How to test your alternator

car alternatorTest car alternator Car alternator, if you don’t know what one is then take a look at our article what is an alternator. Your car alternator will not last forever, and when it does go, it can leave you stranded somewhere with a flat battery – and no-one needs that.

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Not everyone has breakdown cover. Perhaps you currently have car problems (battery keeps going flat) and you think it may be that the alternator is to blame !! Whatever your current situation, testing your alternator is a quick and simple job and something you should do to keep your car tip-top and reliable. You don’t need to take your car alternator apart to test it and you don’t need any expensive tools. What you will need (buy one, they are cheap or borrow one) is a volt meter or multimeter. They are very simple to use, here is a photo of one (cost about £10) ; Here is what you do, simple car alternator test ; Make sure all the accessories on your car are turned off (lights, radio etc.) and then rev up the cars engine to a fast idle (say 2000-2500 rpm). Set the volt meter or multimeter to the DC scale and measure the voltage across the battery terminals – red lead of the voltmeter on the positive terminal, black on the negative. You will need someone to rev the engine whilst you connect the meter to the battery. With the engine at a fast idle, the voltage on the meter should read around 14 volts (13.5 to 14.4). The alternator needs to generate a larger voltage than the battery’s rated voltage to overcome the internal resistance of the battery.

The current needed to recharge the battery would not flow at all if the car alternator output voltage was the same. The larger the difference between the alternator output and the battery voltage, then the quicker the battery will charge.

So as you can imagine, if your car is quite old and the alternator and the battery are too, problems can occur with the battery getting enough charge and making the car more difficult to start and less reliable.

If the reading on the multimeter is less than 12 volts then you may have a failed alternator.

Even if you have to buy a Haynes Manualmultimeter and some spanners, it will still probably cost you less than having a garage do the work.

The alternator is bolted to the side of the engine and is driven (turned) by the engine via a belt.  The belt turns a shaft inside the alternator when the engine is running and the output (voltage) is sent to the battery via wires. 

Many places sell brand new replacement alternators as well as refurbished or reconditioned alternators.  A reconditioned alternator has had all the internal wear parts such as brushes replaced with new ones. They will have been tested and will come with a guarantee, so don’t worry about using one of these, they are good as new and cheaper. I have also fitted second hand (used) alternators to a number of my cars. These have been sourced from scrap yards or vehicle dismantlers. They normally come with a receipt and can be changed within a short period of time if they turn out not to be working. I have always asked the supplier to test the alternator before I have paid for it. When buying parts it is always very important to keep the receipts incase a problem occurs. Receipts are always worth keeping as they show a history of your car which prospective buyers like to see when they are thinking of buying your vehicle.

If your garage ever tells you that you need a new 
alternator, ask them ;

a) have they performed a load test on it ?
b) what were the voltage readings ?
c) was the alternator belt worn or slipping ?
d) was there excessive noise from the alternator

These simple questions will let your mechanic know that you know a bit more about your cars electrical system than just how to turn the lights on. This should mean that you get given the best service and are treated better by the garage and not feel like you have been ripped off when work is done to your car.

Consider some breakdown cover to annoying eventualities like this.  Have a read of our unbiased reviews of the car breakdown services currently available (opens in new window).

This clip shows the sound that your alternator should NOT be making if it is working fine.

One more check ;
Next, turn on the heater, the rear window de-mister, the radio, the headlights and anything else that draws power from the battery.

Now rev up the engine again and look at the multimeter .

It should still be reading around 14 volts. If it reads lower than 13 volts the chances are that the alternator is faulty and will need to be replaced before it catches you out.

What to do if the alternator has failed
Replacing an alternator is not a difficult job, however it varies from car to car as to how difficult the alternator is to access and remove the belt. If you purchase a Haynes Manual (cost about £13), then it will show you with instructions and photos how to undertake this change on your specific car in the easiest way and it will tell you what tools you need.

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