Failing Alternator Basics: How to Tell Whether the Alternator or Battery is the Issue

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How to diagnose a bad alternator

If you think about it, it’s kind of amazing that cars are as reliable as they are. They’re complex machines with hundreds and hundreds of moving parts all relying on each other in order to not leave you stranded. That being said, there are still plenty of parts that will inevitably need to be changed out if the car lasts long enough, and that includes the two most important elements of your charging system: The alternator and battery.

The alternator, or sometimes referred to as a generator, does just that — generates electricity off of the engine accessory belt in order to keep the battery charged. Your car’s battery isn’t just there for starting, it also provides electricity for the spark plugs, ECU, and all other electronics on the vehicle.
When either the alternator or battery are failing, the two will often show similar symptoms, and replacing the battery might just mask a failing alternator for another few weeks before your car won’t start again. So here is what you need to look out for when diagnosing why your battery isn’t charging or holding a charge.

What are the Symptoms of a Failing Alternator?

Usually your first indication is that your car might struggle to crank or will only click when you turn the key. This could, however, just be a worn-out battery or loose connection, so people usually check there first. Batteries tend to only last 3-5 years depending on the climate, so more often than not it is a battery issue.

One easy way to check if the issue is your alternator is with the headlight test: Either grab a friend to help or you can see yourself by parking facing your wall with the engine running (in a well-ventilated area). Turn the headlights on and rev the engine in neutral or park. If the headlights and/or interior lights dim, your alternator is likely failing. You may also notice this when

If your car won’t start, you can take your battery to a local auto parts store and have them test the battery for you. They will charge the battery fully and test the battery to make sure it can accept and hold a full charge. If they test your battery as good, it could be your alternator why the battery drained.

How to Get Home With a Bad Alternator:

Duracell Lithium Ion Jump StarterThat really depends on how far gone the alternator is. You may be able to get a jump from another person (or use a battery jump box, very handy to have), but if your battery has too low a charge and isn’t getting much or any help from the alternator, then you’re not going to make it far.
If you can give your battery a full charge, you may be able to get where you need to go. Otherwise you’re looking at getting towed back to a shop or home. AAA has saved me many times!

How to Repair a Faulty Alternator:

The most common reason an alternator fails is that the brushes inside wear out, or possibly a bad diode. There are rebuild kits you can buy to fix and rebuild them, but in my opinion you might as well just buy a replacement unit with a warranty. For most cars alternators aren’t especially expensive, meaning the labor involved in replacing them is usually the worst part.
Unfortunately, there is only one way to know for certain if your alternator is bad, and that is to have it tested. Once you remove it, you can take it to a local autoparts store to be tested as well. They will put it on a machine that will spin it up in order to see if it is producing any electrical current. This will further help make sure you found the root of the issue before dropping cash on a replacement when there was another issue instead.
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Garrett Davis
Garrett has something of a sickness when it comes to cars, working on everything from Jeeps, to sports cars, to over-engineered German nightmares. Currently he is embroiled in an Audi Allroad offroad project, and is slowly losing his grasp on sanity.

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