The seven symptoms
- Battery warning light on while driving — that lamp monitors the charging system, not the battery.
- Headlights that dim at idle and brighten when you rev.
- The car starts fine from a jump, runs, then dies again shortly after the cables come off.
- A whining or growling noise from the accessory belt area that rises with engine speed.
- Electrical gremlins: flickering dash, slow power windows, audio cutting out under load.
- A burning-rubber or hot-electrical smell from the belt drive.
- Repeated dead batteries on a battery that tests healthy.
The test that confirms it
Put a multimeter across the battery terminals with the engine off: a healthy, charged battery reads about 12.4–12.7 volts. Start the engine and read again. You should see roughly 13.7–14.7 volts.
Below 13.2 volts running means the alternator is not charging properly. Above about 15 volts means the regulator has failed the other way and is overcharging — which will cook the battery and can damage electronics. A reading that barely changes between engine off and running is the classic dead-alternator result.
Now switch on headlights, blower and rear defroster. Voltage should stay above roughly 13 volts. If it sags into the twelves under load, the alternator can no longer meet demand.
Alternator or battery?
| Clue | Points to battery | Points to alternator |
|---|---|---|
| Slow crank, then fine all day | Yes | No |
| Runs after jump, dies once cables removed | No | Yes |
| Battery light on while driving | No | Yes |
| Lights dim at idle, brighten on revs | No | Yes |
| Only fails after sitting overnight | Yes | No (or parasitic drain) |
Before you replace it
Check the belt and tensioner. A glazed serpentine belt or a weak tensioner will slip under load and mimic a failing alternator exactly. Check the main charging cable and the engine-to-body ground strap too — corrosion there produces low charging voltage with a perfectly good alternator.
On many modern cars the alternator is controlled by the engine ECU with a variable-voltage strategy, so voltage legitimately fluctuates. Scan for charging-system fault codes before condemning the unit.
Replacement cost
The unit itself typically runs from around $150 for a remanufactured common-application alternator to $400 and up for OE units on European cars. Labour ranges from under an hour on an accessible inline-four to several hours where the alternator sits low in the engine bay behind the subframe.
Remanufactured units from a reputable rebuilder with a lifetime warranty are usually a sound buy. Very cheap no-name alternators frequently fail again inside two years, and you pay the labour twice.
Frequently asked questions
- How long can you drive with a bad alternator?
- Once the alternator stops charging you are running on battery alone — typically 20 to 60 minutes with lights and accessories on, and much less at night in cold weather. Treat it as a get-to-the-shop situation, not a wait-until-payday one.
- Can a bad alternator kill a new battery?
- Yes, in both directions. An undercharging alternator leaves the battery in a chronic state of partial charge, which sulphates the plates. An overcharging one boils off electrolyte. Always test charging voltage after fitting a new battery.
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