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Distributor Pick-up - Inductive Diagnostics

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).

As the metal rotor spins, a magnetic field is altered which induces an Alternating Current (AC) voltage from the pick-up. 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 voltage produced by the pick-up will be determined by several factors, these being :

Engine speed --- the voltage produced will rise when cranking

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 attenuate when connected to the amplifier circuit, but will produce perfect AC when disconnected and tested under cranking conditions.
A annular inductive type distributor pick-up.

Distributor Pick-up - Hall Effect Diagnostics.

This form of trigger device is a simple digital on / off switch which is recognised and processed by the ignition control module.

The trigger has a rotating metal disc with openings; this passes between the electromagnet and the semiconductor. The effect of a magnetic field that is able to pass through one of the 'windows' will stop the flow of voltage. When the 'window' is closed the flow is reinstated.

Technical Information

A semiconductor has the ability to be a conductor or an insulator depending on whether the semiconductor 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.

The effect of a magnetic field that is able to pass through one of the 'windows' will stop the flow of voltage. When the 'window' is closed the flow is reinstated. This action will produce a digital square wave that is understood by the ECM or amplifier and will not need the extra Schmitt trigger circuitry to convert the analogue signal into a digital signal. Schmitt triggers are used with permanent magnetic pick-up amplifiers.

This circuit may also be referred to as an Analogue to Digital (A to D) convertor.

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

When the signal is observed or plotted against a primary waveform it can be seen that the coil fires when the voltage from the Hall effect trigger drops to zero volts. This occurs when the 'window' on the metallic rotating vane opens.

A diagram of a Hall effect pick-up, the terminal marked 0 is the Hall voltage.
A Hall effect distributor.