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.
The rectification of the AC voltage generated by the alternator is performed by the alternator rectifier/diode pack. Close analysis of the rectified AC voltage reveals the characteristic ripple present across the vehicle battery while the alternator is charging.
By using the AC coupling feature in PicoScope, you can reject the DC portion of the battery voltage to display the ripple centred on zero volts.
Now, with only the ripple present, you can evaluate the performance of the alternator rectifier/diode pack and the integrity of the alternator phase windings.
Depending on the charging system configuration, it may be difficult to obtain sufficient voltage ripple in order to diagnose the alternator rectifier pack. Vehicles with a long cable that runs between the alternator and battery or with larger heavy duty batteries may all demonstrate minimal ripple. In scenarios like these, you can load the vehicle battery by switching on headlights, heated screens and heater blowers and connect your blue test lead directly to the alternator’s main positive terminal.
With a conventional charging system (with no PCM control) the formation of the ripple and the values obtained are directly proportional to the electrical load placed upon the vehicle’s battery. The output of the alternator will increase to compensate for the electrical load, increasing the amplitude of the ripple which will be useful in the evaluation of the alternator and rectifier pack.
Refer to the vehicle’s technical data for specific test conditions and results.
Typical values with engine idling:
The example waveform illustrates the rectified output from the alternator.
This waveform shows that:
The waveform above demonstrates a typical example of either a failed alternator phase winding or rectifier diode pack. (Either fault will produce near identical waveform)
The noise level obtained at point 1 (across the vehicle battery) may hinder diagnosis given the influence of other operational circuits whilst the engine is running. Moving your blue test lead from the battery positive terminal to the alternator main battery terminal (point 2) will help minimize this noise and reveal a more concise waveform.
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.
We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.
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: https://www.picoauto.com/library/waveform-library
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.
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: https://www.picoauto.com/support/
February 10 2017
I DON’T HAVE THE PATTERN LIKE THE ONE YOU HAVE ON THE DEMO. CAN YOU SHOW ME WHAT THE PATTERN LOOK LIKE IF IT WAS BAD