When to Replace an Alternator


The alternator was integrated into the electrical system of the engine on vehicles in the 1960s, replacing the old generators. Although the alternator works much like a generator, its main function is to handle electrical loads during engine operation and to restore a charge to the battery that the battery used to start the vehicle. While most engine starting failures are automatically blamed on the battery, a severely drained battery can cause stress to an alternator and overwork it, which will compromise the life of the alternator.

The Alternator

  • The alternator is often overlooked as an electrical system problem in most vehicles. Anytime a vehicle fails to start, the battery and or the engine starter are usually considered the culprits. Most often, if a battery is replaced, the amps and volts will sustain and restore a certain amount of current back to the car. Because of this, many people automatically think they've solved the problem. Until they drive five or 10 miles down the road, that is.

    A bad alternator will fail to charge the battery and demands being placed on the electrical system, such as radio, air blower, lights and power windows, will rely on the voltage of the battery. Because the battery is a temporary power source, it will inevitably fail and you will be left on the side of the road waiting for a tow service or a roadside repair. The typical lifespan of an alternator, without incident of other electrical problems, should last between 75,000 and 100,000 miles. Aftermarket replacement alternators may last much less. This is because they are often re-manufactured and use cheaper interior components, but the price of them is significantly less than the original equipment.

The Battery

  • The primary job of the battery is to start the engine. It has to produce enough amperage to turn the starter against the flywheel and crank the engine. Once it has done its job, the alternator takes over where the battery left off. It provides an electrical recharge to the battery for the spent amps to start the car and then assists the load demand to the battery for other electrical components to operate.

    A properly functioning alternator does not fully recharge a battery. If the battery has to constantly be recharge or continuously fails to start a vehicle, chances are the battery needs to be replaced. By simply charging the battery or jump starting it, places the alternator in a compromising position. It has to provide more charge to the battery and can overheat and overwork.

Other Variables to Consider

  • Since the alternator uses a belt-driven pulley to supply power to the battery and electrical system, another overlooked component is the drive belt itself. If not supplying the correct amount of tension, the belt can slip while turning along the pulley system and not provide the alternator with enough current to handle the load demand. This can cause the battery to fail because the demand on the alternator is not able to sustain recharge back to the battery or loads. Another component that can fail on an alternator is the voltage regulator.

    Most alternator nowadays use an internal voltage regulator, so if it fails, replacing the alternator with a remanufactured one is a more cost efficient option than paying labor to have it rebuilt. However, older alternators used an external voltage regulator that could simply be replaced as a separate component without having to remove and replace the alternator.

Testing and Maintenance

  • Having an annual or semi-annual electrical system check is advisable, especially if you're planning to travel or you live in an area that experiences severe winter weather. The electrical system check can be done by almost any repair facility and automotive parts stores. Often times, a nominal fee or no fee at all is extended for the service in the hopes that if you need the battery or alternator replaced, you'll have the service done there or buy the parts needed from them. The electrical system check will reveal life status of the battery with the loads on and off and will also go beyond the battery. It will notice if there is an unusual diode ripple often detected from an alternator near the end of its life. If this occurs, removing the alternator for a more thorough inspection is advisable.

    Again, most repair facilities and parts stores have alternator testers, but require the removal of the component to test it. Again, these places will most often perform this service for little or no money on an alternator already removed from the vehicle. However, almost none of them will offer to remove the alternator, test it and then replace it for free.


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