Signs of a bad alternator

Alternator or battery: the fastest way to tell

The jump start row is the most useful line on this table, and it costs nothing to run. What the car does after the cables come off is the answer.

The eight signs, roughly in the order they appear

The battery warning light comes on while you are driving. Not at startup, which is a normal bulb check, but partway through a trip. This is the earliest reliable sign and the most ignored, because the light says battery and the battery seems fine.

Headlights that dim at idle and brighten when you rev. Sit at a red light at night and watch them. A charging system holding proper voltage keeps the lights steady regardless of engine speed.

A whine or a growl that rises and falls with engine speed rather than road speed. That is usually the alternator bearing, and sometimes a failing diode, which can also produce a faint hum through the radio.

Electrical accessories behaving oddly for no reason. Windows moving slowly, the radio resetting, the screen rebooting, the dash flickering, dashboard warnings appearing in clusters. Low voltage upsets modules in ways that look unrelated to each other.

A hot or burning smell, like scorched wire or hot rubber. That is either an alternator working itself to death or a belt slipping against a seized pulley.

The car starts off a jump and then dies again within a few miles. This one is close to definitive.

The battery goes flat repeatedly, and a new battery does the same thing within weeks.

And finally the car stops while driving and will not restart at all, which is where every ignored version of this ends up.

The jump start test, which costs nothing

This is the single most useful thing you can do before spending money, and it needs no tools beyond a set of cables and a second car.

Jump the car. Once it is running, disconnect the cables and let it idle.

If it keeps running comfortably, revs cleanly, and the lights stay bright, the alternator is generating power. Your problem is the battery, or something draining it while the car is parked.

If it dies within a minute or two of the cables coming off, or dies a mile or two down the road, the alternator is not carrying the load. The car was running on the jumper cables, then on stored charge, and when that ran out it stopped.

That result does not tell you whether it is the alternator itself, the belt driving it, or the wiring to it. What it does is separate charging faults from storage faults, which is the fork where most of the wrong parts get bought.

One thing not to do: the old trick of disconnecting the battery terminal while the engine runs to see if it keeps going. It was a reasonable test on a 1975 pickup. On a modern car it can spike voltage badly enough to take out control modules, and that repair is in a different price bracket entirely.

What actually fails inside one

An alternator is a small generator. The engine spins it with a belt, a rotor spins inside a set of windings, and that produces alternating current, which a set of diodes converts to the direct current the car uses. A regulator holds the output steady at around 14 volts regardless of engine speed or load.

Four things wear out. The bearings, which is the whine, and usually the first audible sign. The brushes, which are small carbon contacts that wear down over about a decade of use and produce charging that comes and goes. The diodes, where one failing diode leaves the alternator producing perhaps two thirds of its rated output, which is enough to look fine on a short daytime drive and not enough at night in the rain. And the regulator, which can fail low, leaving the battery slowly starved, or fail high, which overcharges and cooks a battery in weeks.

That partial failure mode is worth dwelling on, because it is the one that gets misdiagnosed. An alternator producing 60 percent of its output is not dead. It will pass a casual voltage check at idle in a quiet parking lot and fail on the highway with the wipers, lights, heater and rear defroster all running. Testing under load is what finds it.

The cheaper things that look identical

Before anyone quotes you an alternator, three things should be ruled out, and all three are far less expensive.

The serpentine belt and its tensioner. The belt drives the alternator pulley, and a glazed belt or a tensioner that has lost its spring lets it slip under load. Classic signature: the warning light appears at idle and clears when you rev. A belt costs a small fraction of an alternator.

The battery cables, terminals and grounds. Corrosion at a battery post adds resistance, and resistance in the charging path produces exactly the voltage readings a failing alternator produces. Cleaning a terminal takes minutes.

The battery itself. A battery with a shorted internal cell pulls system voltage down while the alternator works perfectly, lighting the same warning lamp. Load testing the battery separately is what catches this.

We check all three before quoting an alternator, because the alternative is you paying for the expensive part and still having the fault.

What we do at the car

We come to wherever the car is and test rather than infer. Resting battery voltage, then a load test on the battery. Charging output at idle and at raised rpm, with the electrical load switched on and off, which is what exposes an alternator that is weak rather than dead. Voltage drop across the main charging cable and the grounds. AC ripple, which is the specific test that catches a failed diode. Belt and tensioner condition. And a scan for charging codes, since on most modern cars the engine computer manages charging rate and records why it stopped.

You get the readings and a written price. The diagnosis is $95 and most take under an hour.

The alternator itself we quote by make and model once testing confirms it, because part cost and the labor to reach it vary enormously between cars, and a headline price that turns out to be wrong on your car is worth nothing to you.

Mobile alternator work across NYC

We replace alternators at the curb, in driveways and in parking garages across Flushing and the rest of Queens, Brooklyn, Manhattan, the Bronx, Staten Island and Long Island. No tow, and no leaving the car somewhere for two days.

Text us the year, the make, and what the car does after a jump start. That last detail narrows this more than anything else you can tell us, and we usually reply in under five minutes.

Common questions