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