Testing your car’s alternator or battery requires a digital multimeter with a voltage setting. There are many people who worry how to tell if your car’s alternator or battery is failingsince it can be difficult for them to know which of the two components is the one that is failing.
How to tell if your car’s alternator or battery is failing
Your car battery has a capacity of 12 volts and you should have at least that much to start the car. If the battery voltage is low, the alternator needs to be checked to rule out a problem with the charging system before replacing the battery.
You can test the system with a multimeter to determine the battery and alternator output, giving you a better idea of the condition of your battery and alternator.
- Turn the dial on your multimeter to the 20V position. This will allow you to read the voltage of your battery and alternator.
- Put the tester’s red test lead on the positive (+) terminal of the battery, and then put the black test lead on the negative battery post. Look at the reading on the meter, it should be at least 12 to 13 volts. If the reading is less than 12 volts, checks the alternator to determine if it is not fully charging the battery.
- Start the car and place the red lead of the multimeter on the “B” terminal located on the back of the alternator. Lay the black wire on the ground, not on the alternator box. Write down the reading on the meter, the reading should be around 13.5 volts and can be as high as 15 volts. If the reading is outside of this range, remove the alternator for further testing.
- Test the battery again while the engine is running. If the voltage reading on the battery is higher than 13.5 volts, the alternator is trying to charge the battery and the battery needs to be tested further. Most auto parts stores will test your battery for free if you remove it from the vehicle.
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