Alternator, how it works, symptoms, testing, problems, replacement

Updated: March 20, 2016
Alternator. Click for larger photo

An alternator is a generator of electric power in your car and is a major component of your vehicle's charging system. Whenever your engine is running, the alternator charges your battery and supplies additional electric power for the vehicle's electrical systems. An alternator is attached to the engine and is driven by a drive belt (also known as a serpentine belt).

An alternator is a maintenance-free unit. On average, an alternator can last for 8-12 years without any repairs. If an alternator fails, the car may still run on battery power alone for a short time, but will die as soon as the battery charge is depleted. Replacing an alternator with a new unit is quite expensive, but there are alternatives. Read more below.


Battery warning light
Battery-shaped warning light
indicates a problem with a charging system

The most frequent symptom of a problem with your vehicle's charging system is a battery-shaped warning light (in the photo) or the "CHARGE" icon that comes on while driving. Normally this warning light should come on when you turn the ignition, but should disappears as soon as the engine is started. If it stays on, there is a problem with your charging system.

The charging system warning light doesn't point directly to a failed alternator, although alternator problems are very common. Your mechanic will need to do further testing to pinpoint the defective part.

Sometimes when the alternator becomes weak, you may notice that your car's headlights and dash lights become dim at idle, but get brighter at higher RPMs.

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How an alternator is tested

Charging system and battery tester
Computerized battery and charging system tester

Your mechanic can test the state of your charging system with the battery and charging system tester (in the photo). A battery and charging system test (AVR test) can cost from $30 to $50. The test can show if the charging system is weak or not working at all. It also can detect if one of the diodes inside the alternator has failed.

If the charging system failed the test, your mechanic will need to do further diagnostic to see if it's the alternator or something else causing the problem. Other charging system problems include a loose drive belt, faulty wiring or blown fuse, defective ignition switch, etc.

Checking the alternator output voltage
Checking the alternator output voltage with a multimeter

If no charging system tester is available, your mechanic can do a simple voltage test. The test involves checking the battery voltage with the engine off and with the engine running. The battery voltage should increase once the engine is started, as the alternator supplies additional power (see the photo). If the battery voltage does not increase once the engine is started, there is a problem with the charging system.

Should an alternator be replaced or rebuilt? What is cheaper? Read .

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