Distributor pick-up (Hall effect) - voltage

The purpose of this test is to investigate the operation of a Hall Effect distributor pickup based on the output voltage and frequency during cranking and engine run conditions.

How to perform the test

View connection guidance notes.

  1. Use manufacturer’s data to identify the distributor pick-up circuit.
  2. Connect PicoScope Channel A to the distributor pick-up signal circuit.
  3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  4. Start the scope to see live data.
  5. Crank and, if possible, run the engine at idle.
  6. With your waveform on screen stop the scope.
  7. Turn off the engine.
  8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

  • A cyclic digital waveform switching between two voltages; a low voltage just above 0 V and a high voltage just below 2.5 V.
  • The switching frequency increases with increasing engine speed.
  • There are no drop-outs or anomalies within the waveform.

Further guidance

This form of trigger device is a simple digital 'on/off' switch which produces a digital output that is recognised and processed by the ignition control module. The trigger has a metal disc with openings that rotates between the electromagnet and the semiconductor.

A semiconductor has the ability to be a conductor or an insulator depending on whether it sees or is shielded from the magnetic field. This magnetic field is switched on and off by the rotating disc that travels between the two objects.

A magnetic field that passes through one of the windows stops the flow of current through the semiconductor. When the window is closed, the flow resumes. This action produces a digital square wave that the ECM or amplifier understands without needing any the extra circuitry to convert the analogue signal into a digital one. (Permanent magnetic pick-up amplifiers use a Schmitt trigger, while other types use an Analogue to Digital, or A to D, converter.)

The sensor, because of its convenient output, is used in many other applications including road speed sensors and speedo drives.

The sensor have its characteristic three connections: a live supply at battery voltage, an earth and the output signal.

The square wave, when monitored on an oscilloscope, varies in amplitude, but this is not considered a problem as it is the frequency that is important, not the voltage level. The output can also be measured on a multimeter that has a frequency facility.

When the signal is observed or plotted against a primary waveform, the coil can be seen to fire when the voltage from the Hall effect trigger drops to zero. This occurs when the window in the metallic rotating vane opens.


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


  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • PicoScope Battery Clip


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Guided test: Distributor pick-up (Hall effect) - voltage