Primary ignition vs crankshaft position

The purpose of this test is to monitor the output voltage from the Crank Angle Sensor and the ignition's primary trace to identify the cause of any impending misfires at higher engine revs.

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. Use manufacturer data to identify the output/signal circuit for the crank sensor.
  4. Connect PicoScope channel B to the crank sensor output/signal circuit.
  5. Start the engine and allow it to idle.
  6. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  7. Start the scope to see live data.
  8. Increase engine RPM until high RPM misfire occurs.
  9. With your live waveforms on screen stop the scope.
  10. Stop the engine and turn off the ignition.
  11. Use the Waveform Buffer and Zoom tools to examine your waveform.

Example waveform

Waveform notes

In this waveform we can observe the output voltage from the Crank Angle Sensor (shown in red) at the same time as monitoring the Ignition's Primary trace (shown in blue). The main reason for evaluating these two waveforms together is to identify the cause of any impending misfires at higher engine revs.

The picture shows the 'missing tooth' reference point and the primary induced voltage. The offset between these two points will vary between different vehicle manufacturers as the 'missing tooth' is not always in the same position.

As the engine speed is increased, the distance between the reference point and the induced voltage alters due to the engine's ignition timing advance. The gap in the red trace is due to the 'missing tooth' in the flywheel or reluctor and is used as a reference for the Electronic Control Module (ECM) to ascertain the engine's position. Some systems use one reference point per revolution while others use two. The trace from the CAS should maintain a constant voltage at a given engine speed, while the primary ignition trace shows the firing of the ignition circuit.

Should the engine start to misfire at speed, ensure that the CAS signal is not breaking down: this may be seen as an intermittent trace or as a reduction in amplitude. If the CAS output remains constant, the primary picture may falter: this could be due to either a fault coil or amplifier.

Waveform Library

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

Example inductive sensor

Example ignition coil

Example ignition amplifier

Further guidance

Should the engine start to misfire at speed, ensure that the crank sensor signal is not breaking down: this may be seen as an intermittent trace or as a reduction in amplitude. If the crank sensor output remains constant, the primary picture may falter: this could be due to either a faulty coil or ignition amplifier.

 

Please see individual waveform topics:

  • Ignition primary
  • Crankshaft position sensors (inductive)

GT390-1

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

  • 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: Crankshaft vs Primary Ignition