Signs Your Alternator Is Failing
The alternator does two jobs: it powers everything electrical while the engine runs, and it recharges the battery. When it fails, the car runs on battery alone — and a car battery will not last long doing an alternator's work.
Six warning signs
1. The battery warning light comes on while driving. This light is frequently misread as a battery fault. It is actually a charging system warning, and the alternator is the usual culprit.
2. Headlights dim at idle and brighten when you rev. Classic low-output symptom.
3. A whining or grinding noise from the front of the engine that rises with revs. Usually a failing alternator bearing.
4. Repeated flat batteries. If a new battery goes flat within weeks, suspect charging rather than the battery.
5. A hot electrical or burning rubber smell. Either the diode pack is overheating or the auxiliary belt is slipping on the pulley.
6. Odd electrical behaviour — flickering dash, radio resetting, power steering feeling inconsistent at low revs.
Is it the battery or the alternator?
The quick field test: if the car starts fine after a jump but dies again shortly after the leads come off, the alternator is not charging. If it starts on a jump and then runs and restarts normally all day, the battery was the problem.
The proper test is measuring charging voltage at the battery terminals. Around 12.4 to 12.7 volts with the engine off is a healthy battery. With the engine running you should see roughly 13.8 to 14.6 volts. Anything below 13.2 volts running means the alternator is not doing its job.
How long can you keep driving?
Only far enough to get somewhere safe. Once the battery drops below around 11 volts the engine will cut out, and on most modern cars that means losing power steering assistance and brake servo vacuum at the same time. That is not a situation to take onto a motorway.
Check the belt first
Before condemning the alternator, always check the auxiliary belt and tensioner. A glazed, slipping or snapped belt produces exactly the same warning light and is a far cheaper fix. We test the whole charging circuit before recommending a single part.