Products suited to this guided test*

Alternator AC ripple – with ECM control

The purpose of this test is to view the rectification of the alternator output voltage, where the alternator output is 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:

  • The (AC coupled) alternator ripple waveform is centred around 0 V, as the DC component (which may range between 12.6 to 18 V) is removed.
  • Low ripple with low electrical loads (peak to peak voltages < 100 mV) and vice versa.
  • No significant downward spikes, noise, or missing peaks (dropouts) in the ripple.

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. Accurate control of alternator output and charging is essential to the operation of Start-Stop systems, which place increased demands on starter batteries.

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.

Unlike voltage regulator systems, in which alternator output depends on electrical load, ECM controlled alternator outputs depend on a variety of additional parameters, including the battery’s temperature and estimated states of health and charge.

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 can vary with engine speed, electrical load, battery condition and time since cranking. 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.

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 dashboard instrumentation.
  • Loss of Start-Stop functionality.

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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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.

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