The Main Symptom of a Bad Alternator

The most telling sign of a failing alternator is the battery/charging warning light on the dashboard (battery icon or “ALT/GEN”) staying illuminated while the engine is running, often accompanied by dim or flickering headlights. This light indicates the vehicle isn’t charging properly, which is the alternator’s primary job. Below is a deeper look at what that means, how to confirm it, and what else to watch for.

Why the Warning Light Points to the Alternator

Modern vehicles monitor charging system voltage; if alternator output drops too low (undercharging) or spikes too high (overcharging), the battery light turns on. While a faulty alternator is the most common cause, the light can also be triggered by a slipping/broken serpentine belt, corroded connections, a blown charging-system fuse, or a failing battery that drags the system down.

What Voltages You Should See

With a healthy alternator and good connections, you should typically see around 13.7–14.7 volts across the battery with the engine running. Engine off, a fully charged battery reads about 12.6 volts. If you see less than roughly 13.0 volts running (especially with accessories on), the system is likely undercharging; above about 15.0 volts suggests overcharging, both of which can illuminate the charge light. Note: some vehicles use smart charging and may briefly run closer to 12.5–13.5 volts under light load, but sustained low voltage under load is abnormal.

Other Corroborating Symptoms of a Bad Alternator

Beyond the dash light, several consistent behaviors strengthen the case that your alternator is failing. These reflect inadequate or unstable power supply while the engine is running.

• Dimming or flickering headlights and dashboard lights that change with engine RPM.
• Weak or erratic electrical accessories (slow power windows, infotainment resets, blower fan slowing).
• Engine stalling or rough running at idle as voltage to the ECU, fuel pump, or ignition drops.
• A dead or repeatedly drained battery after driving; the car runs after a jump but dies shortly later.
• Squealing noise or hot, burning-rubber smell from a slipping or seized belt/pulley.
• Whining or grinding noises from the alternator that rise with engine speed (bearing or diode issues).
• Diagnostic trouble codes such as P0562 (System Voltage Low) or P0563 (System Voltage High), or “Battery/Charging System” warnings.

These symptoms, especially when they cluster together, point toward inadequate alternator output or mechanical drive issues that prevent the alternator from charging effectively.

Quick Checks You Can Do

A few simple observations and measurements can help you confirm whether the alternator is at fault before committing to repairs.

1. Watch the battery/charging light at start-up and while driving; note if it brightens at idle and dims with higher RPM.
2. Measure battery voltage: about 12.4–12.7 V engine off; around 13.7–14.7 V running. Persistently below ~13.0 V running indicates undercharge.
3. Turn on loads (headlights, rear defroster, A/C). Voltage should remain near or above ~13.3 V; a dip into the 12s suggests alternator trouble.
4. Inspect the serpentine belt for cracks, glazing, looseness, or contamination; ensure the tensioner keeps it tight and the alternator pulley spins freely.
5. Check alternator and battery connections for corrosion or looseness; verify related fuses/fusible links.
6. Scan for codes with an OBD-II tool; look for voltage-related DTCs or manufacturer-specific charging faults.
7. If available, test AC ripple (excess ripple indicates bad diodes) or have a parts store perform a charging-system test.

If voltage tests and inspections point to low or unstable output with an intact belt and good connections, the alternator or its internal regulator is the likely culprit.

When It’s Not the Alternator

A weak or sulfated battery can also trigger the battery light and cause dim lights or stalling, especially in cold weather. Likewise, a loose or slipping belt, corroded terminals, or a failing battery cable can mimic alternator failure by starving the system of charge. Rule these out with a battery load test, belt inspection, and a close look at wiring before replacing the alternator.

Summary

The main symptom of a bad alternator is the battery/charging warning light staying on while the engine runs, often with dim or flickering lights. Confirm by checking system voltage: healthy charging is roughly 13.7–14.7 V with the engine running. If voltage is low or unstable and the belt, battery, and connections are sound, the alternator is the most likely source of the problem.

What sound does a failing alternator make?

A failing alternator can make a whining, grinding, or rattling noise, often caused by a worn-out bearing, and a squealing or screeching sound due to a loose or worn serpentine belt. You might also hear a clicking or buzzing sound from internal electrical components or see a burning smell and dimming lights as other symptoms. 
Sounds to Listen For

• Whining or Grinding: This is a common sign of a worn or damaged bearing inside the alternator. 
• Squealing or Screeching: A high-pitched squeal, especially when accelerating or turning, usually indicates a loose or worn serpentine belt that drives the alternator. 
• Rattling or Growling: Similar to a grinding noise, this can also point to issues with the internal bearings. 
• Clicking or Ticking: Worn internal components like brushes or diodes can cause a clicking sound. 
• Buzzing or Crackling: Electrical issues within the alternator, such as with the voltage regulator, can create buzzing noises. 

