Primary (current)

The purpose of this test is to monitor the primary current on an external coil based ignition system.

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

  1. Connect a low amp current clamp to channel A of your PicoScope.
  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. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  5. Start the scope to see live data.
  6. With your live waveform on screen stop the scope to stop the capture.
  7. Stop the engine and turn off the ignition.
  8. Use the Waveform Buffer and Zoom tools to examine your waveform.

Waveform notes

The example waveform shows the current limiting circuit in operation. The current switches on as the dwell period starts and rises until the requisite 5-10 amps (depending on system) is achieved within the primary circuit, at which point the current is maintained until it is released at the point of ignition.

The dwell will expand as the engine revs are increased. This is to maintain a constant coil saturation time, hence the term 'constant energy'. If time rulers are placed at the beginning of the dwell period and the induced voltage line, the coil saturation time can be measured. This will remain exactly the same regardless of engine speed.

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

Example ignition coil

Further guidance

The purpose of the ignition amplifier is to switch the relatively high primary current of approximately 5 to 10 amps to earth when the component receives a signal from either the pick-up or Electronic Control Module (ECM).The output from the coil is determined by the speed of the switching. The faster this occurs, the greater the induced voltage into the coil's primary winding. The amplifier can best be described as a solid-state relay, allowing a small current to switch a much higher current. Due to this high amperage, the component will get hot, and a liberal application of heat transfer grease should be applied to the mating surface to dissipate the heat. Without this precaution the component will overheat and fail. The earth connection is crucial to the correct operation of the amplifier. An earth path waveform and topic can be found in the main menu.

The amplifier can also be referred to as a 'module' or an 'igniter' (Japanese term).


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

  • 20 A / 60 A DC (low amps) current clamp


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Guided test: Primary Current