Primary ignition and current (using the 10:1 attenuator)

The purpose of this test is to check primary ignition voltage and ignition coil 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. Plug a 10:1 Attenuator into Channel A on the PicoScope.
  2. Connect the low amp current clamp to Channel B on the PicoScope. Select the 20 A range, switch on and zero the clamp. Observe the current directional arrow on the clamp, incorrect direction will invert your waveform.
  3. Connect both channels to the vehicle ignition coil negative.
  4. Minimise the help screen with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  5. Start the scope. 
  6. Run the engine at idle speed to capture your waveform.
  7. With your live waveform on screen stop the scope.
  8. Turn off the engine.
  9. Use the Waveform Buffer and Zoom tools to examine your waveform.

Waveform notes

In the Non-current limiting waveform example Channel B shows a rise in primary current taking just over 5 ms and reaching around 5.5 amps maximum.

Channel A shows a primary voltage with good characteristics reaching a peak voltage which is over 300 volts. 

Your waveform may show a Current limiting system. In this waveform example on Channel B you can clearly see the peak current is limited by the ignition system. You will also see the current build time is much shorter.

Channel A once again shows a good primary voltage. In this waveform you will also see the characteristic jump in voltage at the point the system limits the current rise.

Note; values are system specific.

Waveform Library

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

Example ignition coil

Further guidance

Technical Information

The primary ignition is so called as it forms the first part of the ignition circuit. Through the ignition coil, it drives the secondary High Tension (HT) output. The primary circuit has evolved from the basic contact breaker points and condenser to the distributorless and coil-per-cylinder systems in common use today. All of these ignition systems rely on the magnetic induction principle.

Magnetic Induction

This principle starts with a magnetic field being produced, as the coil's earth circuit is completed by either the contacts or the amplifier providing the coil negative terminal with a path to earth. When this circuit is complete, a magnetic field is produced and builds until the coil becomes magnetically saturated. At the predetermined point of ignition the coil earth is removed and the magnetic field collapses. As the field inside the coil 250 to 350 primary windings collapses it induces a voltage of 150 to 350 volts.

The induced voltage is determined by,

  • The number of turns in the primary winding.
  • The strength of the magnetic flux, which is proportional to the current in the primary circuit.
  • The rate of collapse, which is determined by the speed of switching of the earth path.

Dwell Period

Dwell is measured as an angle, with contact ignition, this is determined by the points gap. The definition of contact ignition dwell is the number of degrees of distributor rotation with the contacts closed.

As an example, a 4 cylinder engine has a dwell of approximately 45 degrees, which is 50% of one cylinder's complete primary cycle. The dwell period on an engine with electronic ignition is controlled by the current limiting circuit within the amplifier or control unit.

The dwell angle on a constant energy system expands as the engine speed increases, to compensate for a shorter period of rotation and maximise the strength of the magnetic field. The term constant energy refers to the available voltage produced by the coil. This remains constant regardless of engine speed, unlike contact ignition where an increase in engine speed means the contacts are closed for a shorter time and gives the coil less time to saturate .

The induced voltage on a variable dwell system remains constant regardless of engine speed, while it reduces on contact systems. This induced voltage can be seen on a primary waveform.

Both dwell periods 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 on the induced voltage line the coil saturation time can be measured. This will remain exactly the same regardless of engine speed.

Within the primary voltage waveform there are several sections that need closer examination. In the waveform shown the horizontal voltage line at the centre of the oscilloscope begins fairly constant at about 40 volts, but then drops sharply into what is referred to as the coil oscillation. The coil oscillation period should display at least 4 peaks counting both upper and lower. This can also be seen in Figure 2.

There is no current in the coil primary circuit until the dwell period Figure 3, which is when the coil is earthed and the measured voltage drops to zero. The dwell period is controlled by the ignition amplifier, and the length of the dwell is determined by the time it takes to build up approximately 8 amps. When this predetermined current has been reached the amplifier stops increasing the primary current and maintains it until the earth is removed from the coil at the precise moment of ignition.

The vertical line at the centre of the trace, called the induced voltage, is above 200 volts. The induced voltage is called magnetic induction. At the point of ignition, the coil earth circuit is removed and the magnetic field or flux collapses across the windings. This in turn induces an average voltage between 150 to 350 volts Figure 4.


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

  • 10:1 Attenuator


  • Premium 6-way breakout lead set


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


  • Back-pinning Probe Set


  • Small Crocodile/Gator Clips


  • Flexible Back-pinning Probe


  • PicoScope Battery Clip


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Guided test: Primary Voltage vs Current