Alternator AC ripple – without ECM control

The purpose of this test is to check the rectification of the alternator output voltage, where the alternator output is not regulated by the Engine Control Module (ECM).

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. Switch on electrical consumers and increase engine RPM whilst observing your waveform.
  6. With your waveform on screen stop the scope.
  7. Turn off the engine.
  8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

  • An average voltage centred around 0 V (due to the AC coupling of the scope).
  • A continuous series of ripples having a consistent amplitude.
  • No excessive or uneven downward spikes between ripple pulses.
  • No repetitively missing ripples or anomalies, where there are the same number of good ripples between every occurrence.
  • There may be noise spikes or voltage jumps but their appearance should be random and not regular, after a fixed number of pulses.

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

When the engine is running, an alternator generates electrical energy to supply the vehicle’s on-board electrical systems and replace the battery charge consumed during cranking.

The alternator converts mechanical 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 are rectified by an arrangement of diodes to give a DC output.

The maximum output is limited by a voltage regulator, which varies the alternator output relative to the system voltage; when the system voltage is low, the regulator increases the alternator output, and vice versa.

The rectification of the generated AC current creates a continuous series of voltage pulses, a ripple, within the alternator’s output. Periodically missing pulses or disruptions within the ripple indicate a problem with either the windings or the rectification diodes. Sharp spikes, usually downward, between the pulses indicate diode failure and the presence of unrectified AC voltage in the circuitry.

The alternator output will vary with engine speed and electrical load. However, a consistent ripple must be maintained throughout these variations.

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.

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.

Typical symptoms of faulty alternator would be:

  • Battery warning light illumination.
  • Malfunction Indicator Lamp (MIL) illumination.
  • Diagnostic Trouble Codes (DTCs).
  • Rough idle.
  • Possible engine misfire.
  • Loss of battery state of charge or state of health.
  • Erratic or malfunctioning dash board instrumentation.

Alternator, or related, faults that can cause the above symptoms are:

  • Diode faults, caused by heat and vibration or the inclusion of moisture into the circuitry.
  • Short or open circuits, or high resistances, in the stator windings.
  • Poor battery states of health or charge.
  • Short or open circuits, or high resistances, in battery or earth cables and/or connections.
  • Alternator drive mechanism faults, including pulley, belt condition and tension, or freewheel issues.

Diagnostic trouble codes

Selection of component related Diagnostic Trouble Codes (DTCs):








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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 - without ECM control