How to Replace a Faulty Alternator

February 15, 2012

Learn the role an alternator plays in an engine, the 3 main causes of faulty alternators, and how to replace alternator units on your own.

Engine Alternator Detail

Your car's alternator provides electrical current to your vehicle after the battery helps to start the engine. It also helps to charge your battery while the engine is running. Think of an alternator as a type of power generator for your car. Occasionally, an alternator will need to be replaced. Here is a guide to help you replace the alternator in your vehicle.

What You Will Need

  • A new or rebuilt alternator
  • A new serpentine belt (optional)
  • A socket set or wrenches
  • WD-40 or other lubricant spray

Purchase the New Alternator
Visit your local car parts store and purchase a new or rebuilt alternator. In many cases, the car parts store may not actually carry a new alternator for your vehicle. However, most car parts stores stock a significant number of rebuilt alternators. In order to purchase the new alternator, you will have to pay a deposit until you give them the old alternator from your vehicle. Once you take the old alternator core to the parts store, they will refund the deposit.

Disconnect the Battery Cable
Disconnect the negative terminal from your car battery to avoid being shocked while working.

Remove the Serpentine Belt
Loosen the serpentine belt by a loosening the bolt that holds it tightly in place. If the serpentine belt appears crack or damaged, this would be a good time to replace it.

Remove Old Alternator
Disconnect the alternator wiring harness and loosen the nuts or bolts that hold it to the mount assembly. Remove the alternator from your vehicle.

Install the New Alternator
Install the new alternator and attach it securely to the mound with the nuts or bolts you removed earlier. Connect the wiring harness as well.

Reattach or Install the New Serpentine Belt
Loop the serpentine belt back over the pulley of the alternator, or reinstall a new serpentine belt. Make sure to follow the path correctly and make sure the belt is tightened securely.

Reconnect the Battery
Reconnect the negative terminal of the car battery and tighten the terminal down with a wrench or ratchet socket.

Test the New Alternator
To test the installation of your new alternator, start your engine and allow it to run for a few minutes. If your vehicle has an electrical output indicator or a gauge with a picture of a small car battery on it, check the indicator and make sure that it is considerably past the midway point. Turn on several accessories such as the air-conditioning and the radio, and make sure that the indicator does not move back into the negative zone.

You can also disconnect one of the terminals from your battery at this point to check and see if the vehicle continues to run. If the vehicle continues to run, the alternator has been installed correctly and is functioning. If the vehicle engine cuts off, there was a problem in the alternator's installation, or the new alternator may be faulty.

What an Alternator Does

The alternator is a basically an electrical generator that is run by your vehicle's engine. The alternator is linked to the vehicle's crankshaft by a flexible drive belt, which spins the alternator's drive wheel along with the crankshaft. As the central shaft of the alternator rotates, it spins specialized magnets inside a coil structure housed in the alternator's body. This generates an electrical current which is then used by the vehicle's systems and engine to perform important functions, such as powering the ignition and engine management systems, as well as the headlights, cooling fan and other electrical accessories included in the car. The alternator also charges the vehicle's battery. Since the alternator relies on the motion of the engine's crankshaft to spin its generator, a car requires a battery to store the necessary electricity to start the engine.

Alternator Components

  • The drive belt and pulley system.The alternator is driven by a flexible rubber and fabric drive belt (often incorrectly called a "fan belt") which is attached by metal pulleys to the engine's crankshaft. These components spin the generator. If the alternator's drive belt fails, the generator will stop spinning and the vehicle will no longer run correctly once the battery is depleted
  • The alternator shaft and bearings.The rotating assembly of the alternator is comprised of a metal shaft mounted inside the alternator, normally using two sets of ball bearings. The bearings allow the shaft to spin freely. If the bearings or shaft fail, the alternator may begin to make a grinding sound, or stop spinning altogether. If these components fail, it often causes failure of the drive belt as well. Once the alternator stops spinning, it no longer produces the electricity needed by the motor and the car will no longer run
  • Brushes and the rotating assembly.The alternator houses special magnets and components that generate the electricity used by the car. The alternator's "brushes" are one of the most common wear components in the alternator's rotating assembly. As the brushes become worn, the alternator becomes less and less efficient at producing electricity. Eventually the alternator will no longer produce enough electricity to charge the battery effectively and the car will no longer run properly
  • The voltage regulator and electronic components. Many modern alternators are equipped with internal voltage regulators that limit the amount of power allowed to flow from the generator into the vehicle's electrical system. This prevents damage to the engine's fragile electrical components. Most cars include a "failsafe" mechanism that isolates the alternator from the car's electrical components should the voltage regulator fail. If these components fail, the car will no longer be supplied with the necessary current and the car will stop running

3 Causes of Faulty Alternators

Poor Connections
If the wires and other connecting devices that draw power from the various systems through the alternator do not connect properly, or if they come out of alignment, the electrical power will not be able to travel through the various systems it needs to reach.

Belt Break Down
The alternator belts are responsible for keeping the alternator functioning efficiently and smoothly. These belts wear out over time. If the alternator belts are damaged or broken, the alternator itself will likely not work either.

Glaze Damage
Alternators themselves can become cracked or "glazed," which is a process when the alternator becomes coated in grease, debris and other materials that prevent it from adequately doing its task. In order to determine if this is the problem, open up hood and examine the alternator by hand. The glazed or cracked alternator will show visible signs of damage.