How to Confirm That Your Alternator is Failing

When your alternator fails, your car’s battery will completely drain, and you won’t be able to start your car. A failing alternator is an issue that can leave you stranded somewhere. That’s why every mechanic recommends:

  • Keeping an eye out for the symptoms of a failing alternator
  • Diagnosing the issue
  • Replacing the alternator as soon as it’s necessary.

Signs of a Bad Alternator

Old new alternator

It’s common for an alternator to go bad due to:

  • Failed diodes messing up the alternator’s charging output and causing the electric current to drain out of the battery
  • Worn needle bearings not letting the rotor spin as fast as it should
  • Failing voltage regulator sending either too much or too little electric current to the battery

When there’s an issue with the alternator, certain symptoms will crop up. They include:

  • Stalling engine
  • Trouble starting the engine
  • Dead battery
  • The “GEN” or “ALT” light coming on in the dashboard
  • Reduced electrical power
  • Dim or flickering headlights when the engine is idling
  • A rattling or grinding noise coming from the engine bay (only when accompanied with the other symptoms listed here)

Ruling Out Other Parts First

Before you check your alternator with a cigarette lighter voltage gauge, you want to make sure all of the related parts are in good working order. Sometimes the alternator itself is fine, but a related part, like a loose drive belt, is causing the issue.

We recommend doing a quick visual inspection of the following parts:

  • The drive belt for any damage or looseness
  • The lead running between the alternator and the battery for poor connection or damage
  • The wiring harness for looseness
  • All the wires connected to the alternator for any looseness or fraying
  • The alternator fuse (if your car has one)

Checking the Alternator

Chevy alternator

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

Checking the Electrical System Voltage

  1. Plug the gauge into the cigarette lighter.
  2. Turn the battery on without starting the engine.
  3. Take note of the gauge reading.

The gauge reading is your battery’s at rest voltage. It should be between 12.6V and 12.9V. If it’s between 12V and 12.5V, your battery is not fully charged. If it’s less than 12V, then there’s something wrong with your battery or alternator.

It’s easy to pin the blame on the battery. Sometimes the culprit is indeed the battery, but there’s a chance that it’s the alternator. Luckily, it’s easy to determine if the alternator is the problem.

Testing the Alternator

Now, start your engine and then take note of the gauge reading. When you start the car, the alternator starts powering the battery.

A functioning alternator and battery should show a reading of 13.9V-14.2V. If your reading falls between this range, but it was below 12.5V in the previous step, then it’s likely that your battery is bad.

If the reading is between your battery’s at rest voltage and 13.8V, your alternator is failing. If the reading is below your battery’s at rest voltage, then the alternator is definitely bad.

Please contact us with any questions you may have about this diagnostic process.