How to check your car's alternator
Updated 12 Oct 2019
If you’ve recently replaced your battery, but are still having issues starting your car, the problem may lie with your alternator.
The alternator supplies the vehicle with power while the engine is running and also charges up the battery.
This is an integral part of your vehicle’s operation, and if it fails, can leave you stranded. Here is a step-by-step guide on how to check the output of your alternator.
The first step is to find yourself a multi-meter. These are relatively inexpensive and can be picked up from an auto parts store, such as Supercheap Auto.
You will need to set the meter to volts by turning the dial to the voltage icon. The next step is to locate the battery in your vehicle.
This is usually located under the bonnet of the car, but can be located in the boot, or in some European vehicles, under the passenger seat!
Once you have located the battery, take a reading of the battery voltage with the car turned off, to give you a base reading.
Take the black and red probes of the multi-meter and place them on the corresponding battery terminals. You should get a voltage reading displayed on the screen of the multi-meter. A good battery should sit around the 12 volt mark.
Once you have the battery voltage, it's time to start the car. Once the vehicle is running, place the probes on the battery terminals.
A good alternator will be putting out around 13.9 to 14.3 volts. If the reading is below this, it may be that your alternator is on its way out.
If the voltage is similar to your first reading when the vehicle is off, then the alternator is not working at all and needs to be replaced.
One thing to watch out for on newer vehicles is that they may have a battery monitoring system.
This is a system that monitors the output of the battery and adjusts the operation of the alternator to suit. These systems increase the fuel economy of your car, and also increase the life of your alternator.
This system can give you an incorrect reading when measuring the output of your alternator, so it is very important to check the battery voltage first, before testing the alternator.
The easiest way to tell if your vehicle has this system, is to check the negative battery cable.
If there is a black connector on the negative terminal, or a circular connector around the negative battery cable, your vehicle is equipped with this feature.
If your vehicle is having starting issues and is equipped with this system, it is recommended that you have your vehicle inspected by a professional. Meanwhile, here are some pointers for when your battery light comes on.
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Finding a passion for cars from a young age, Joel carried out work experience as a mechanic whilst at school before starting an apprenticeship after finishing year 12.
After almost 10 years on the tools and in customer service, he moved into the IT realm as a Data Analyst and In-House mechanic at AutoGuru.