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Alternator test with an oscilloscope – voltage ripple diagnostics

11.07.2024

Alternator test with an oscilloscope – voltage ripple diagnostics

Vehicle electronics diagnostics are becoming increasingly complex as electronic systems in modern cars become more complex. Undoubtedly, the alternator, which is a key component of a vehicle’s electrical system, plays an important role in maintaining proper voltage levels and charging the battery. In this article, we will discuss a method for diagnosing alternator voltage ripple using an oscilloscope, which will help detect potential problems and prevent serious failures.

What is an alternator?

An alternator is a device that converts mechanical energy into electrical energy, generating three-phase alternating current (AC), which is then converted to direct current (DC) to charge the battery and power the vehicle’s electrical systems. Current rectification is carried out using a 6-diode rectifier bridge, which can be found in the diagram below.

The charging current of a given alternator depends on its condition and rpm as well as the rpm that the engine gives.

There are two main types of alternators found in vehicles:

  1. Alternators controlled by ECU (Engine Control Unit). : These are integrated into the vehicle’s engine management system, allowing for more precise control of alternator voltage and performance. The ECU monitors various vehicle parameters and adjusts alternator operation accordingly.
  2. Non-controlled alternators: These traditional alternators operate autonomously, regulating the output voltage using a built-in regulator. Although less advanced than ECU-controlled alternators, they are still widely found in many vehicles.

Why is voltage ripple diagnostics important?

If we use a multimeter to measure, we will be able to detect a problem with the alternator as long as the charging voltage is abnormal. In some cases, the average alternator voltage remains normal, and we can only find abnormalities by evaluating the alternator voltage waveform.

Excessive voltage ripple, or alternator output voltage pulsation, can be a symptom of damage to rectifier diodes, voltage regulator or other alternator components. These problems can result in unstable voltage, which negatively affects the operation of the battery, as well as other components of the electrical system.

Regular diagnosis of voltage ripples with an oscilloscope allows early detection and repair of these faults, preventing more serious failures. An alternator operating with a defective diode or a short circuit within the windings may overheat. In some cases, this can increase the risk of fire.

Procedure for testing the alternator with an oscilloscope

Connecting an oscilloscope

  • Connect the oscilloscope probe to the alternator output, to the output voltage terminal or to the battery terminals. Attention should be paid to safety during measurements, due to the drive components in motion. The measurement is taken with the engine running and at slow speed. For greater readability of the waveforms, it is a good idea to increase the current draw, for example, by turning on the lights or the fan blower.
Scope DT is a great oscilloscope that allows you to measure signals of car components. Includes help with model oscillograms for many off-components.

Oscilloscope configuration:

  • Channel Configuration. The channel input should operate in AC mode, that is, variable component registration.
  • Sensitivity settings. The sensitivity configuration of the oscilloscope depends on the expected voltage level. Typically, we set the oscilloscope to a sensitivity within approx. 50 mV per plot.
  • Time base selection. To accurately analyze the signal, the time base should be approx. 2 ms/plot (about 20 ms of waveform visible simultaneously on the screen).

Interpretation of results

After starting the engine, the alternator output voltage waveform will appear on the screen. Regular ripples of no small amplitude indicate normal alternator operation, while excessive fluctuations, especially in the form of repetitive patterns, may suggest problems. In the following section, we will show the shape of the waveforms of correct and on typical alternator problems.

The correct course can be superimposed on interference, which in most cases comes from other working electrical components like solenoid valves, ignition coils and injectors. Therefore, these types of signals should not negatively affect the evaluation of the course. The amplitude of the ripple shown below is approx. 50 mV.

1: Correct alternator ripple waveform free of interference. 2-4: Correct signals with superimposed interference from other components of the vehicle installation.
1: Correct alternator ripple waveform free of interference. 2-4: Correct signals with superimposed interference from other components of the vehicle installation.

Malfunctioning alternator

Depending on where the fault occurs and the nature of the fault, it will affect the shape of the mileage. Examples of the pulsation waveforms that a faulty alternator can generate are shown below. The amplitude of these waveforms is greater, from 100 to even several hundred mV.

1: A break in the rectifier diode. 2: Short circuit in the rectifier diode. 3: Short circuit in the excitation diode. 4. Short circuit between stator windings.
1: A break in the rectifier diode. 2: Short circuit in the rectifier diode. 3: Short circuit in the excitation diode. 4. Short circuit between stator windings.

An alternative to the oscilloscope

If you do not have an oscilloscope or have not yet mastered the skills to use it, there is also another method to diagnose the alternator, as well as the battery and starter. That way is the DBT-12+ advanced battery tester. In addition to quickly and safely checking the battery, the device is capable of recording the waveforms of the running starter and alternator voltage ripple. We will get the same data as performing an alternator test with an oscilloscope. It is noteworthy that the measurement is carried out fully automatically while carrying out the so-called ” measurement “. Installation test, that is, the test of the starter and charging circuit preceded by a battery test. The mechanic does not need to perform additional steps, and the mileage of interest can be seen in the computer program and on printed reports.

The DBT-12+ tester generates waveforms useful for analyzing the performance of a charging system by replacing an oscilloscope. We will also diagnose the car's starter.

Summary

In conclusion, the alternator voltage ripple test using an oscilloscope is an effective method for detecting faults in a vehicle’s electrical system. Regular examination of the alternator allows for early detection and the possibility of repairing potential problems, which prevents more serious failures and ensures long-term, reliable battery performance. We encourage you to regularly evaluate voltage pulsation waveforms with an oscilloscope or an advanced tester. The measurement takes minutes, and can greatly speed up diagnosis.

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