When your battery light remains on after you’ve started your car, it means you have an alternator issue. You’re going to want to test your alternator and this guide is going to teach you how to do this.

It’s pretty easy to check if your alternator is working properly, all you need is a multimeter. However, some new cars are fitted with alternators that are controlled by the onboard computer. These modern alternators don’t use a voltage regulator like the older models. Everything is managed by the computer. This includes alternator diagnostics.

If you own a late model car that has an alternator with computerized management, it’s even easier to check for alternator faults. You will need onboard diagnostics (OBD). Simply connect the OBD to your car and check the fault codes. This will give you all the details of any faults that the computer has logged.

For cars fitted with a traditional alternator, you will perform the conventional testing methods using a multimeter. Before we begin, it would be a good idea to gain a little knowledge about the alternator and your car’s electrical system.

Understanding your Alternator

An alternator supplies electricity to all the electrical components on your car and keeps the battery charged. The power stored in the battery is used to start the car. It is important to understand the relationship between the battery and the alternator, they work together to keep your car functioning as it should. A problem with the alternator will affect the battery. Similarly, a weak battery may also affect the alternator.

When testing your alternator, you also need to test the battery.

How does an alternator work?

 An alternator works like an inductive AC motor, just in reverse. Unlike a motor that converts electric energy into mechanical energy (rotation), an alternator converts the mechanical energy from the engine into electric energy.

The main components (rotor and stator are basically the same as an induction motor). The stator is connected to a pulley which is turned by the engine via an alternator belt. The spinning stator induces an electromagnetic force (EMF) between the rotor and the stators.

The magnetic force causes a flow of electrons, creating an alternating current (AC). A rectifier converts the AC power to DC, which supplies the battery and fuse box of your car. The voltage is controlled by a voltage regulator.

Testing Your Battery and Alternator

Start out with a basic inspection of the alternator, wires, and belt.

When the engine is running, check for irregular movement and noises from the alternator. A loud screeching noise when you increase engine RPM is usually an indication of a worn or loose alternator belt. A whining noise at the alternator means you could have a damaged alternator bearing.

With the engine off, check the tension of the alternator belt. Pressing against the belt with your fingers should not result in any movement of the belt. Check that all the wires and connectors at the alternator and battery are in good condition. Replace any damaged wires or connectors.

Check that the battery is clean. If you see a blue powdery substance on the battery connections clean these with hot water. Adding a little baking soda to the water will clean it more effectively.

Step 1: Using your multimeter, check the battery voltage with the engine off. Set the multimeter to DC voltage (DCV) with a calibration higher than 15V, this is usually DCV 20. Connect the black probe of your multimeter to the negative point on your battery and the red probe to positive. If your battery is in good condition it should be around 12.2 to 12.8 volts.

Step2: Conduct a battery load test. If you have a load tester, this is easy – connect the tester to the battery and press the “test” button. Since few DIYers own a battery load tester, you will need to use your multimeter. This means getting someone to help you. With the car in park or neutral, turn the key so that the starter motor cranks the engine. While the starter is motor is turning, the battery voltage should not drop below 11V.

If your battery voltage is below 12V with the engine off, or it drops below 11V under load, you should replace your battery.

Step3: With the engine idling normally, test the voltage at the battery as described above. When the engine is running, you should have a voltage reading of 13.8 to 14.5.

Step4: Switch on the lights and air conditioner to full power. The voltage reading may change slightly but should remain between 13.5V and 14.5.

If the voltage, with the engine running, drops below 13V or rises above 15V, your alternator needs to be professionally tested as it will probably need to be repaired or replaced.

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