8 Signs Your Alternator Is Failing (and How to Tell It's Not the Battery)

Dim lights, a battery light or a car that dies while driving? Learn the warning signs of a bad alternator and how mechanics test it.

RCJ Mobile Mechanics

10/12/20261 min read

a car engine with a black hood
a car engine with a black hood

Many people replace a battery only to find their car dies again a few days later. In a lot of those cases, the real problem was the alternator all along.

What the alternator does

The battery starts the car. Once the engine is running, the alternator takes over. It powers the electrical system and recharges the battery. If the alternator stops charging, the car runs on battery power alone until the battery is flat.

Warning signs of a failing alternator

  1. Battery warning light on the dashboard while driving

  2. Dim or flickering headlights, especially at idle

  3. Electrical accessories acting strangely: slow power windows, a weak radio, a dash that flickers

  4. The car dies while driving, often after the lights go dim

  5. Repeated flat batteries, even after fitting a new one

  6. A whining or grinding noise from the front of the engine

  7. A burning smell, from a slipping belt or overheating wires

  8. Trouble starting after the car has been driven, not just in the morning

Battery or alternator?

A quick test tells the story. With the engine off, a healthy, fully charged battery reads around 12.6 volts. With the engine running, a working charging system usually shows roughly 13.8 to 14.7 volts at the battery. If the voltage doesn't rise when the engine is running, the alternator or its wiring is likely the problem.

We also test the battery under load, because a battery can show good voltage but still be too weak to crank the engine.

Why alternators fail

  • Worn bearings

  • Worn brushes or a faulty voltage regulator

  • Heat, which is a big factor in Jamaica

  • Oil or coolant leaks dripping onto the alternator

  • A worn or loose drive belt

Don't ignore it

A failing alternator can leave you stranded without warning, and running a battery completely flat over and over shortens its life. If your battery light comes on, get the charging system tested as soon as possible.

Need help with this? RCJ Mobile Mechanics comes to you in Portmore, Spanish Town, Kingston, St. Andrew and across St. Catherine. Call or WhatsApp (658) 207-6565 or book a mechanic online.