Bad alternator symptoms: is it the alternator or the battery?

You head out early for work, the car won't start, a neighbor gives you a jump and you buy a new battery. Two days later, stuck in traffic on the General Cañas highway, the battery light comes on and the car starts acting up. Often the problem wasn't the battery at all but the alternator, the part that charges it. Here's how to tell the two apart and what to check before you spend money.

What does the alternator do?

The alternator makes electricity while the engine is running: it recharges the battery and powers the lights, the radio, the A/C and the car's computer. The battery only gives the push to start the engine; after that, everything depends on the alternator.

That's why a bad alternator ends up leaving you with a dead battery: the car keeps running for a while on whatever charge is left, until it runs out.

What are the symptoms of a bad alternator?

The clearest sign is the battery light coming on while the engine is running. These are the most common signs:

1. Battery or charging light on while you drive. Some cars show it as "ALT", "GEN" or "CHARGE".
2. Lights that dim or flicker, especially at idle, and brighten when you rev the engine.
3. A battery that keeps going flat, even a new one.
4. The engine dies while you drive, or stumbles when you turn on the A/C, lights and radio together.
5. Slow power windows, a radio that resets or gauges that act strangely.
6. A whine or squeal that rises with engine speed, which can come from the belt or the alternator's bearings.
7. A burning or hot-rubber smell near the engine.

None of these proves on its own that the alternator is at fault. Have a mechanic confirm it before replacing the part.

How do I tell the alternator from the battery?

Notice when the car fails: trouble starting points to the battery; trouble while driving points to the alternator.

What happens	Most likely
Won't start, but everything works fine once it's running	Battery
Starts with a jump and dies soon after, or won't start again the next day	Alternator
Battery light on with the engine running	Alternator or belt
Lights dim at idle and brighten when you rev	Alternator
Battery several years old, corroded terminals or a swollen case	Battery

The quickest test is a multimeter. With the engine off, a healthy battery reads around 12.6 volts; with the engine running, the alternator should bring it up to roughly 13.5 to 14.5 volts. If the reading stays the same or drops with the engine running, the alternator probably isn't charging. Exact ranges vary by car, so have your mechanic or a battery shop confirm it with their equipment.

What else can cause the same symptoms?

You don't always need a new alternator. Before you buy one, ask for these to be checked too:

• The alternator belt (or serpentine belt): if it's loose, cracked or slipping, the alternator can't spin properly and won't charge.
• The battery terminals and cables: a corroded or loose terminal cuts off charging even when everything else is fine.
• The ground strap from the engine to the body.
• The fuses for the charging system.
• A hidden drain, like a light that stays on or a badly installed accessory, that empties the battery while the car is parked.

In Costa Rica, heat, heavy rain and short trips in traffic also make the battery and alternator work harder. If your car has a lot of years on it and the alternator has never been replaced, it's one of the parts that wears out with time.

Should I buy a new, remanufactured or used alternator?

All three can work well, as long as the part comes from a reliable source with a warranty. A remanufactured alternator is a used one rebuilt with new wear parts, and it usually costs less than a new one. We cover the differences in our guide to new, used, OEM or aftermarket parts.

What do I need to get an alternator quote?

The same model can use different alternators depending on the engine and equipment, so the more details you give us, the better:

• Make, model, year and engine size.
• The VIN or chassis number. Here's where to find your car's VIN.
• A photo of the old alternator, especially its label and pulley.
• Whether you want new, remanufactured, or to see options.

We import it to order from the US: it usually arrives in 1 to 3 weeks, and the price we quote already includes freight, duties, IVA and our service.

In short

If the car fails while it's running, the battery light comes on, or a new battery goes flat again, suspect the alternator. Before replacing it, have the voltage measured and the belt, terminals and cables checked. Once your mechanic confirms it's the alternator, request your free quote and we'll send you the price in colones for your car.

Frequently asked questions

Can I drive with a bad alternator?

Only briefly, and as short a trip as possible, to the shop. The car runs on what's left in the battery and can die at any moment, including on the highway.

Can a bad alternator damage the battery?

Yes. If it undercharges, the battery gets drained deeply again and again, which shortens its life. If it overcharges, that can damage it too. That's why it's worth checking both.

How long does an alternator last?

It depends on the car, how it's used and the climate, so there's no fixed number. Many last for years, but it's one of the parts that most often wears out on high-mileage cars.

How long does an alternator take to arrive?

Usually 1 to 3 weeks, because we import it to order from the US. If it's urgent, tell us when you ask for a quote.