Alternators, how to replace a faulty one

Changing your car alternator

Avoid high costs when replacing your car alternator by doing it yourself!


how to replace your alternator, alternator repairs

Alternator Problems

There are a few tell-tale signs that your alternator may be on the way out. If you notice any of the following signs then I suggest you check your alternator over to see if it would be wise to replace your car alternator.

Head lights dimming while driving

If you notice your headlights are not what they used to be and seem to be dimmer or flicker then this could be either a fault with the head lamps, bulbs or perhaps its just the alternator playing up.

Strange idling

You may notice you car juddering or the revs randomly fluctuating. This can be a range of things but before you do anything “expensive” check the alternator first.

Charge won’t hold

One of the more noticeable ways of telling if you have a faulty alternator is if the battery won’t hold a charge. It’s always best to check your alternator first with a multimeter than to buy a new battery. I made this mistake wasted money on a new battery that wasn’t needed.

Battery Light flicks on and off at low RPM’S

This could be because the alternator is struggling to keep a consistent flow of power. This may be down to the bearing going bad on the alternator are marks on the belt it’s self.

Battery Light is on all the time

9/10 times this is down to the alternator or a bad battery. Check both and you will soon find out what’s at fault. Oh, and don’t drive for too long if your battery light is on. Because if it’s a modern car and the alternator has gone you’ll last about 10 minutes if that before the battery is drained.

These can all be signs that your alternator may be about to give in. Of course there may be other reasons why these faults have a risen in your car but its best to test your alternator and learn how to replace your car alternator if necessary.

Before you begin

Changing an alternator at home can seem like a very daunting job, but we assure you 9/10 times all you need to do is remove the belt a few bolts and cables and swap the alternator over. In some cases you will need to remove the bumper (especially if it’s a Renault).

If this is the first task you’re going to undertake and have not touched a socket set yet, then please make sure you read this guide carefully and make sure you are comfortable doing the job, looking through your car’s manual may help out. Personally this was the 2nd job I did on my car as the serpentine belt had snapped and the bearing of the alternator had seized. The 1st job I had done was swapping a faulty head light bulb, only do the job you feel comfortable doing.

Tools you will need to test, replace or fix your car alternator

There are plenty of options you have when it comes to alternators. You can start by testing them to see if the alternator is at fault and if you are sure it’s faulty then it comes to the fun bit you can either replace your car alternator or decide to fix your car alternator. Personally I would choose to replace it every time. I tried fixing one once and spent around £20.00 on a bearing and brushes to find out it didn’t work at all, the brushes are really hard to find these days as most people send their alternators off for refurbishment. The funny thing is the replacement alternator cost me £30.00 from a scrap yard and lasted the life of the car (100,000+ miles) so yeah, sometimes it’s better to just replace your car alternator.

The correct service manual for your vehicle’s make and model.

You should most definatly invest in one of these it will help you in the long run. I get all mine from The Haynes website

Tools and equipment as outlined in your vehicle’s service manual.

This can vary from vehicle to vehicle as there may be certain pipes/hoses or parts of the car that make it more difficult for you to access and remove the alternator. This is a simple check list of the tools you may require to remove your car alternator. Please double check with your manual if any specialist tools are needed.

 A Socket set

If you don’t have one by now I suggest you go out and buy one there dirt cheap and last quite a long time. For this job id recommend a standard set. The bolt will be around 14mm – 20mm depending on the car your working on. You may need a small extension bar for some hard to reach bolts but normally the socket tool will do the trick. Something like this would do the job, there are a lot more cheaper options out there, so look around before committing.

A prying bar

Alternator repairs, prying bar, belt repairs, serpentine belt repairs, drive belt repairs

Prying Bar

Not always needed but in some cases it helps with removal and refitting of the alternator and alternator belt.

Wrench Set

The alternator tensioner can sometimes have a strange setup. Where the use of a wrench and Allan key is needed. There is no generic size for these either. I suggest looking at the tensioner first if you can and seeing how big it is. The last one I fixed was quite large and a normal wrench set wouldn’t fit the socket. See the below picture:

Drive Belt replacement, serpentine belt replacement, alternator replacement

How to change a serpentine belt

As you can see it requires the use of a wrench of the outside and a Allan key in the middle you turn one clockwise and the other anticlockwise to release tension. It can be quite stiff if its not been changed in a while.

Hex/Torx Allan keys

Just pick up a full set of keys they come in handy for all kinds of jobs. You can pick hex keys up at pound shops these days so there not too expensive. I forked out for a HEX, TORX combi and it’s served me well for sometime it was under £10.

The Correct Replacement Alternator

Now this is not as always straight forward as it may seem as for some bizarre reason some manufacturers of cars like to change alternators as they please and well lets just say because it fits one car it may not fit the same. I’ve seen cars that always use Bosch alternators and then for some strange reason one year they decided to use Valeo. So you really can’t just guess this. One of the best ways I know, on how to find out what you car needs is to visit euro car parts. Enter your registration number and look for the alternator that bests matches your car. If you live in the Cheshire area and want a 2nd hand alternator you can always try Car Transplants. 9/10 they have the parts you need but their customer service isn’t very good. Just get in and out as fast you can.

