Alternator AC ripple/diode test – without PCM control

The purpose of this test is to check the regulation and rectification of the alternator output voltage.

How to perform the test

View connection guidance notes.

  1. Connect PicoScope Channel A to the alternator B+ terminal.
  2. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform..
  3. Start the engine and allow it to idle.
  4. Start the scope to see live data.
  5. With your live waveform on screen stop the scope.
  6. Turn off the engine.
  7. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

Regular peaks and troughs, with peak to peak voltages around 100 mV.

No significant uneven downward spikes. 

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

The alternator’s function is to generate and supply electrical energy to the vehicle’s on-board electrical systems and to replace battery charge consumed during engine cranking and start-up.

The alternator converts mechanical energy (rotation) to electrical energy by causing a magnetic field to rotate within a fixed set of windings. The changing magnetic field induces AC voltages within the windings, which the alternator’s circuitry, consisting of diodes and a solid-state voltage regulator, rectifies to produce a regulated DC voltage output.

For an accurate and reliable signal always connect at the alternator B+ terminal: it is convenient to measure the ripple directly at the battery positive terminal; however the battery can dampen the waveform such that problems can be missed.

With regard to the alternator ripple waveform, uneven peaks show a problem with the windings or rectification diodes. Faults of this nature may not be noticed directly by the vehicle operator as the alternator may lose only a small percentage of its output.  Sharp spikes, usually downward, indicate diode failure and the presence of unrectified AC voltage in the circuitry.

The even waveform shape must be maintained throughout all electrical loads and engine speeds. Turning on electrical consumers and increasing the engine speed will increase the alternator load which can provoke faults that are not evident at low loads. If the peak to peak output voltages are above 500 mV, the offending voltage spikes may disrupt other electrical systems, in particular those systems dependent on an AC signal.

Typical complaints would be:

  • erratic or malfunctioning dash board instruments.
  • rough idle or even engine misfire.

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

  • Multimeter Probes


  • 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