When does the alternator need to be replaced?

Updated: May 20, 2020.
The 12-volt battery in your car stores electric power to start the engine. The job of the alternator is to recharge the battery when the engine is running. The alternator is the main part of the charging system. If the alternator doesn't work, your engine may still run for a short time until the battery power is depleted.
Car Alternator Car alternator.
An alternator is bolted to the engine and is driven by a drive belt. It doesn't need any maintenance and only needs to be replaced if it stops working or if there is some other problem with it.

How long does the alternator last? On average, an alternator fails after 100,000-150,000 miles. In cars that are driven daily, an alternator tends to last longer.
Tip: one of the reasons an alternator can fail prematurely is when an engine splash shield is broken or missing and road dirt and moisture can get into the alternator.

What are the symptoms of a bad alternator? Sometimes an alternator could fail completely with no warning. Other times, it gradually becomes weaker and starts giving intermittent problems first. If an alternator stops working, the red battery-shaped warning light comes on while the engine is running. Normally this warning light comes on only when the engine is stopped and the ignition is ON.
Battery-shaped charge warning light Charge warning light.
In some cars, the message "Check Charging System" might also appear on the driver's display.
One of the possible symptoms of a weak or failing alternator is when the interior lights or headlights become dimmer when the engine idles, but get brighter when it's revved. Another symptom is when a good battery is constantly undercharged.

Of course, these symptoms could be caused by many other issues (e.g. corroded battery terminals, wiring problems or a loose or damaged drive belt, etc.). To confirm a bad alternator, it must be tested.
How do mechanics test an alternator? Auto repair shops have a hand-held tester (in the photo) that can test the battery and the charging system. After the test, you will get a printout showing the charging system performance. The only problem is that if the alternator has an intermittent problem, it may pass the test when tested, but stop working again later.
Car Battery Tester Testing the charging system and the battery capacity with a battery tester.
Charging system test In this car, the charging system test shows that the alternator is OK.
If an old alternator is suspected of having an intermittent problem, it's usually replaced as a precaution. Whenever there is a problem with an alternator, the alternator wiring connectors must be checked for corrosion and looseness.

Your mechanic can also use a voltmeter to test the alternator output (charging) voltage with the engine running. Typically it's done with the headlights turned on, to make sure the alternator works under load. A good working alternator should provide 13.5-14.2 Volts. In some cars, the charging system voltage might go up to as high as 15 Volts. In some modern cars, the alternator is not charging constantly, but rather comes on and off as commanded by the computer (PCM) as a fuel-saving measure. In this case, the testing procedure might be different.

An alternator might also need to be replaced if one of the two bearings inside the alternator becomes noisy. A bad alternator bearing typically produces a whining or growling noise. Typically mechanics diagnose bad alternator bearings using a mechanic's stethoscope.
New Alternator New aftermarket alternator.
In some cars, an overrunning alternator decoupling (OAD) pulley can also fail or start making a whining noise. In many cases OAD pulley can be replaced separately, but might require a special tool.

How much does it cost to replace an alternator? There are several repair options: an OEM alternator (part) is more expensive ($380-$680). A new aftermarket or rebuilt alternator is cheaper ($180 to $420). The price typically includes a core charge that you will get back after returning the old part (core).
Tip: some modern vehicles with Engine Stop-Start system that shuts down the engine when a car is stopped have a special heavy-duty alternator. It's always important to verify that an aftermarket alternator will fit.
Labor charges to replace an alternator can vary from $89 to $250 depending on the difficulty and the location of the alternator. In an average compact car with a small engine, it's a fairly easy job. In some vehicles with limited space under the hood, many parts may have to be removed to get to the alternator, and this will cost more.
Another option is to have the alternator rebuilt by a local Starter & Alternator shop. It's cheaper ($100-$170 without installation) but takes more time.
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Many automotive wreckers sell used alternators; they are usually not very expensive $40-$120, but the condition is unknown. If an alternator was stored outside in humid conditions, it might be corroded inside.
If you are planning on replacing an alternator yourself, make sure to follow the manufacturer's instructions and safety precautions. The battery must always be disconnected before replacing an alternator.
If you want to go even further and rebuild the alternator yourself, rebuild kits are available online for $25-$85. Rebuilding an alternator is a time-consuming precise electrical job that must be done according to manufacturer's instructions.

Should a drive belt be changed whenever an alternator is replaced? It's a good idea if the drive belt is old. The belt has to be removed to replace an alternator anyway. This means that in most cases, there is no extra labor and you will only have to pay extra for a new drive belt, which is not very expensive.

Should the battery be replaced whenever an alternator is replaced? It's not necessary, however it's not uncommon for an old battery to fail when an alternator stops working. This happens because when the battery is completely drained, it deteriorates sooner. Read more: When does a car battery need to be replaced? If your alternator has been replaced and the battery is discharged, your mechanic can re-test the battery after it has been fully recharged. Usually, it takes 20-40 minutes of running the engine or driving to recharge a good battery.

Alternator recalls: Several car makers, including Fiat Chrysler, GM and Nissan have issued recalls related to alternators in the past, check with your dealer or look it up on the NHTSA website.