The alternator charges the battery and powers the electrical system while the engine runs. The classic signs of a bad alternator are a battery warning light, dim or flickering lights, a battery that keeps going flat, a whining noise from the front of the engine and electrical accessories acting up. A running engine with a healthy charging system usually shows about 13.5 to 14.7 volts at the battery.
Bad alternator symptoms
- Battery or charging warning light on the dash, sometimes shown as a battery symbol or the letters ALT or GEN.
- Dim, flickering or pulsing lights, especially at idle or when accessories are switched on.
- Dead battery that starts fine with a jump but goes flat again after driving.
- Whining or grinding noise that rises with engine speed, from worn alternator bearings; see whining noise when accelerating.
- Burning smell from an overheating alternator or a slipping belt; see burning smell from car.
- Electrical glitches: slow power windows, a resetting radio or several warning lights at once.
- Stalling or rough running as voltage drops, and codes such as P0562 system voltage low or P0882 on the transmission module.
How to test an alternator
- With the engine off, measure battery voltage: about 12.6 volts means fully charged.
- Start the engine and measure again: about 13.5 to 14.7 volts means the alternator is charging.
- Turn on the headlights, blower and rear defroster: voltage should stay above about 13 volts.
- Below about 13 volts running, or the same as engine-off voltage, means the alternator or its wiring is not charging. Above about 15 volts means the regulator is overcharging, which damages batteries.
- Many auto parts stores will test the battery and alternator on the vehicle.
Alternator or battery?
A battery that will not hold a charge overnight but shows normal charging voltage while running is usually the battery itself. A battery that dies while driving, or charging voltage that stays near 12 volts, points to the alternator, a slipping or broken serpentine belt, or a corroded charging cable or fuse.
What causes alternators to fail
- Worn bearings and brushes with age and mileage
- Oil or coolant leaks dripping onto the alternator
- A slipping belt or a failing belt tensioner
- Overloading from a weak battery that the alternator must constantly charge
Can you drive with a bad alternator?
Only briefly: the car runs on battery power until the battery is drained, then stalls, often without much warning. Turn off accessories and drive straight to a repair shop. Low voltage also upsets engine and transmission modules, so harsh shifting or limp mode may clear once the charging system is fixed.
More guides: serpentine belt · how long car batteries last · battery light on · how to jump start a car · car battery dies overnight · alternator or battery? · bad ground symptoms.
Trouble codes: P0563 System Voltage High · P0620 Generator Control Circuit · P0622 Generator Field Circuit · all OBD2 codes.
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