Alternator AC ripple/diode test – without PCM control

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.

Connection guidance

Connection for diagnostic work will of course vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice;

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from,

  1. Breakout leads.
  2. Back -pining probes.

Testing Sensors and Actuators; (to include relevant circuit/connectors).

  • When testing a sensor, it is desirable to gain access at the control unit.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

  1. Connect PicoScope Channel A to the vehicle.
  2. Minimize the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  3. Start the engine and allow it to idle.
  4. Select GO or press the space bar to see live data.

Example waveform

Waveform notes

The shape of the waveform is a reflection of the three phases of output from the alternator. The ripple must show even peaks with no significant uneven downward spikes. Uneven peaks show a problem with the rectifier diodes. The even shapes must be maintained throughout the RPM range.

Measure the peak to peak voltage, above 500 mv may begin to disrupt other electrical systems, in particular any system using an AC signal.

Waveform Library

The Waveform Library has 23 examples of  alternator ripple.

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select 'Alternator ripple/diode test'.

Further guidance

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)


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5 comments | Add comment

January 09 2018

Bernard: You can add the scans to our Waveform Library. This can be accessed via the “File” menu within PicoScope Automotive. For more information on access to the Waveform Library please see:

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.

Bernhard List
May 31 2017

I have scans of bad diodes. How can I post them or write to me and I send them to you. Btw. by just sending those scans to the manufacturer I got a new alternator at no charge within a few days

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 - without PCM control