How do you test an alternator

Have the alternator and regulator checked

Testing the alternator: this is how professionals test and measure

The car's alternator (LiMa) normally provides the required direct current and charges the car battery at the same time. If that doesn't happen, the car's power supply will break down sooner or later. That is why the function of the alternator is monitored from the cockpit via the charge control lamp (LKL). When the ignition is switched on and the engine has not yet been started, the display lights up because all consumers are only supplied with energy via the battery. If the engine is started, the charge control lamp goes out again because the power, i.e. the supply and charge current, now comes directly from the alternator.

If the charge indicator light continues to glow, glow or flicker while driving, you should immediately schedule an appointment in the workshop to have the alternator checked. Find out here how an alternator can be tested and find reliable specialist workshops for testing and repairs in your area. At the same time, you can compare the costs of repairing the defective alternator and make an appointment online with the workshop of your choice.

Checking the alternator is a professional job

Various defects can be considered as the cause of the defect in the alternator. Only by carefully and systematically testing and measuring the alternator can you find out exactly where the fault is to be found. These systematic work steps, the necessary specialist knowledge and the measuring devices are convincing reasons to have the alternator checked quickly and safely in the workshop. The workshop also offers the best conditions in the event that the alternator has to be removed or replaced. But before the alternator is really changed, the fault must first be found. It may not be necessary to change, but the damage can also be repaired this way.

It all starts with a visual inspection of the alternator

The inspection by the expert begins with a comprehensive visual inspection of the alternator and all components such as belt drive, alternator regulator and wiring. The specialist examines all cables, ground straps and connections on both the alternator and the car's battery. The tension of the V-belt is also examined. Its play should not be more than two centimeters at the longest free space. Testing the battery is also part of the diagnosis because the alternator and car battery are always viewed as one system. The expert also pays attention to unusual noises from the alternator during troubleshooting. Whistling or a regular clacking indicates defective bearings or a defective freewheel.

Testing the alternator without a meter

If troubleshooting during the visual inspection of the alternator is unsuccessful, the experts test the DC alternator with regard to frequently occurring defects. Here, the damage profile gives the specialist important clues. The check includes, for example, the charge control lamp itself, any interrupted cables or corroded connections or a defect in the generator's regulator. If the charge indicator lamp lights up or glows while driving, a short circuit to ground could be the problem.

Testing the alternator with a measuring device

If the problem is not found during the optical and mechanical test, the expert will pick up his measuring device. This will measure all components of the alternator.


Check the alternator's direct current

The use of the multimeter begins with measuring the charging voltage that is emitted by the alternator. The charging current of the alternator is measured once while idling and once at a speed of 4,000 revolutions per minute. If no DC voltage is measured in one of the two test measurements, the alternator is defective.

Is the alternator or regulator defective? Testing the voltage difference

If everything is okay at this point, the specialist measures the voltage difference between the alternator and the battery. These values ​​also allow conclusions to be drawn about the cause of the error. The alternator has between 13.8 volts and 14.4 volts. If the voltage arriving at the battery is more than one volt lower when the loads are switched on, the fault is most likely to be found in the cabling. If, on the other hand, the voltage on the alternator is significantly higher, the problem is more with the alternator regulator. The alternator regulator - regulator for short - regulates the charging current and thus protects against excessive voltages in the on-board network.

Measuring the alternator with special equipment

If the defect in the alternator still does not show up after all these tests and efforts, the workshop uses an oscilloscope and clamp meter to measure individual special currents on the car. An oscilloscope visualizes the course of the measured values. The focus is on the charging current, the so-called quiescent current and the excitation current. The person skilled in the art can also see on the oscilloscope whether the so-called harmonic ripple is within the ideal range. This tells you whether the car's voltage regulator is working as it should. Every mistake should have been discovered by now at the latest.

Advantages when testing in the workshop

The work steps involved in checking and testing the alternator, the necessary know-how and special equipment clearly show that the test can only be carried out reliably in a specialist workshop. Also, only the workshop offers the best conditions for removing and changing the alternator. If it should be necessary to change the alternator, you can have the costs of the change calculated on FairGarage. The workshop offers can be compared online without obligation and it is also possible to make an appointment with the workshop of your choice with just a few clicks.