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 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-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • 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.
  5. With your live waveform on screen select STOP or press the space bar to stop your capture.
  6. Turn off the engine.
  7. Use the WAVEFORM BUFFER and ZOOM tools to examine your waveform.​


You can use the Scale Rulers located at the top left corner of the voltage scale to measure the peak to peak voltage of the waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

This ripple shows even peaks indicating even rectified output from the three alternator phases. The peak to peak voltage is around 106.9 mV.

There are no significant uneven downward spikes which would indicate diode faults. 

Waveform Library

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

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, above 500 mV and diode spikes 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.
  • rough idle or even engine missfire.


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


  • Back-pinning Probe Set


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


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Guided test: Alternator AC ripple/diode test - without PCM control