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How to Diagnose a Failing Alternator at Home

What happens when your alternator completely fails? Your car will run on its battery until the battery is drained. Then the car won't start, and you might be stranded. We recommend checking your alternator and replacing it if needed as soon as you notice the symptoms of a failing alternator.

The Most Common Tell-Tale Signs That Your Alternator is Going Bad

Learning how to recognize the symptoms of a failing alternator is the first step towards preventing battery failure. Keep an eye out for the following symptoms:

  • An indicator light titled “GEN” or “ALT” coming on in the dashboard
  • Dim or flickering headlights at idle
  • Reduced power to electrical accessories such as the power windows.
  • A rattling or grinding noise coming from the engine bay (this could mean a variety of things, so you'll have to do some troubleshooting.
  • Engine won't stay running
  • Engine won't start
  • Dead battery

Why Do Alternators Go Bad?

Mitsubishi alternator

There are several common reasons alternators fail, which are:

  • Bad bearings: Inside the alternator are needle bearings that enable the rotor to spin in the stator. Sometimes these bearings will break down over time from dirt and heat. When the bearings no longer work, the rotor will stop spinning as fast as it should, and may eventually seize. The best indicator of bad bearings inside the alternator is a rattling or grinding noise.
  • Leaky diodes: Sometimes the diodes will fail, and as a result the alternator’s charging output becomes wonky. Bad diodes can also cause electric current to drain out of the battery when the car isn't running.
  • Faulty voltage regulator: The voltage regulator prevent voltage that is too high from reaching the battery. Sometimes the voltage regulator will fail and misread the electric current. As a result, electric current with normal voltage levels doesn't make it to the battery. The battery can also be overcharged if the voltage regulator fails.

Whatever the reason, the entire alternator will need to be replaced. It’s quite easy to find an OEM replacement. We sell genuine OEM Mitsubishi parts, including alternators. Look up your Mitsubishi model in our catalog and then you’ll find the right part number for your car. Here’s also a list of our top selling OEM Mitsubishi alternators:

  • Part No. M1800A141D: For 2007-2013 Outlanders
  • Part No. M1800A145D: For 2004-2012 Eclipse, Galant, Lancer, and Outlander models
  • Part No. M1800A002D: For 2005-2007 Lancers

How to Diagnose a Bad Alternator

Image Credit: CarGurus

First, you need to confirm that your car’s electrical system isn’t getting enough voltage. All you need is a cigarette lighter voltage gauge. You can get one of these things for under $10 on Amazon.

But before you test the alternator, make sure there aren't any problems under the hood. Check these things:

  • The lead from the alternator to the battery and make sure both ends are secure.
  • The wiring harness that plugs into the alternator to make sure it is fully connected, and that none of the wires are loose or frayed.
  • The drive belt. Make sure it isn't loose or damaged.
  • The alternator fuse. (Only some vehicles have an alternator fuse. This fuse actually protects the alternator control circuit, not the alternator.)

Assuming everything checked out above, simply plug the voltage gauge into the cigarette lighter. Then turn your ignition on without starting the engine. This will provide power to the cigarette lighter outlet.

The voltage meter will then show your battery’s at rest voltage. It should be between 12.6V and 12.9V. If it is between 12V and 12.5V, your battery is not fully charged. (If it is less than 12V, your battery is completely flat.) This could mean that you have a bad battery, or the alternator could be going bad. The next step will point you in the right direction.

Now start the car. The alternator will start charging the battery. For most modern vehicles, your voltage gauge should show 13.9V - 14.2V with the engine idling. (This number varies a bit by vehicle. Google your vehicles battery charging voltage to be sure.)

If your vehicle's charging voltage is between the battery's resting voltage and 13.8 V, your alternator is weak. It is charging the battery a little, but not enough. It needs to be replaced. If the voltage gauge shows a normal charging voltage, and the battery won't fully charge, the battery is weak and should be replaced.

If the voltage shown starts dropping below the resting voltage of the battery, the alternator is really weak or completely dead.