Alternator AC ripple/diode test

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

Plug a BNC test lead into Channel A on the PicoScope, place a large black clip on the test lead with the black moulding (negative) and a large red clip onto the test lead with the coloured moulding (positive).

Position the clips onto the battery ensuring the correct polarity: red to positive (+) and black to negative (-) as shown in Figure 1.

As you will see in the preset scope picture and the example waveform on this page it is essential that the voltage range is set to alternating current (AC) for this test.

Example waveform

Waveform notes

The example waveform illustrates the rectified output from the alternator.

This waveform shows that:

  • The output is correct and that there is no fault within the phase windings or the diodes (rectifier pack).
  • The three phases from the alternator have been rectified to direct current (DC) from the original alternating current (AC) and that the three phases that contribute towards the alternator's output are all functioning.

If the alternator was suffering from a diode fault, long downward tails will appear on the trace at regular intervals and 33% of the total current output will be lost. A fault within one of the three phases will show a similar picture to the one illustrated but will be three or four times the height, with the base to peak voltage in excess of 1 volt.

The voltage scale at the side of the oscilloscope is not representative of the charging voltage, but is representative of the upper and lower limits of the DC ripple. The amplitude of the waveform will vary under different conditions, with a fully charged battery showing a flatter picture, while a discharged battery will show a greater amplitude until the battery is charged.

Technical information

The objective of the charging circuit is to provide a regulated voltage to charge the battery and replenish the current consumed by the vehicle's electrical circuits. The alternator is a fairly recent addition to the motor vehicle, replacing the dynamo which was fitted until the 1970s.

The output from a dynamo was determined by engine speed and, unlike the alternator, it had negligible output when the engine was at idle. It was not unknown for the charging warning light to flicker at idle and for the dynamo to require frequent brush changes. These brushes were considerably larger than those found on alternator as they carried the total current output, unlike alternator brushes that carry only the field current. This current energises the electromagnet to produce the output.

The field current is approximately six to eight amps.

The rating of the alternator tends to be vehicle-specific, as a base model has less electrical demand than a vehicle with typical top-of-the-range accessories such as electric front and rear heated screens, heated mirrors, additional lighting, heated and electrical adjusted seats.

The alternator output, as the name implies, produces an alternating current (AC) output, which is rectified to direct current (DC) to provide the correct type of voltage to replenish the battery, keeping it at full charge.

The alternator has three internal windings wound 120 degrees between phases and requires nine diodes in a 'bridge' configuration to rectify the output. The voltage is controlled by a solid-state regulator that maintains the voltage at a predetermined setting of about 13.5 to 15 volts. The output current is determined by the requirement at the time. For example, a battery that has just been subject to prolonged cranking will see a higher output from the alternator than if the battery were fully charged.

The regulated voltage can be measured on a multimeter, but this reading can appear correct even if the alternator has a diode fault that reduces the output by 33%. The only true way to monitor the alternator output is to observe the output waveform on an oscilloscope.

Ford Focus: 'Smart' Alternator

The charging system used on the Ford Focus is unlike any other charging system currently in production.

Ford uses what is termed a 'smart charge' system. With a conventional charging system, the battery is charged at a voltage that is determined by the voltage regulator, with all the electrical load being drawn from the alternator-fed battery.

Smart charging enables the voltage supply from the alternator to vary depending on the temperature of the battery's electrolyte. A cold battery responds better to a higher voltage than a hot battery, which responds better to a slightly lower voltage. The temperature of the electrolyte is calculated by monitoring the air intake temperature when the engine was last stopped and the current intake air temperature. From these two measurements, the battery's temperature can be calculated and the appropriate charge sent to the battery.

The alternator has two connections to the Engine Management Module (ECM), to monitor and control the output. This monitoring also allows the Idle Speed Control Valve (ISCV) to be operated when high electrical demands are made when the engine is at idle. The ECM also controls the engine run relay, which only allows circuits with a high current demand to be activated when the alternator is charging. Until then, the components remain inactive.

The ECM is now responsible for switching off the dashboard-mounted 'charging light'. When starting the engine with a conventional alternator, the unit is activated as soon as the ignition is switched on, but a 'smart charging' system will only initiate the alternator once the engine has started. This action avoids an unnecessary waste of voltage on a vehicle with a discharged battery and also avoids the extra effort involved in cranking an engine with an operational 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

  • PicoScope Battery Clip


  • Large Dolphin/Gator Clips


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


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


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Guided test: Alternator AC Ripple/Diode Test