How to Replace an Alternator

Your alternator supplies your car’s battery and auxiliary systems with electrical current, and a failed alternator left alone will eventually lead you with a dead battery, or a dead car on the side of the road. Keep reading to learn the signs of a damaged alternator and find out the general steps required for a DIY alternator replacement.

How to Tell If an Alternator is Bad

There are some well-established warning signs of a bad alternator. Keep in mind that many failures in the charging system can also be the wiring between the alternator and the battery, but it’s best to always start with the alternator. Because the alternator supplies current to the vehicle’s electrical systems, this is where you’ll notice most that something is amiss.

  • Dead Battery: Your car battery will die without a working alternator to keep it charged. If your battery dies, you should test the battery (free at AutoZone). If the battery is good, then there is likely something wrong with the alternator.
  • Dim Lights: If you notice your headlights or other lights dimming or flickering, this can indicate that they aren’t getting enough current. Generally, the dimness will worsen as the alternator wears.
  • Warning Light: Most cars are equipped with what is otherwise known as the battery light. This light often has a misconception that it means the battery is bad. What it actually means is that the alternator is not properly charging the battery.
  • Sluggish Accessories: Another possible sign is if electrical accessories or features like power windows move more slowly than they typically do.

Tools to Do the Job Right

Before you get to work, you’ll want to gather all the necessary components. Choose a level, safe work area. In addition to a replacement alternator, you’ll need some tools:

  • Hand tools like ratchets, sockets, and wrenches
  • Belt tensioner tool
  • Gloves and safety gear
  • Battery tender or memory saver, if necessary
  • Jack, jack stands, and wheel chocks, if necessary

Automotive designs vary greatly from car to car. You may need something else, and many cars won’t need the last two items to get the job done, although some do. It all depends on how your make and model was made. If you don’t have a tensioner tool, borrow one from us through the AutoZone Loan-A-Tool program.

How to Replace Your Alternator

1. Disconnect the Negative Battery Cable

This step is crucial if you want to avoid a short, and trust us, you do. Before you go about disconnecting the negative cable, consult a vehicle repair manual to determine if a battery maintainer should or should not be used to avoid drivability and/or module re-programming issues. Make sure that the ignition is in the ‘OFF’ position too. Now, disconnect the negative (ground) battery end and cover it up to avoid making contact during the job. The negative battery terminal is identified with a Minus sign ( – ).

Now is a good time to clean, inspect, and test the battery. A weak or failing battery can cause premature failure of the replacement alternator. If you’re not sure how to test it, bring it to AutoZone for a free test.


Make sure that the battery is fully charged when this job is completed. Do NOT use the newly installed alternator to ‘charge up’ a low voltage battery. This can cause damage to the alternator!

2. Locate the Alternator

The alternator is mounted to the engine, either the front or the side depending on the vehicle. If the alternator must be accessed from underneath the vehicle, a hydraulic jack, jack stands, and wheel chocks will be needed to lift the vehicle and maintain a safe work environment.

3. Inspect Related Components

While you’re looking at the alternator, inspect the following related parts of the starting and charging system:

Drive Belt: This should be changed every 90,000 miles, and while your replacing your alternator is a perfect time to replace this belt if its time is soon to pass. Look out for cracks, frays, and overall wear and tear. A worn drive belt will produce noise and reduce the performance of the alternator. Inspect and replace any additional belts as needed.

Drive Belt Tensioner: If so equipped, the belt tensioner can be inspected while removing the drive belt. It should feel ‘smooth’ without ‘sticking’ or ‘binding’ as it swings back-&-forth with the belt tensioner tool. The belt tensioner must not be extremely tight or loose. Also check the alignment of the drive belt and pulley(s). A worn belt pulley (idler) or belt tensioner will cause belt slippage, squalling sounds, uneven belt wear and poor performance of the alternator and the other drive belt accessories such as the power steering pump.

Electrical Plug Pigtail Connector: Ensure the alternator pigtail and harness are clean and intact. A loose terminal or damaged pigtail connector can cause electrical and charging problems. Replace all damaged or worn components. Use a small amount of water proof grease (dielectric grease) on the connector and alternator plug-in during installation.

Battery Terminal B+ Voltage Wire: The alternator 12v battery wire can become worn or damaged over time. If it appears ‘burned’, has a cracked or melted terminal end, damaged or missing wiring insulation or feels ‘stiff’, the battery wire should be replaced. Failure to do so will reduce alternator performance and may even cause an electrical fire.