Other Symptoms

• Dimming Lights: Opens in new tabHeadlights, interior lights, or dashboard lights may dim when the alternator isn’t supplying enough power. 
• Burning Smell: Opens in new tabA burning smell of rubber or electrical components can be a sign of a failing alternator or a slipping belt. 
• Dead or Difficult-to-Start Battery: Opens in new tabA failing alternator won’t recharge the battery, leading to a drained battery and difficulty starting the car. 
• Dashboard Warning Light: Opens in new tabThe battery or charging system warning light on your dashboard will often illuminate. 

If you notice these sounds or symptoms, have a professional inspect your vehicle to diagnose and fix the problem.

How to tell if it’s battery or alternator?

To tell if you have a bad battery or alternator, start the car and use a voltmeter to check the voltage. If the voltage is consistently above 14 volts, your alternator is likely good and the battery is the problem. If the voltage is below 13.5-14.5 volts, the alternator is likely failing and not charging the battery properly. Alternatively, you can observe the engine’s behavior: if it shuts off when you disconnect a jump-start cable or if the lights dim significantly and then brightens, the alternator is likely bad. 
Signs of a Bad Battery 

• Slow or no engine crank: The engine makes a “rurr, rurr, rurr” sound but doesn’t start.
• Clicking sounds: when trying to start the engine.
• Dimming headlights or interior lights .
• Frequent need for jump-starts .
• Illuminated battery or check engine light: on the dashboard.
• A swollen or bulging battery case.

Signs of a Bad Alternator 

• Car stalls or shuts off frequently: after starting.
• Headlights flicker or are overly bright .
• Dim or bright lights: that change with engine RPMs.
• Whining, squealing, or growling noises: from the engine.
• A burning smell of rubber or wires.
• Electronic features like the radio or power windows malfunction.
• A dead battery that keeps dying.

How to Test with a Voltmeter

1. Start the engine: and let it run. 
2. Connect the voltmeter: to the positive (+) and negative (-) terminals of the battery. 
3. Check the voltage reading . 
   • Good Alternator: Voltage should read between 13.5 and 14.5 volts. 
   • Bad Alternator: Voltage is low (below 13.5 volts), pulsing, or much higher than normal. 
   • Good Battery: Voltage should be around 12.8 volts with the engine off. 

If You Suspect a Bad Alternator
A failed alternator cannot charge the battery. To confirm, you can try this test with the engine running: carefully loosen and disconnect one of the battery terminals. 

• If the engine continues to run, your battery is bad.
• If the engine shuts off, your alternator is bad.

What is the most common problem with an alternator?

The most common issues with an alternator include worn bearings, malfunctioning voltage regulator, and corroded cables.

How do I know when I need a new alternator?

You need a new alternator if you see a battery warning light, notice dim or flickering lights, experience difficulty starting or stalling, hear strange noises like a whine or grinding, smell burning rubber, or have continuously dead batteries. These signs indicate the alternator isn’t properly charging your car’s battery and electrical system.
 
Visual and Electrical Symptoms

• Battery Warning Light: The most common sign is a glowing battery-shaped warning light on your dashboard. 
• Dim or Flickering Lights: Headlights, interior lights, and other electrical components may dim or fluctuate because the alternator isn’t providing consistent power. 
• Malfunctioning Accessories: Power windows, radios, or air conditioning might work poorly or not at all. 
• Repeatedly Dead Batteries: If your car’s battery keeps dying, the alternator may not be recharging it sufficiently while the engine is running. 

Performance and Starting Issues

• Difficulty Starting: The engine might crank slowly or struggle to turn over. 
• Engine Stalling: The vehicle can stall while driving due to insufficient electrical power to components like the spark plugs. 

Auditory and Olfactory Clues 

• Strange Noises: Listen for whining, screeching, or grinding sounds coming from the engine, particularly from the alternator area.
• Burning Smell: A smell of burning rubber or wires can indicate the alternator is overheating.

Next Steps
If you notice any of these symptoms, it’s crucial to have your alternator checked. 

• Battery and Charging System Test: Opens in new tabVisit an auto parts store like AutoZone.com or a trusted mechanic for a free starting and charging system test to confirm the issue. 
• Voltage Check: Opens in new tabA professional can use a multimeter to check the battery’s voltage when the engine is running; it should read around 14 to 14.5 volts.