How much does it cost to replace an alternator?

Alternator cost  £50-£600
Garage fee savings  £50-£300

How much could I save by replacing my own alternator?

Alternator replacement is very much model-dependent, with some replaceable by the average home mechanic within the hour but others requiring more than two hours. A main dealer can charge you up to around £150 for an hour's labour, so this is a job well worth doing yourself, with a Haynes Manual by your side.

Find a manual for your car here and start saving with Haynes

What does an alternator do?

A car alternator and drivebelt

A car alternator is responsible for generating power for electrical systems and recharging the battery.

The alternator is driven by the auxiliary drivebelt (sometimes it gets its own alternator belt), which is powered by the engine’s crankshaft. The alternator 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.

Replacing an alternator? A basic toolkit is all you need, and the job should take less than two hours, but you'll require some experience to do it. Your Haynes Manual will take you through the job.

What happens when an alternator goes wrong?

“A bad alternator will cause your headlights to dim, other car electrics will start to malfunction and you’ll have to jump-start your car to get it started”

Is the starter motor turning over slowly? Do you have to keep charging the battery, even though it was replaced recently or you know it's in good condition? Your alternator may be faulty, which means 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.

Other clues for when an alternator goes bad include the electric windows slowing down or the headlights being dimmer than normal. Basically, if you’re having electrical issues, but a new battery hasn’t fixed it, your alternator could well be to blame.

An alternator’s bearings cause most failures (it starts to get noisy when the engine is running), but the drivebelt that powers the alternator can also snap or become loose. If it's the latter, 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.

How long does an alternator last?

An alternator is designed to last the lifetime of the vehicle, which should be 20+ years if it's well maintained. However, nothing's guaranteed and parts can fail.

In the first instance, check the drive belt (as suggested above), because that is most likely to fail before the alternator itself.

Can an alternator be repaired?

An alternator is usually unserviceable and will need to be replaced. Without a functioning alternator, a car battery isn't much use. It'll 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.

How an alternator works

An alternator comprises a voltage regulator, a rotor, a stator and diode, with the electricity being produced by the rotor spinning within the stator. The diode converts the AC current to DC and the voltage regulator ensures the current is manageable for the battery.

The DC electricity 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.

How to test an alternator

How to test an alternator

Alternator testing is possible for 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). 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 engine revs up to 2,000rpm. If the reading is still below 13v, then your alternator is more than likely failing.

A modern, 'smart' alternator cannot be tested in this manner and requires kit such as an oscilloscope that a home mechanic is unlikely to possess.

Replacing 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 alternator replacement:

  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

Do I need to change my car alternator?

An alternator in an engine bay

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 to replace a car alternator

Mainly basic tools are required for replacing an alternator, 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