How a car’s alternator works

where alternator is used

A car’s alternator is responsible for generating power for various systems (headlights, radio, heater motor, heated rear window etc) as well as recharging the battery, and without it your car won’t run for long.

The alternator is driven by the auxiliary drivebelt (sometimes it gets its own alternator belt), which is powered by the engine’s crankshaft. It turns mechanical energy into electrical energy in the form of an AC current, which has to be turned into DC current via diodes for use in the car’s electrical systems.

This task requires experience. A basic toolkit should suffice though, and the job should take less than two hours, depending on your model.

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.

How to change your alternator

Watch this video to see how it's done. Find the full step-by-step task for your model.

A very brief summary of the task:

  1. Disconnect the negative lead from the battery
  2. Depending on your model, you may need to remove other engine parts to gain access to the alternator. Haynes will tell you how to do this
  3. Disconnect the alternator’s electrical connections and mounting bolts. Remove it
  4. Make sure the new alternator matches the old one and has a pulley fitted. Install it and replace any components removed earlier

Why you should change your alternator

are alternator repairs expensive

An alternator is crucial to a car’s operation, especially in a modern car where electronics such as processors govern so many systems. Without the electricity generated by the alternator, the car won’t run for long.

The alternator keeps the battery charged. This is important not only for starting the engine but also for running power-heavy systems when the car is running, such as heated seats, heated screens, ventilation motors etc.

Tools you will need

Mainly basic tools are required for this job, although you’ll need a torque wrench and you may need to raise the car to remove the undershield/wheelarch liner.

  • Floor jack (not your car’s emergency jack)
  • Axle stands
  • Ratchet and socket set
  • Torque wrench
  • Flat-bladed/Phillips/torx screwdriver

How much does an alternator cost to replace?


Alternator  $90-$1100
Garage fee savings  $350