Signs of a Bad Alternator - How to Tell if Your Alternator is Bad

The alternator on your vehicle is a generator that produces direct current (DC) voltage which is used to keep your battery charged and your electrical system working properly. But when your alternator starts to fail, you may notice a few specific symptoms before it leaves you stranded. Keep reading to learn more about the purpose of your car’s alternator, as well as the symptoms of a failing alternator, and how to check the alternator to determine if it’s time for it to be replaced.


The alternator is responsible for all of the electrical power that is used by the vehicle when it is being driven. Alternators are driven by the engine's serpentine belt or V belt to generate power for the radio, lighting, and on-board computer systems on your vehicle, but also provide power to the battery so that it is fully charged and ready to start your vehicle next time. Once the engine has been started, the alternator takes over power delivery to the entire vehicle, which also includes the ignition system that keeps the vehicle running. Though the alternator does help restore the amount of power the battery expends on starting the engine, it is not necessarily a battery charger. It can take up to 11 hours of continuous operation to fully recharge a battery that is at 75% capacity using only the alternator. An alternator that has to recharge a severely discharged battery, or a battery that is not holding a charge, can result in stress and heat that can damage the alternator. That's why it's a good idea to replace your battery at the first sign of a problem to reduce the risk of a more expensive repair.

How Does an Alternator Work?

Alternators use internal magnets or wound field coils that act as an electromagnet to produce a rotating electrical field. This allows them to generate alternating current (AC) voltage using mechanical power produced by the engine. That AC voltage is then passed through the alternator rectifier to convert it to DC power that can be used by the vehicle’s various electrical systems. Most modern alternators also include a built-in voltage regulator that limits the voltage output of the alternator to prevent overcharging the battery or damaging on-board electronics. Some newer vehicles may also have alternators that are regulated externally by the engine control module (ECM).


using a multimeter to test battery under hood

What are the Signs of a Bad Alternator?

Though they don’t always fail in the same way, signs of a bad alternator are typically consistent from one vehicle to another. When your alternator is failing, it may be hard to tell the difference in a bad alternator vs. a bad battery. The most common alternator failure symptoms are listed below:


  • Dim or especially bright headlights or other vehicle lighting
  • Lighting that seems to swell in brightness with increased engine speed
  • Slow window motors, weak power locks, or other electrical accessories with reduced capacities
  • A battery that won’t stay fully charged or difficulty starting the vehicle after it has been running
  • A battery that smells of sulphur or is leaking due to overcharging
  • Squeaking, squealing, or grinding noises coming from the alternator pulley due to a failed bearing
  • An engine that can be jump-started but slowly dies as battery voltage lessens
  • An active battery warning light on the dash or an amp meter that reads on the “D”, or discharge, side of the gauge

If you’ve noticed any of these symptoms in your vehicle, it may be a good idea to check your alternator and connections, or to have your alternator tested at your local O’Reilly Auto Parts free of charge.

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using a multimeter on an alternator under the hood of a car

How to Test an Alternator on a Car

The easiest way to test an alternator yourself without even removing it from the vehicle is to use a multimeter to read voltage across the battery posts. Alternators are designed to charge typically between 13.5 and 14.5 volts. If it is significantly higher or lower, your alternator has likely failed. Below are the steps to perform an alternator multimeter test.


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  1. Set the multimeter to VDC (DC voltage) above the amount of voltage you anticipate – since you may see voltage readings of over 14 volts, most multimeters should be set to 20 volts DC.
  2. With the key turned off, check the voltage across the posts of the battery. It should be around 12.5 volts if it is fully charged and in good condition.
  3. Now is also a good time to check your battery terminals for corrosion or loose connections, as this can prevent the alternator from properly recharging the battery. If you notice terminal corrosion, it is recommended that you remove it using a battery brush and/or battery terminal cleaner.
  4. With your multimeter leads connected to the battery terminals, start the engine and read the voltage – with the engine running, car battery voltages should be over 13 volts, and preferably closer to 13.5-14.5 volts.
  5. You can also place a load on the alternator by turning on the headlights and high-beams, turning the blower motor to high, and turning up the radio – if the power at the battery supplied by the alternator falls below 13 volts under load, the alternator should likely be replaced.

Other factors may also need to be considered when testing or checking your alternator before you decide to replace it:


  • Check the condition and tension of the engine drive belt
  • Check the connections at the back of the alternator and at the battery, as well as the condition of the wire insulation between the alternator and battery or wiring harness
  • Listen for unusual noises from near the alternator – a mechanic’s stethoscope or even simply a piece of fuel hose can help you pinpoint the source of any engine or alternator noise
  • Look for oil leaks near the alternator that may have allowed oil to contaminate the unit's internal windings or destroy its brushes and prevent it from charging
  • Check the condition of the battery's ground wire or strap to the body, engine, and chassis

If these alternator tests are inconclusive, or if you’re not comfortable doing this yourself, feel free to bring your car, or just the alternator, by your local O’Reilly Auto Parts. Our Parts Professionals can test the alternator on or off the vehicle, and will always help you find the best replacement option if your alternator has failed.


in the act of removing the wiring on an alternator under the hood

How to Replace An Alternator

If your alternator has failed, you’ll likely want to make this repair as quickly as possible. Not only can an overcharging alternator damage your battery or electrical accessories over time, but an undercharging alternator can easily leave you and your car stranded.


Replacing most alternators is a relatively straightforward task that usually involves simply disconnecting the battery and removing the serpentine belt, wiring harness connections, and retaining or mounting bolts. In many cases, alternators are held in place by as few as two or three bolts. Though your alternator may be difficult to access or get in and out of the engine compartment, most alternator replacements can be completed in an hour or two with only common hand tools, helping to keep your alternator replacement cost as low as possible. Shop replacement alternators online or at your local O’Reilly Auto Parts to restore your vehicle’s charging system.


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If you’ve completed your testing and are ready to replace your alternator, shop our selection of alternators for your vehicle or visit your local O’Reilly Auto Parts today. Our Parts Professionals will be more than happy to provide free alternator testing, as well as find you the best replacement alternator and offer any advice you may need for your repair. It's also a great idea to inspect and consider replacing the serpentine belt, idler, and tensioner while you already have the engine's accessory drive disassembled.


If you’ve come this far and aren’t certain you want to replace your own alternator, we can also recommend a local service shop that can replace your alternator and help get you back on the road, using the right parts from O’Reilly.