The usual signs of a bad alternator are a battery warning light while driving, dimming or flickering lights, a battery that keeps going flat, and a car that dies while running. A mobile mechanic can confirm it with a charging test and, on most cars, replace the alternator where the car is parked in one to three hours.
Symptoms of a failing alternator
- Battery or charging light on the dash while driving
- Dim or flickering headlights, brighter when you rev the engine
- Electrical glitches: weak blower, flickering radio, power windows slowing down
- Dead battery, again and again, even a new one
- Car stalls while driving, often after the lights dim
- Whining or grinding from the front of the engine (worn bearings)
- Burning smell from an overworked alternator or a slipping belt
Confirming it's the alternator
Battery and alternator problems look alike. A proper test checks:
Battery resting voltage and a load test
Charging voltage with the engine running, roughly 13.7 to 14.7 volts on most cars
Output under load, with lights and blower on
The belt and connections
Our guide to bad battery or bad alternator? explains the voltage check.
The belt matters too
The alternator is driven by the serpentine belt. A worn, cracked or loose belt can make a good alternator undercharge, and it often squeals while doing it. See serpentine belt squeal.
How mobile alternator replacement works
Disconnect the battery
Release belt tension and remove the belt
Unbolt and unplug the old alternator
Fit the new or remanufactured unit, refit the belt, reconnect
Test charging voltage at idle and under load
On some vehicles the alternator sits low or behind other parts, which adds time.
What it costs
Broadly $400 to $1,000 installed for many cars and light trucks, depending on the part (new vs. remanufactured), the vehicle and how hard it is to reach. High-output units and some European cars cost more. Get a written quote.
New or remanufactured? Remanufactured alternators are rebuilt with new wear parts and often carry good warranties at a lower price. Ask which you're getting and what the warranty covers.
Can I drive with a bad alternator?
Not far. The car runs on stored battery power until it runs out, then shuts down, possibly in traffic. Turn off everything you can and head somewhere safe, or have a mechanic come to you.
Related problems
If the car only clicks or won't crank, the starter is worth checking too. And if you're seeing other warning lights, our guide to dashboard warning lights explains what each one means.
Frequently asked questions
Will a new battery fix a bad alternator?
No. The new battery will drain just like the old one.
How long does an alternator last?
Often a long time, sometimes the life of the vehicle, but heat, heavy electrical loads and age wear them out.
Can a mobile mechanic replace an alternator in a parking lot?
On most vehicles, yes. It's a standard mobile repair.
Book an alternator test
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