Contamination on the Alternator like Oil, Grease, or Antifreeze: A small amount of outer surface contamination is normal. However, if the alternator is covered (soaked) in engine oil or other fluid types, the source of the contamination must be found and repaired. Contamination can shorten the life expectancy of the alternator and possibly void the manufacturers warranty.

4. Alternator Removal

Once the electrical connections, drive belt and any interference items are inspected, disconnected and removed, the old alternator can be unbolted from the engine and mounting brackets.

5. Match it Up!

Compare the old alternator to the replacement alternator. Note the electrical connector plug-in position and the design (shape). Make sure everything matches before installing. This also applies to the position of the alternator mounting bolt holes.

NOTE: Some alternators will appear different due to consolidation by the manufacture. Check the installation paperwork that came with the replacement alternator first before assuming you have the incorrect unit.

6. Alternator Installation

Carefully align the working alternator mounting holes and loosely install the mounting bolts and nuts. Ensure all the bolt and/or nut threads are started and tighten them evenly. On some vehicles, the electrical connections will need to be installed first before permanently tightening the alternator for access reasons.

Place the drive belt in position and ‘rotate’ the belt tensioner so the belt can be installed. You may need to refer to the under-hood belt routing diagram or consult the repair guide. You can also check with an associate at your neighborhood AutoZone.

Due to tight clearances between the pulleys, it is usually best to place the belt onto the pulley(s) that have ‘a lip’ (usually the grooved type) and slip the belt onto the final pulley without a ‘lip’ (usually the smooth type). The belt tensioner will only travel (swing) so far which can make installing the belt difficult. Never pry or force the belt onto the pulley. Vehicles with a manual tensioner or the belt tightens with the alternator, there should about ½ inch of deflection in the belt between the pulleys with the greatest distance between them.

7. Finishing Up

Once the alternator, drive belt, and electrical connections are reinstalled, scan the work area and make sure nothing has been overlooked. Remove all loose items and tools. Follow the proper steps for the battery maintainer if one was used.

Ensure the battery posts and battery cable end connections are clean and in good condition. Replace as necessary to ensure tight connections.

Install new felt washers on the clean battery posts. Re-attach the battery cable ends to the battery. Positive first, the negative (ground) last. Apply a film of battery post grease to the top and sides of battery cable ends and battery posts. This will help prevent acid build up and corrosion in the future.

Now, it’s important to test the alternator’s output. This can be done for free at AutoZone, and can also be done at home with a multimeter. You can learn more on how to properly test here. Place the multimeter probes on the battery terminals. With the car running, you should be seeing somewhere between 13.25 to 15 volts with a fully charged battery. If you are not, and you are only seeing battery standing voltage (around 12.2-12.5) then your alternator is still not properly charging.

It’s important to note, this does not mean your brand new alternator is defective! From here, the wiring leading from the alternator, the connections, and possible fusible links must be checked out. Every vehicle’s wiring in this area is slightly different, so consult your repair manual, or online repair source to help.

Also, if the job appears to be too big, or you need help troubleshooting, you can contact one of our Preferred Shops in your area, that can help you do the job.

Remember: the battery should be fully charged. A low voltage battery can damage the replacement alternator.

With the vehicle level and the final steps completed, start the engine. Listen for odd noises, the smell of burning plastic, or the sight of smoke. You can bring the vehicle to AutoZone and have the alternator tested on the vehicle to ensure the charging system is performing correctly.

Some of these steps may vary slightly depending on your exact year, make, model, and engine. Always refer to your service manual to determine the required components and to find the location of your alternator and tensioner.

Now that you know how to replace an alternator, it’s time to pick up all the tools and equipment you need. For the best prices on name-brand alternators, affordable hand tools, and precision replacement parts, visit your local AutoZone.

If the job is too big for you, seek out one of our Preferred Shops to help you do the job.

Do you need a more in-depth look into your vehicle to help you with your project? ALLDATA, the leading provider of automotive repair information, is now providing DIYers with the same information that the pros use with ALLDATAdiy’s single-vehicle subscriptions.

Learn More

Advice, how-to guides, and car care information featured on and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

FREE Loan-A-Tool® program requires returnable deposit. Please note that the tool that you receive after placing an online order may be in a used but operable condition due to the nature of the Loan-A-Tool® program.

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