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

Connect the low-amp current clamp to Channel A of the PicoScope. Select the 20 A range if appropriate and switch the current clamp on. Press the zero button before connecting the clamp to the circuit.

The current clamp should be placed directly onto the coil's supply cable and not around the loom that will also contain the negative (or negatives depending on the ignition system). The connection is illustrated in Figure 1.

The waveform will show a curving line that indicates the speed at which the coil is being saturated. The flatter the line, the longer it is taking to magnetize the coil. The waveform flattens out for a time, where the current is being maintained by the amplifier once it has reached its requisite current. The current is held until the amplifier releases the earth path and the waveform then drops vertically. This vertical line is equally important, as a sloping line indicates that the amplifier is not switching fast enough and the induced voltage will suffer as a result.

With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings.

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.

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


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