Alternator - current and voltage at idle - 12 V

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

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control unit.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

  1. Connect PicoScope Channel A to the vehicle battery terminals.
  2. Connect the high amp current clamp to PicoScope Channel B, switch on and zero the clamp.
  3. Connect the clamp onto the battery positive lead, observe the current directional arrow, if incorrect the waveform will be inverted.
  4. Start the engine and run at idle.
  5. Switch on electrical auxiliaries (headlights, heaters, etc.).
  6. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  7. Start the scope to see live data.
  8. With your live waveform on screen stop the scope.
  9. Turn off the engine.
  10. Use the Waveform Buffer and Zoom tools to examine your waveform.

Waveform notes

These known good waveforms have the following characteristics:

A stable battery voltage (Channel A) around 14.5 V, with no spikes or significant ripples.

An AC waveform (Channel B) with a DC average around 48 A and even peaks and troughs.

Waveform Library

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

Further guidance

The function of the alternator is to provide energy to power any electrical systems operating when the engine is running. Furthermore, the alternator must have sufficient output to replace charge within the battery after cranking and engine start.

Increases in alternator output have a number of adverse effects on vehicle operation:

  • The requirement for a stronger magnetic field places more resistive torque on the engine, a drain on engine output which leads to an increase in fuel consumption and an inevitable increase in emissions. This has led to the development of smart alternator systems (please see our guided test for smart alternators).
  • Heat becomes an issue for the alternator, with manufacturers using ducted air cooling or water-cooled alternators to regulate their temperature.

An alternator with a faulty control regulator can increase voltage proportional to RPM, causing damage to the battery and to connected electrical systems.

Symptoms / causes of a faulty alternator;


  • Noise and vibration from dry or damaged bearings due to dirt ingress or heat
  • Loss of output from loss of electrical contact between brushes and slip rings due to frictional wear.


  • Reduced or loss of output due to heat / vibration effects on windings and rectifier
  • Excessive output leading to component and battery damage due to faulty control unit, again caused by heat or vibration.


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

  • 200 A / 2000 A (high amps) DC current clamp


  • PicoScope Battery Clip


  • Large Dolphin/Gator Clips


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


  • Premium Test Lead: BNC to 4 mm, 3 m


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Guided test: Alternator - current and voltage at idle - 12 V