

  • Multiple, possibly random, misfire across cylinders.
  • Hesitation upon high torque demand.
  • Rough running at engine speeds.
  • Difficulty starting.


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.

Help us improve our tests

We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.

Add comment

Your email address will not be published. Required fields are marked *

Guided test: Primary Current