Alternator AC ripple/diode test – with 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.

How to perform the test

View connection guidance notes.

  1. Connect PicoScope channel A to the alternator B+ terminal and earth.
  2. Run the engine and turn on electrical systems, lights heaters etc.
  3. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  4. Start the scope to see live data.
  5. With your live waveform on screen stop the scope.
  6. Turn off the electrical systems and the engine.
  7. Use the Waveform Buffer, Zoom and Measuring tools to examine your waveform.

Example waveform

Fidure 2

Voltage ripple measured at the alternator’s main positive terminal.

Waveform notes

Refer to the vehicle’s technical data for specific test conditions and results.

Typical values with engine idling:

  1. Voltage measured at the alternator’s main positive terminal (AC-coupled) centered on 0 volts
  2. Characteristic voltage ripple created during the rectification of AC to DC voltage
  3. Signal ruler indicating the upper level of the uniform voltage ripple
  4. Signal ruler indicating the lower level of the uniform voltage ripple
  5. The ruler legend recording the numerical voltage values relative to the position of the signal rulers
  6. Optional voltage scale (+- 500 mV) required to improve the resolution of the ripple on intelligent charging systems.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, Alternator AC ripple / Diode Test.

Further guidance

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.
  1. Voltage measured across vehicle battery (AC-coupled)
  2. Voltage measured at Alternator main battery terminal (AC-coupled)
  3. Voltage spikes exceeding 1 V (peak to peak) indicating diode or phase winding failure
  4. Sequential loss of alternator output as a result of either rectifier or phase winding failure
  5. Correct uniform output from the alternator due to good phase windings or functioning proportion of the rectifier/diode pack

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.

Suitable accessories

  • Large Dolphin/Gator Clips


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Guided test: Alternator AC ripple/diode test - with PCM control