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

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 rebuilding 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.