SEVERITY: MODERATE

Alternator Problems: Signs Your Alternator Is Failing

The alternator charges your battery and powers all electrical systems while the engine runs. When it fails, the battery takes over — but a battery alone only lasts 30-60 minutes. Recognizing alternator failure early prevents getting stranded.

What Is It?

The alternator converts mechanical energy (engine rotation via belt) into electrical energy (AC current, rectified to DC). It typically outputs 13.5-14.5 volts. When output drops below 13V, the battery begins discharging.

Common Causes

Worn Brushes

Very Common

Carbon brushes contact the rotor to transfer current. They wear down over time, reducing output. Often the first component to fail.

Failed Voltage Regulator

Common

The regulator controls alternator output voltage. A failed regulator may cause overcharging (burning out bulbs) or undercharging (dead battery).

Bad Diode(s)

Common

Diodes rectify AC to DC. A failed diode reduces output capacity and can cause a parasitic drain when the engine is off.

Worn or Loose Belt

Common

The serpentine belt drives the alternator. A worn or loose belt slips, reducing alternator speed and output. You may hear squealing.

Failed Bearing

Moderate

The alternator bearing supports the rotor. A failing bearing produces a grinding or whining noise that changes with engine RPM.

How to Diagnose

  1. 1

    With engine running, measure voltage across the battery terminals. Should be 13.5-14.5V. Below 13V = alternator undercharging.

  2. 2

    Turn on headlights, AC, and radio — voltage should stay above 13V. If it drops, the alternator can't handle the load.

  3. 3

    With engine off, voltage should be 12.4-12.7V (fully charged battery). Below 12V = battery is discharging.

  4. 4

    Listen for grinding, whining, or squealing from the alternator area.

  5. 5

    Check the serpentine belt for wear, cracks, or looseness.

  6. 6

    Look for the battery/charging warning light on the dashboard — this directly indicates a charging system fault.

Estimated Repair Cost

$20 (belt) to $500+ (alternator replacement)

When to See a Mechanic

If the charging light is on or voltage is below 13V with the engine running, have the alternator tested. Most auto parts stores will test it for free. Driving with a failing alternator risks getting stranded when the battery dies.

Frequently Asked Questions

How do I know if my alternator is bad?

Signs: dimming headlights, battery warning light, dead battery despite being new, electrical accessories acting erratically, grinding/whining noise. Test: voltage at battery should be 13.5-14.5V with engine running.

Can a bad alternator damage the battery?

Yes — both undercharging (deep discharge damages the battery) and overcharging (boils electrolyte, warps plates) can permanently damage a battery.

Related Parts

Shop alternators →Shop serpentine belts →Shop batteries →

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