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.

Connection guidance

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.

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.

In modern charging systems that utilize PCM control, the formation of the ripple and the values obtained are no longer directly proportional to the electrical load on the vehicle’s battery. The PCM will decide when the charging will commence based on a number of factors, such as battery temperature and engine load, in order to reduce the effect on emissions and fuel consumption. In addition to this intelligent charging control, the introduction of Stop-Start systems has seen an increase in battery capacity, which in turn affects and quenches the alternator voltage ripple.

It may, therefore, be beneficial to measure directly at the alternator’s main positive terminal to obtain sufficient voltage ripple to be able to diagnose the alternator rectifier pack. You can remove the surface charge from the vehicle’s battery by keeping the engine switched off and switching on the headlights for 30 seconds. This will help stimulate battery charging when the engine is started. Once you have started the engine, continue to load the vehicle battery by switching on additional loads such as heated screens and the heater blower.

How to perform the test


  1. Connect a BNC test lead to Channel A on your scope.
  2. Install the clips on your test leads and connect them to the battery terminals: BLACK to the negative terminal, RED to the positive terminal.
  3. Start the engine
  4. Start capturing in PicoScope Automotive by clicking the Go button in PicoScope or pressing the space bar on your keyboard.

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.

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.

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.

Failed rectifier/diode pack or phase winding

  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