Distributor pick-up - inductive

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

The connection will vary depending on the distributor set up. This may either have the amplifier mounted directly onto the body of the distributor or be remotely mounted in various locations.

If the amplifier is remotely mounted:

Plug a BNC test lead into Channel A on the PicoScope, place a Back-pinning Probe on the test lead with the black moulding (negative) and a Back-pinning Probe onto the test lead with the coloured moulding (positive). Back probe the two wire multiplug on the inductive pick-up fly lead coming out of the distributor's body, as illustrated in Figure 1.

If the amplifier is mounted on the distributor body:

Remove the amplifier attach two small fly leads to the pick-up's output connections, and refit the amplifier (if you wish to test the pick-up whilst the engine is running).
Plug a BNC test lead into Channel A on the PicoScope, place a small black clip on the test lead with the black moulding (negative) and a small red clip onto the test lead with the red moulding (positive). Attach the two clips to the two short fly leads.

As you will see in the preset scope picture and the example on this page the waveform has been stabilised by using a falling trigger.

Example waveform

distributor pick-up inductive waveform

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).

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 from as low as 2 to 3 volts when cranking to over 50 volts, at higher engine speeds.
  • 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.

Technical information

This particular type of pick-up generates it's 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 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.
This particular waveform is known as a sinusoidal, or sinewave.

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

  • Engine speed - the voltage produced will rise from as low as 2 to 3 volts when cranking to over 50 volts, at high engine speed.
  • 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.

There are two types of permanent magnetic pick-ups these are called Annular and Limb. The distinguishing factor between the two types is that the limb pickup has only one reluctor point where as the annular will have a reluctor point per cylinder.

The metal rotor (pick-up teeth) will invariably be mounted on the distributor shaft and driven from the camshaft. They can be tested in several ways using either a multimeter or an oscilloscope.

Figure 2 shows a diagram of an annular inductive type distributor pick-up.

AT019-2

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

  • Back-pinning Probe Set

    £48.00

  • Flexible Back-pinning Probe

    £7.00

  • Small Crocodile/Gator Clips

    £5.00

  • Premium Test Lead: BNC to 4 mm, 3 m

    £48.00

  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)

    £179.00

Share your experience

If you have any suggestions to improve this guided test please do so using the 'add comment' button.

2 comments | Add comment

Steve Smith
October 21 2016

Good catch on the text, we’ve made that adjustment. Sadly we do not have the specific details of the vehicle used within this Guided Test.

John Grady
March 28 2016

just a correction..the pickup is not “invariably mounted on the distributor shaft”..the spinning metal points are mounted there,  called a reluctor.

The pickup is mounted stationary within the distributor.

Thank you for the voltage readings ; what pickup this is,  would help the information..

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Guided test: Inductive Pick-up