What is the lifespan of an alternator?
A car battery is, on its own, not much use. It would only last about 30 miles in daytime driving conditions, even less at night when you need the headlights. The battery in a car is not there to provide endless power like the batteries in a torch. Instead, it’s there to receive power, hold onto it for a bit, and then send it out into the car. But it can only do that if the alternator is in good health.
The alternator is what generates your car’s electricity. Driven by the engine, the alternator is in effect a small generator that spins internally As it does, it generates electricity.
This is used by the car for various things, and it’s sent to the battery to keep it topped up for extra duties the alternator can’t help with, such as starting the car, or keeping things operating when the engine is turned off.
An alternator is designed to last the lifetime of the vehicle.
How will I know when the alternator goes bad?
Is the car hard to start? Do you have to keep charging up a battery that you know is in good condition? If you’re nodding along, it could be because your alternator is on the way out.
If the electric windows have slowed down or the lights seem to be dimmer than normal, they are also signs an alternator is on its way out. Basically, if you’re having electrical issues, but a new battery hasn’t fixed it, your alternator could well be to blame.
You can test older-style 'fixed-voltage' alternators with a multi-meter. Set it to 20V, start the car, put the red probe on the positive terminal of your alternator (consult your Haynes manual) and put the negative to earth (not the negative on the alternator, use a body bolt or chassis part). The reading should be around 13-14v. If it’s much higher, the voltage regulator is failing.
If the reading is much lower, try bringing the car up to 2,000rpm and see what the reading is. If it’s still below 13v, then your alternator is more than likely failing.
A modern, 'smart' alternator cannot be tested in this manner and requires equipment such as an oscilloscope that a home mechanic is unlikely to possess.
When to change your alternator
“Your headlights will dim, other car electrics will start to malfunction and you’ll have to jump-start your car to get it started”
An alternator’s bearings cause most failures (it starts to get noisy when the engine is running), but the drivebelt can snap or become loose.
When the alternator isn’t working your battery isn’t being charged (you should see a red warning light or other message on your dashboard) and it’ll go flat quickly - your headlights will dim, other car electrics will start to malfunction and you’ll have to jump-start your car to get it started. But this won’t work for long before the battery fails completely.
If the alternator glitch has been caused by drivebelt failure, you should be able to get away with tightening or replacing the belt, but if the alternator is faulty it should be replaced without delay.
All cars are slightly different, so if it is time to change your alternator, use our before you begin checklist, and find your car for specific instructions.