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 you 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. You 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.

How an alternator is tested

Chaeging 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.

Alternator replacement vs rebuilding

Replacing an alternator could be costly: $420 to $850. An original (OEM) part bough from a dealer is more expensive. An aftermarket or a rebuilt unit is cheaper. Another alternative is to have your alternator rebuilt. The way it works is your mechanic can remove the alternator and send it to the nearest alternator/starter rebuilder shop. Once the alternator is rebuilt, your mechanic will install it back. It may take more time, but it's usually cheaper, since you only pay the cost to remove and install ($70-$120) plus the rebuilder's charge ($80-$150). Rebuilding an alternator at home is difficult and takes a lot of time, but not impossible. Alternator rebuild kits are available online for $15-$50. Whenever the alternator is replaced, it's good idea to change the drive belt too. A drive belt is not very expensive, and by replacing it together with the alternator, you can save on labor, as the drive belt has to be removed to replace the alternator. Read more about the drive belt.

How to make your alternator to last longer

Often an alternator can fail prematurely when a protective engine undercover or shield is damaged or missing. This is happens because sand and water from the road can get inside the alternator and cause it to wear faster. If your engine undershield is damaged, have it replaced to keep the engine compartment clean and dry. A coolant or oil leak can also damage the alternator. Similarly, if you have to shampoo the engine compartment, the alternator must be protected from water and detergent.

How an alternator works, common problems

Battery warning light
Alternator cutaway. Image courtesy of Robert Bosch GmbH

A typical AC car alternator has two windings: a stator (stationary outside winding) and a rotor (rotating inner winding). A voltage supplied through the voltage regulator to the rotor winding energizes the rotor and turns it into a magnet. The rotor is rotated by the engine via a drive belt. The magnetic field produced by the rotating rotor induces AC electric current in the stationary stator winding. Diodes are used to convert AC current into DC current used in the vehicle's electric system. The output voltage is controlled by the voltage regulator (photo below). Typically, a voltage regulator is built-in into the alternator.

Battery warning light
Voltage regulator. Image courtesy Robert Bosch GmbH

The most common alternator problems include worn carbon brushes (the two "legs" in this photo), worn contact rings (the two copper cylinders at the back of the rotor in the cutaway image) and a failed voltage regulator.
Bad outer and inner alternator bearings (large and small silver cylinders in the cutaway image above) can produce a whining noise. When the alternator is rebuilt, the bearings, voltage regulator, brushes and some other parts are typically replaced with new ones.

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