Primary ignition (using the 10:1 attenuator)

The purpose of this test is to check primary ignition voltage on an external coil based ignition system. Looking at coil oscillation, burn time, dwell period and peak voltage.

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.

Note: This helpfile refers to a 10:1 attenuator. If you are using a 20:1 attenuator please adjust the Probe settings for the relevant channel. These settings can be found under the Channel Options button, then: Probe > 20:1 Attenuator.

How to perform the test

  1. Connect a 10:1 attenuator to channel A of PicoScope.
  2. Connect channel A to the negative terminal on the ignition coil and to earth.
  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.
  7. Stop the engine, turn off the ignition.
  8. Use the Waveform Buffer and Zoom tools to examine your waveform.

Waveform notes

The ignition primary waveform is measuring the negative side of the ignition coil. The earth path of the coil can produce over 350 volts.

Within the primary picture 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 oscillations. The coil oscillation period should display at least 4 peaks counting both upper and lower. 

There is no current in the coil's primary circuit until the dwell period, 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 reach the requisite 5-10 amps, depending on system. 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. This is 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 produced by magnetic induction. At the point of ignition, the coil earth circuit is removed and the magnetic field or flux collapses across the coil primary windings. This induces an average voltage between 150 to 350 volts. 

Waveform Library

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

Example ignition coil

Further guidance

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's earth is removed and the magnetic field collapses. As the field inside the coil's 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 module.

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.

GT386-2

Disclaimer
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

    £64.00

  • Premium 6-way breakout lead set

    £269.00

  • Back-pinning Probe Set

    £40.00

  • Flexible Back-pinning Probe

    £3.00

  • Large Dolphin/Gator Clips

    £10.00

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Guided test: Primary Voltage