Products suited to this guided test*
  • Back-pinning Probe Set

  • Red Sprung Hook Probe

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

  • *At Pico we are always looking to improve our products. The tools used in this guided test may have been superseded and the products above are our latest versions used to diagnose the fault documented in this case study.

Alternator phase wires

How to perform the test

Connect BNC test lead into Channel A and Channel B of the scope and then connect a small black clip to one the negative ends (black moulded connector). Stack the second BNC cable on to the first. Place the black clip on to the battery negative, as shown in Figure 1.

On the positive cable ends (red moulded connectors), connect a Back-pinning Probe. Locate the voltage regulator multiplug and back-probe two of the three phase wires. In our example, the regulator is mounted on the seat frame and the phase wires are all coloured yellow. Connect as Figure 2.

With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings.

Example waveform

Waveform notes

The example waveform illustrates the three phases before rectification. Each of the three phases can be seen switching between -4 volts and +12 volts giving a peak to peak voltage of 16 volts.

This waveform:

  • shows that the output is correct and that there is no fault within two of the three phase windings.
  • confirms that the voltages from two of the three phases are equal.

Connect the oscilloscope to the other phase to observe the other output voltage.

The alternator's three phases are wound at 120 intervals, enabling it to produce a high enough current output while still retaining a compact size. The picture shows the phases before rectification, where the alternating current (AC) is converted through a series of diodes into the direct current (DC) required to charge the battery.

With a four-channel scope, it is possible to use a third channel for the third phase, allowing all three phases to be viewed at the same time.

Technical information


  1. Rotor
  2. Stator
  3. Alternator Case
  4. Regulator/rectifier
  5. Ignition switch
  6. Battery

Faulty waveforms - troubleshooting

Ensure that the three phases are present and that they all have similar amplitudes.
If one of the phases is missing, it may have an open circuit that can be tested for continuity using a multimeter.

Pin data

See technical data.


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.

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Guided test: Alternator Phase Wires