The alternator is the part that makes electricity while the engine runs. It powers your lights, radio, and electronics and recharges the battery as you drive. Common signs it's failing include a battery or charging warning light that comes on while driving, headlights that dim or flicker, a whining or grinding noise from the front of the engine, electrical features acting up, and a battery that keeps going dead even after it's replaced. If the charging light comes on while you're driving, the car is running on battery power alone and may stall, so head to a safe stop. Hillside Auto Clinic at 26122 Western Ave, Lomita, CA 90717 can test your charging system; call (310) 956-5794 or schedule.
What the alternator does
Your battery's main job is to start the engine. Once the engine is running, the alternator takes over. It's driven by the serpentine belt, and it produces the power your car uses while you drive, including headlights, wipers, the blower fan, the infotainment screen, and the engine computer. Whatever is left over goes back into recharging the battery.
When the alternator weakens, the car starts pulling power from the battery instead. The battery can only carry the load for so long, which is why alternator problems often look like battery problems at first.
Warning signs of a failing alternator
- Battery or charging light on while driving. On many cars the light is shaped like a battery, and on some it reads ALT or GEN. It usually means the charging system isn't keeping up, not just that the battery is low.
- Dim or flickering lights. Headlights or dash lights that dim at idle and brighten when you rev the engine, or that pulse while you drive, can point to weak charging output.
- Electrical features acting strange. Power windows moving slowly, a radio that resets, a blower fan that slows down, or a screen that flickers can all be signs of low voltage.
- Whining, growling, or grinding from the engine bay. A worn bearing inside the alternator can make a whine that rises with engine speed, or a rougher grinding sound.
- A smell of hot wires or burning rubber. An overworked alternator can run hot, and a slipping belt on the alternator pulley can smell like burning rubber. Our guide to burning smells covers other causes.
- A battery that keeps dying. If you've needed several jump-starts, or a newer battery went flat again within days, the alternator may not be recharging it.
- Stalling or trouble restarting. As voltage drops, the engine computer, fuel pump, and ignition system may not get the power they need, and the car can stall.
- Several warning lights at once. Low voltage can cause unrelated lights to come on together, such as ABS, traction control, or the check engine light.
What to do if the battery light comes on while driving
If the charging light comes on while you're on the road:
- Turn off what you don't need, like the A/C or heater fan, the radio, seat heaters, and phone chargers, so the battery lasts longer.
- Watch the temperature gauge. On many cars the same belt that turns the alternator also turns the water pump. If the belt broke, the engine can overheat quickly. See our guide to engine overheating.
- Avoid shutting the engine off until you're somewhere safe to stop, since the car may not restart.
- Get to a safe spot or the shop. If the lights are dimming fast, the gauge is climbing, or the steering suddenly gets heavy, pull over and arrange a tow.
How far you can drive depends on the battery's charge and how much power the car is using. It could be a short distance, especially at night with the headlights and wipers on, so don't count on making a long trip.
Alternator or battery? How to tell the difference
The two work together, so one can look like the other. Some general patterns:
- A car that won't start but runs fine after a jump, then starts normally for days, often points to the battery or a drain while the car is parked.
- A car that starts after a jump but dies again soon after, or shows the charging light while running, points more toward the alternator or the charging circuit.
- Lights that dim while driving point more toward the charging side than the battery.
Our post on car battery warning signs covers the battery side in more detail. The only way to know for sure is a test of both the battery and the charging system, which takes the guesswork out before any parts are replaced.
Other causes that look like a bad alternator
Not every charging problem means a new alternator. Other common causes include:
- A worn, loose, or slipping serpentine belt, which can't spin the alternator properly.
- A weak belt tensioner that lets the belt slip under load.
- Loose or corroded battery cables and ground straps, which block current even when the alternator is fine.
- A blown fuse or fusible link in the charging circuit.
- Wiring or connector damage at the back of the alternator.
- A failing battery that can't hold a charge, which makes the alternator work harder.
Why alternators wear out
Alternators have bearings, brushes, and electronic parts that wear with time and miles. Heat under the hood, oil or coolant leaks dripping onto the alternator, and a battery that's on its way out can all shorten its life. Lots of short trips around Lomita and the South Bay with the A/C, lights, and accessories running also put a steady load on the charging system.
What we check
A technician starts by testing the battery's condition and charge, then measures charging output with the engine running, both at idle and with electrical loads turned on. We inspect the serpentine belt, tensioner, and pulleys, check the cables, grounds, and fuses in the charging circuit, and listen for bearing noise at the alternator. We'll show you what we found and explain your options before any work is done.
Hillside Auto Clinic is open Monday–Friday 8 AM–6 PM and Saturday 10 AM–3 PM (closed Sunday). Visit us at 26122 Western Ave, Lomita, CA 90717, call (310) 956-5794, or email hillsideautoclinic@gmail.com.
Call or schedule
Seeing a battery light, dimming headlights, or a battery that won't stay charged in Lomita, Torrance, Harbor City, or anywhere in the South Bay? Call (310) 956-5794 or book online.
FAQ
How long can I drive with a bad alternator?
It depends on how charged the battery is and how much power the car is using. Some cars run for a short while on battery power alone, but it can be a very short distance at night or in the rain. It's best to drive straight to a safe stop or the shop.
Can a bad alternator drain a new battery?
Yes. If the alternator isn't recharging the battery, even a brand-new battery will run down. In some cases a faulty alternator can also draw power while the car is off.
What does a failing alternator sound like?
A whine that rises and falls with engine speed, or a growling or grinding sound from the front of the engine, can come from worn alternator bearings. A squeal on startup is more often the belt.
Will a jump-start fix a bad alternator?
No. A jump-start can get the engine running, but if the alternator isn't charging, the battery will run down again and the car may stall.
Does the battery light always mean the alternator is bad?
Not always. The light means the charging system isn't working the way it should. The cause can be the alternator, but also a slipping belt, loose cables, a blown fuse, or wiring trouble. Testing tells you which one it is.
