How to test an alternator

A person using a multimeter connected to a car battery to check the alternator performance

This image shows the only alternator test that home mechanics can safely do. In this example, a digital multimeter is connected to the battery terminals while the engine is running. The reading shown here reflects an acceptable charging voltage.

All other tests require equipment, skills, knowledge and an alternator wiring diagram, which few home mechanics possess. Moreover, tests to assess the condition of the windings require that the alternator be dismantled.

Moreover, testing the rectifier requires the use of an oscilloscope to create a visible waveform of a charging current. Such a waveform could contain valuable diagnostic data about AC ripple currents in a charging current. Ripple currents are so-called because they appear as “ripples” in a waveform of a charging current.

Diodes work like one-way check valves to block alternating current from flowing back into the windings. Thus, one or more defective diodes allow harmful alternating current to enter the vehicle’s electrical system. This can damage electronic control modules and/or other sensitive electronic components.

As a practical matter, these kinds of tests are best performed by specialist mechanics. They have the required equipment, as well as the knowledge and skills required to interpret test results correctly. They also have the correct alternator wiring diagrams, and the required equipment to test an alternator after a repair.

NOTE: Please note that the above information only applies to conventional alternators that contain voltage regulators.

On the other hand, because they do not contain voltage regulators, smart alternators cannot be tested on a test bench. Therefore, they must be installed in a vehicle before they can be tested. Note that the best way to test a smart alternator is to scan the charging system with a diagnostic computer.

NOTE: Smart alternators are very sensitive to overloading, such as might happen when you install one or more auxiliary batteries. If you install auxiliary batteries, be sure to upgrade the smart alternator with one that has a higher rated capacity.

If you suspect your alternator may have failed, we highly recommend that you Find a Mechanic to find a specialist mechanic near you.

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