Symptoms of a Faulty Alternator
A battery warning light, flickering lights or a battery that keeps going flat can all point to the alternator. Here’s how to tell the difference.
The most common symptoms of a faulty alternator are a battery warning light on the dash, dim or flickering lights, a battery that keeps going flat, whining or grinding noises and, eventually, a car that stalls or won’t start. The alternator charges the battery and powers the electrics while the engine runs, so when it fails, the car slowly runs out of electricity.
The warning signs
Battery warning light while driving
The red battery-shaped light is badly named. It usually means the charging system has a problem, not that the battery itself is bad. It may come on steadily or flicker on and off, particularly at low revs.
Dim, flickering or overly bright lights
The voltage should stay fairly steady. Headlights that pulse with engine speed, or that dim at idle and brighten when you rev, can point to the alternator. Over-bright lights or bulbs that blow repeatedly can suggest it’s overcharging, which can damage the battery.
A battery that keeps going flat
If you’ve jump started the car or fitted a new battery and it’s flat again within days, the battery probably isn’t the root cause.
Electrics playing up
Slow electric windows, a weak heater fan or a dim dash are all signs of the car running short of power. Modern cars may start shedding non-essential systems to keep the engine running.
Whining, grinding or burning smells
A whine that rises with engine revs can come from the alternator or its pulley. Grinding can suggest worn bearings. A smell of hot rubber can come from a slipping belt, while a smell of hot electrics is a reason to stop and investigate.
Stalling or cutting out
Once the battery is drained, the engine may lose power to its ignition, fuel and control systems. Stalling has many other causes, so don’t assume, but if the battery light is also on, the two are probably linked.
Battery or alternator: how to narrow it down
With a multimeter across the battery, a healthy battery at rest reads about 12.6V. With the engine running, a charging system should typically show around 13 to 14.5V. A reading stuck near battery voltage suggests the alternator isn’t charging, and a reading well above the range can mean overcharging. Some modern cars use smart charging that varies the voltage deliberately, so one figure on its own isn’t proof either way.
Don’t test an alternator by disconnecting the battery with the engine running. It’s an old trick that can cause a voltage spike and damage the car’s electronics.
Is it always the alternator?
No, and it’s worth checking the simple things before buying parts. A loose or worn auxiliary belt, a slipping tensioner, corroded battery terminals or a poor earth connection can all produce the same symptoms. A proper test covers the belt, the wiring and the output, and mobile car diagnostics can check for stored charging faults on cars that log them.
Can I keep driving with the battery light on?
It’s best not to go far. The car is running on what’s left in the battery, and how long that lasts depends on the battery’s condition and the load on it. Turn off the heated screen, heated seats and any other big electrical loads, and head somewhere safe to stop.
Take extra care if the light came on with a squeal or a bang. If the auxiliary belt has snapped, you may also lose power steering, and on many cars the same belt drives the water pump, so the engine can overheat. If the steering goes heavy or the temperature gauge climbs, pull over safely and switch off. Don’t try to drive it home.
At night, the risk is greater. If your lights are fading, stop where it’s safe rather than ending up with no lights on a dark road.
Getting it fixed
Alternator access varies from car to car. On some it’s near the top of the engine. On others it means removing parts, or working from underneath. We carry out alternator and starter motor repairs at your location, subject to the suitability of the job and location. If the battery has been repeatedly drained, it may have been damaged too, so we’ll test it and tell you about our car battery services if it needs replacing.
When you get in touch, tell us the make, model and year, which lights came on, and whether the car has been jump started recently.