If your buying a 2nd hand alternator from a scrap yard they normally require you to do a trade. Where as for some reason I got charged £5 for it to be “recycled” utter nonsense. Its free scrap metal to them. Just remember this as if you don’t trade they either don’t allow it or put an extra charge on top of the item.

New alternator belt.

You might aswell swap this over while your replacing your alternator there not too expensive. Just make sure before you change your alternator that you write down the way the belt went on. This makes it so much easier to put back on later. If you haven’t because it’s snapped then just remember that the grooved side goes on the grooved pulleys and the smooth side goes on the smooth part of the pulley. Or if your really struggling there is normally a “brief” picture in the Haynes manual on how to put it back on, they normally only go on one way.

Memory Saver

Some people suggest a tool known as a memory saver. Never really used one as I allready have my raido codes to hand. But if your scared of losing any data on your car then maybe it might be best to invest in one. This tool plugs into your cigarette lighter.

Please note:

The alternator is not a “stock” item on a vehicle. The location, parts, tools, and skills needed to change it differ from vehicle to vehicle. We have tried to generalise this guide to suite most vehicles. But if your alternator is slightly different it will state it in your car repair manual.

Although it may seem unnecessary to get a new belt even if the belt looks “good”. It’s common practice to replace these belts every 15,000 to 30,000 miles as they do stretch, tear and get damaged over time. So unless you just got a brand new belt put on it’s probably time to swap it over. It may be down to the belt that your alternator has become faulty.

How to test your car alternator


Before any unnecessary expense you really should test your car alternator. All you need is a multimeter any one will do. You can get some decent ones off amazon or eBay. Or just visit your local Maplins for a basic one. Your looking at around £5-£10 for one no need to break the bank with these.

This is a very simple test to do to make sure your alternator is working correctly. It will also tell you if its just the battery that’s faulty.

To test the alternator attach your multimeter leads to the battery terminals. The red lead goes on the positive terminal and the black lead goes on the negative terminal. Most modern cars are colour coded or have markings on the battery box. An easy way to see whats positive and negative is to follow the cables them self. If one side seems to attach to metal parts of the car this is the grounding cable, the negative cable. Double check with your owners manual if your not sure.

Once they are connected switch your multimeter to the voltage setting. You should get a reading of around 12.50 volts if not then your batterys either dead/dieing or the alternator has not charged the battery up at all.

Now start up the car it should jump to around 13.50 to 14.50 if it doesn’t turn on the headlights and boost the throttle (of course make sure your in neutral). If it doesn’t budge at all then it’s safe to say it’s the alternator that’s the fault here.

Fixing your car alternator

Fixing you car alternator can be both time consuming and complicated. We don’t recommend  fixing an alternator unless you love taking things apart or you have prior experience in fixing a car alternator.

If you want to fix your car alternator then see this guide by Econofix.

Please note you will need to purchase a fair few items to refurbish you alternator and if some parts are just plain broken it would not be worth the cost to repair it. You will also need to have some knowledge on using a soldering iron.

Replacing your car alternator

This is normally the chosen option when it comes to alternators as it’s most likely the most cost effective if you can get hold of a 2nd hand alternator.

If you already have your alternator you can dive straight into this guide but if your off to purchase a 2nd hand one then you will need to remove the alternator first and have it exchanged at a scrap dealer or 2nd hand parts specialist.

Removing the Old Alternator

As mentioned before you can use a tool known as a memory saver insert this now if you have one. As changing an alternator does take an hour or so and in that time it will reset the codes on the radio. Make sure you know your back up radio code.

This guide does not require the use of jacking up your vehicle but this may be the case on some vehicles. I have worked on a few that require you to remove the front bumper as it’s the only way to squeeze the alternator out. You may be lucky. Removing the front bumper is a straight forward task. You don’t nee to remove the vehicles wheels but it is best to jack it up so you can get the screws and bolts under the car. If you need to remove the front bumper see your car manual on how to do this. Believe me you will know if you need to do it.

Remove the Negative Lead From the Battery

Open the hood and locate the battery. It may have a housing cover over it. Remove the top housing so you can get the to the leads. Check both terminals you should either see markings to say is positive/negative or it may be colour coded. If you can’t see anything follow one of the leads and see where it goes. If it connects to parts of the cars chassis and not to any of the main circuit boxes. Then it’s safe to say that’s your negative terminal.

Remove the Alternator Belt

Before you begin make sure you know how the belt is going to go back on do not put it on incorrectly. This is just a brief guide if you wan’t a more in depth guide please visit our “How to replace a serpentine belt guide“.

This can be one of the trickier parts of replacing your alternator. The Alternator belt also known as the drive belt or serpentine belt. Is kept in place by a tensioner. The tensioner helps keep the belt from being too lose or too tight. Some tensioners are harder to remove than others. I find it easier to remove the drivers side wheel to do this but it’s not necessary.

