Products suited to this guided test*
  • 200 A / 2000 A (high amps) DC current clamp

  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)

  • Premium Test Leads: Set of four color leads 5 m (TA199 - TA202)

  • *At Pico we are always looking to improve our products. The tools used in this guided test may have been superseded and the products above are our latest versions used to diagnose the fault documented in this case study.

Alternator voltage and current – 24 V

The purpose of this test is to assess the controlled output of the alternator in relation to electrical load on the battery.

How to perform the test

View connection guidance notes.

  1. Connect PicoScope Channel A to the vehicle battery terminals.
  2. Connect the high amp clamp to PicoScope Channel B.
  3. Switch on and zero the high amp clamp before connecting to the battery positive cable.
  4. Start the engine and run at idle.
  5. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  6. Start the scope to see live data.
  7. Switch on electrical auxiliaries (headlights, heaters, etc.) and vary engine RPM whilst observing your waveforms.
  8. With your waveforms on screen stop the scope.
  9. Turn off the engine.
  10. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.


The orientation of the clamp relative to the wire will determine whether it has a positive or negative output. If a live waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.

Example waveform

Waveform notes

These known good waveforms have the following characteristics:

  • A stable battery voltage (Channel A) around 28.3 V, with no spikes or significant ripples.
  • An AC current waveform (Channel B) with uniform peaks and troughs having a DC average around 27 A.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, Alternator voltage.

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 ECM controlled 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.

Alternators with voltage regulators vary their output dependent on the electrical load, whereas ECM controlled alternator outputs depend on a variety of additional parameters, including the battery’s temperature and estimated states of health and charge. The other advantage of ECM control is that the alternator can be deenergised when not required, reducing torque and heat stress within the alternator and engine loading to improve fuel efficiency.

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 states of charge and/or health.
  • Erratic or malfunctioning dashboard instrumentation.
  • Loss of Start-Stop functionality.

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

  • Reduced output from winding or diode faults, caused by heat and vibration or the inclusion of moisture into the circuitry.
  • Faulty voltage regulators (where applicable) causing alternator output to increase with increasing engine speed.
  • 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: Current and voltage at idle - 24 V