Alternator AC ripple/diode test

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

The purpose of this test is to evaluate the integrity of the alternator phase windings and the performance of the diodes in the Rectifier pack.

Note: The alternator output is dependent upon the charging control circuit, the correct operation of the alternator freewheeling pulley and associated drive belt tension

The test procedure below assumes the conditions mentioned above are all in order and that the alternator phase windings/rectifier pack is functioning correctly.

It is therefore paramount to carry out a basic inspection of all the items above BEFORE measurements are taken to prevent an incorrect diagnosis of the alternator.

How to perform the test



PicoScope settings

  • Channel A: ± 1 V AC
  • Timebase: 2 ms/div
  • Sample rate: 250 Ks
  1. Connect a BNC test lead to Channel A on your scope.
  2. Install the clips on your test leads and connect them to the battery terminals: black to the negative terminal, red to the positive terminal.
  3. Start the engine
  4. Start capturing in PicoScope Automotive by clicking the Go button in PicoScope or pressing the space bar on your keyboard.

Example waveform

Waveform notes

The example waveform illustrates the rectified output from the alternator.

This waveform shows that:

The output is correct and that there is no fault within the phase windings or the diodes (rectifier pack).

The three phases from the alternator have been rectified to direct current (DC) from the original alternating current (AC) and that the three phases that contribute towards the alternator's output are all functioning.

If the alternator was suffering from a diode fault, long downward tails will appear on the trace at regular intervals and 33% of the total current output will be lost. A fault within one of the three phases will show a similar picture to the one illustrated but will be three or four times the height, with the base to peak voltage in excess of 1 volt.

The voltage scale at the side of the oscilloscope is not representative of the charging voltage but is representative of the upper and lower limits of the DC ripple. The amplitude of the waveform will vary under different conditions, with a fully charged battery showing a flatter picture, while a discharged battery will show a greater amplitude until the battery is charged.

Bad waveforms

Technical information

You can read more about alternators in our Online training section under Technical information – The alternator.


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

  • Large Dolphin/Gator Clips


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


Help us improve our tests

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3 comments | Add comment

Maria Johnson
June 23 2017

This seems to be very good and accurate test. Diode test must be performed with the accurate components otherwise it is useless. Thanks for sharing this.

February 13 2017

Hi - If you have PicoScope Automotive open (with your scope connected and an internet connection), look under the File menu you should see the “Waveform Library Browser”, here you can search for both Good and bad waveforms from users around the world.

To access the Waveform Library you will need login information. The same credentials are used to access our automotive forum:

February 10 2017


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Guided test: Alternator AC Ripple/Diode Test