5 Signs Your Alternator May Need to Be Replaced

And what to do if you spot them.

A woman holds open the hood on her Subaru to check her alternator.
Know what to look for to prevent a breakdown.
Bilanol / Shutterstock

If your battery needs replacing, it’s an inconvenience, but when your alternator fails, you can be stuck on the side of the road waiting for a tow truck. The alternator's job is to power your vehicle after it’s been started. It recharges your battery and powers your headlights and other accessories while the vehicle is running. When the alternator fails, the vehicle gets whatever power it can from the battery until the battery is drained or damaged. Follow these warning signs to help you know when to go to the mechanic, or find a safe place to pull over, so you can get the alternator replaced.

1. Illuminated Battery Light

Your dashboard battery light is frequently a sign that you may need a new alternator. “A battery light would indicate a charging issue as the vehicle monitors the voltage,” says Travis Mock, vice president of car care for AAA Automotive Repair Centers. In some cars, the check engine light may also come on, and/or a check charging system message on the dashboard.

The battery light may also come on for other electrical or charging system problems, including:

  • Wiring issues
  • Faulty voltage regulator
  • Worn alternator or drive belt
  • Parasitic drain (when your vehicle’s electrical system continues to draw energy from the battery when the vehicle is off)

If your battery light comes on while you’re driving, you should pull over in a safe place and get the vehicle towed to the mechanic. This is because if the alternator has in fact failed, it’s only a matter of time before your vehicle stalls, which could leave you stopped dangerously in a lane of traffic.

2. Dim or Overly Bright Lights

When the alternator doesn’t produce enough power, “one of the most common symptoms is the dimming of lights or excessive light brightness,” says Mock. The dimming (and sometimes flickering or initial brightening) can occur in your headlights, dashboard light, and/or your interior dome light.

A battery warning light lit up on a close-up of a speedometer.
The battery warning light means you have a problem with your vehicle's charging system.
J.J. Gouin / Shutterstock

3. Trouble Starting or Bad Battery

When the alternator isn’t doing its job, it forces the car to keep the electrical system working by using the battery’s power. Additionally, the alternator should recharge the battery while you are driving, so if it’s not doing that, you might end up with a battery that doesn't have enough charge to start your car. Since the battery isn’t designed to continually carry the load, it may require replacement as well. If you’re having a hard time starting your car, or it won’t start at all, it’s important to have the full charging system checked by a qualified mechanic to confirm if the battery is bad or if it just needs to be recharged, and to make sure that there isn’t an underlying reason you’re having the problem. 


4. Burning Smell from the Engine

“Depending on the alternator issue, it could create an electric burning smell,” says Mock. This could occur because of damage to the drive belt or the alternator itself. 

5. Engine Stalling

If your alternator fails while you are driving, your vehicle may stall, leaving you without power. If your battery can still hold a charge, jump-starting it may get the car started and running again, but not for long. Usually, this will happen in conjunction with a battery light. It’s important to not continue driving after the car has stalled, as it’s just a matter of time before it stalls again.

A mechanic tests a car's battery and charging system.
Bring your car to a professional for testing if you think you might have an alternator problem.
Dragon Images / Shutterstock

How to Tell if the Problem is the Battery or Alternator

Since there is so much overlap between the symptoms of a bad battery and a bad alternator, you need a technician to determine which is causing the problem, according to Mock. It may also be something else entirely. “Faulty or corroded battery cables or connections can also mimic a failing alternator or battery,” Mock says. A mechanic or battery technician can test the system to determine the cause of the problem.

When to Get Your Car Towed

If your battery light comes on or your vehicle stalls, it’s critical that you get your vehicle towed to your mechanic for your own safety. Continuing to drive with a failing alternator could also permanently damage your battery and require you to replace it as well.