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Distributor Inductive Pick-up (Engine Cranking)

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Distributor Inductive Pick-up (Engine Cranking) Waveform Notes

This particular type of pick-up generates its own signal and therefore does not require a voltage supply to power it. Recognisable by its two electrical connections, the pick-up is used as a signal to trigger the ignition amplifier or Electronic Control Module (ECM).

This type of pick-up could be described as a small alternator because the output voltage rises as the metal rotor approaches the winding, sharply dropping through zero volts as the two components are aligned and producing a voltage in the opposite phase as the rotor passes. The waveform is known as a sinewave.

The cranking voltage produced by the pick-up will be determined by several factors, these being:

  • Cranking speed - the voltage produced will be approximately 2 to 3 volts when cranking at a speed of around 250 RPM
  • The proximity of the metal rotor to the pick-up winding. An average air gap will be in the order of 8 to 14 thou, a larger air gap will reduce the strength of the magnetic field seen by the winding and the output voltage will be subsequently reduced.
  • The strength of the magnetic field offered by the magnet. The strength of this magnetic field determines the effect it has as it ‘cuts’ through the windings and the output voltage will be reduced accordingly.

A difference between the positive and the negative voltages may also be apparent as the negative side of the sinewave is sometimes attenuated when connected to the amplifier circuit, but will produce perfect AC when disconnected and tested under cranking conditions. The majority of pick-ups will produce approximately 3 volts peak to peak and this figure is widely accepted to be the minimum requirement to trigger the amplifier / ECM, this voltage will also however be manufacturer and model specific and may differ slightly.