|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 for diagnostic work will 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:
Testing sensors and actuators (to include relevant circuit/connectors):
This example is a known good waveform. The shape of the waveform is a reflection of the three phases of output from the alternator. This ripple shows even peaks with no significant uneven downward spikes.
It is possible and usually much quicker to measure the ripple directly from battery positive. However the battery tends to dampen the signal and problems can be missed. For an accurate and reliable signal always connect at the alternator B+ terminal.
With regard to the alternator ripple waveform, uneven peaks show a problem with the phase coils or rectifier diodes. Faults of this nature may not be noticed directly by the vehicle driver as the alternator may only have lost a percentage of its output.
The even waveform shape must be maintained throughout electrical load and RPM changes. Turning on electrical consumers and increasing the engine RPM will make the alternator work closer to its capacity and therefore show problems that are not clear at a lower output.
Sharp spikes, usually downward indicate diode failure allowing the 'leak' of unrectified AC voltage into the system.
Measure the peak to peak voltage, spikes or anything above 500 mv may begin to disrupt other electrical systems, in particular any system using an AC signal.
Typical complaints would be erratic or malfunctioning dash board instruments.
You can read more about alternators in our Online training section under
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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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