Signs of a bad alternator

Signs of a bad alternator

The first step of knowing if your alternator or battery is bad or starting to go bad is to understand the basic functions.


The battery in your vehicle has the basic function of storing power when the vehicle is turned off.  That being said, if your vehicle starts having an issue while the engine is running, it is typically not caused by the battery.  
The typical signs of your battery starting to fail or being bad are:

  • No Electrical Power
  • No lights on the dash when you turn the key.
  • No sounds.
  • No cranking of the starter when you turn the ignition to the start position.

​Low Electrical Power:

  • Lights might come on in-dash when you turn the key to the on position but they may be dim or flickering.  This is a sign of low battery power (amperage or voltage).
  • Rapid clicking-  A common sound you hear when a battery has some power left but not the correct amount is a rapid clicking noise when you hold the ignition in the crank position. This is your starter trying to engage but not having enough power to do its job.


The alternator in your vehicle has the basic function of creating power when the vehicle is running. As your engine is running, it spins the alternator with a belt creating power (voltage and amperage). Your vehicle using this power to operate all the electronics (gauges, windows, locks, computers, ignition systems, etc). Your alternator typically produces more power than your vehicle is currently using. That extra power goes to your battery to recharge it from the draw from sitting and also the draw from starting the vehicle. 


The typical signs your alternator is failing or bad:

  • Low voltage warning in the instrument cluster.  
  • This could be in the form of a gauge that displays the actual voltage but most common is a red warning in the shape of a battery. If you see this warning, it is telling you that you are using more power than the alternator is producing and will likely have a failure very soon.  If you see this warning, I would recommend pulling over as soon as safely possible and have the vehicle towed home or to your trusted auto shop.
  • The battery is dead- If you can start the vehicle by jump-starting but then it dies when you remove the cables, that is a sign your alternator is not recharging the battery.  This is a sign your alternator is bad but keep in mind it could also be caused by a failed battery or loose connection.  Do not jump to conclusions. Make sure you check the battery, starting, and charging system properly, or have an auto repair shop do so before changing parts to avoid wasting unneeded time or money.
  • Dim or flickering headlights- One of the largest draws of power of your vehicle's headlights.  If the alternator is failing, it will often show up in the flickering or dimming of your headlights.

There are several other signs of a bad alternator or battery but I wanted to give you the basics and keep things simple.  There are noises, smells, and many other things that will serve as clues when diagnosing a problem with your vehicle. Make note of them and share them with your mechanic to help them pinpoint any issues you may be having.
Check out this great video The Engineering Mindset created for an in-depth view on How Alternators Work.