From the top of the car your going to want to locate the tensioner you can feel for it its a pulley wheel located on the belt that is smooth all around. It’s normally just a couple of inches wide and feels like it’s made of plastic/rubber. You can tell it’s a tensioner by the way it’s lent against the belt. You well see it’s keeping the belt in place.

To remove the tensioner you will need a wrench and maybe an Allan torx/hex key depending on the tensioner your working on. First locate the correct wrench and place it around the bolt. Now with your other hand feel for the hex/torx there may be a cap covering the tensioner bolt just tap this off if there is one. Some require a hammer and a screwdriver others just pop right off.

Once you have found the correct parts on the tensioner. With the wrench around the bolt and the torx key in the hole turn the wrench counter clockwise and the torx key clockwise. Be careful not to remove the tensioner completely. It can be quite fiddly to start with but once its loose you will be able to remove the belt with ease. Discard this belt if it’s old.

Remove any necessary parts to reach the alternator

This may just be a few wires or hoses, cable ties can be of use here. Or it may be the bumper that needs to come off, I shall let you decide with this one.

Disconnect all electrical connections (wires) from the alternator

There isn’t many wires that connect to the alternator there maybe 2 to 3 connections. Just be careful when removing the connections so you don’t damage the cables. Make sure you remove the bolt first when removing the big black wire (negative lead). Do not loose the bolt! This bolt is crucial to making the alternator belt work. As I found out when mine wasn’t tightened correctly. You need the bolt to bind the metal clips together to produce a full circuit or your alternator wont work.

Now remove any mounting bolts that hold the alternator in place

This can be quite confusing. Feel around the alternator and remove the bolts one by one there is normally around 3 – 4 bolts make sure you have them all. Now, if your alternator is old and has been sitting in a car for a while chance are it’s bonding to the metal. It may look like it won’t remove or maybe too dirty for you to see the seam. Your going to want to ease your alternator off the engine. With a hammer gently tap the alternator upwards to see if it budges. Then do a bit of wiggling back and forth you’ll soon see the seam between the alternator and the engine frame. It can be quite a pain to remove one at times if it’s old. Some just pop right out. Wiggling it out of the frame is the toughest part, well especially if there is no room for you to work in. Just keep reminding yourself if it went into the car it sure as hell can come out of it.

Checking your Alternators

Now get your new/refurbished/2nd hand alternator and compare the two make sure they look similar and have their electrical points on the same sides. If not you’ve got the wrong alternator and it’s not good. In most cases it wont work, or just wont fit into the car. Always make sure you have the correct product before continuing. One of the main factors is to make sure it has the correct number of pins on the electrical socket.

How to install the replacement alternator

Now this is a lot easier. You should now be very familiar with the car and know what was the best method of getting your alternator out. Now use this new knowledge you have gained to reverse your foot steps and re-install your alternator.

Put the Alternator back in the engine compartment

Locate the engine mounts where the alternator came from and bolt it back up. Some people normally wait till the belt is on to fully tighten the alternator. Its your call just make sure its back in place before you set the belts tension.

Reattach all electrical wires

Make sure you put all the wires back onto the alternator correctly and don’t forget to reattch the bolt to the grounding cable. Make sure it’s snug.

Putting the New Belt Back on

As mentioned above for a more in depth guide see our “How to replace a Serpentine Belt guide“. 

Grab the doodle you drew before if how your serpentine belt is going to go on. Place it over all the pulleys including the tensioner. Make sure you have this on correctly or else your going to have some major problems later on.

Once your happy with the belts location your now going to want to “tension” the belt. Locate your wrench and Allan Key you used to loosen the belt. Now in the reverse order turn the wrench clockwise and the Allan key counterclockwise till you feel the belt tighten up. Try not to over tighten or under tighten the belt. Normally you have a little bit of play in the belt. If in doubt check your Owners manual. You may need a specialist tool to get the best tension for your belt.

If your belt is loose it will make a horrible squeal and if it’s too tight it will damage the bearing on the alternator and maybe the tensioner, so it’s best to get it right.

Reattach all hoses, wires and parts

If you removed any hoses or wises or the front bumper. Well now is the time to put them all back together. If you removed any hoses check them over for any damage. If there damaged replace them. Make sure that if you cable tied up some wires to remove the cable tie and place them in a safe “Comfortable” place, so they don’t obstruct any moving or hot parts.

Reattach the battery

Just reattach the battery negative lead make sure its on snugly so you get full contact with the battery. Do not over tighten the battery just make sure the cable has a strong connection.

Remove the Memory Saver

That is if you used one.

So there we have it! You have now, we hope, successfully replaced your car alternator. Not too hard now was it? Baring in mind a job like this can cost over £200 if taken to a dealership or local mechanic. You should be quite pleased with yourself especially if you bought a 2nd hand part for under £50 that’s a huge saving. Now go treat yourself to a nice cold one.

Regards, The Beat The Mechanic Team.

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