When you crank the key in the ignition and nothing happens but an ominous clicking sound, you may need to replace the alternator. This part turns mechanical energy into electrical energy and is responsible for recharging the car battery while the engine is running. If it begins to act up or fails completely, your car's battery won't recharge and you'll wind up needing to replace that as well. If you suspect your battery is dead and install a new one, you will find that one runs down pretty quickly. This is why it is important to ensure the alternator is not the cause of the issue so you don't risk ruining a brand new battery.
Signs of a failing alternator
When the alternator begins to go bad, the symptoms can be similar to those associated with a dead battery. This is partly because the alternator is not recharging the battery, so the part is not functioning correctly either. In addition to having trouble getting the car to start, you may also notice various electrical components are not working at full strength. For example, headlights may flicker or be dimmer, the radio might stop working or other interior functions will cease.
Check the battery first
Since it's easy to mistake an alternator issue with a battery problem, the first step to diagnosing your alternator is disconnecting the car battery and ensuring it is not the source of the problem. Make sure the car is off and cooled down before you begin. When disconnecting a battery, always start with the ground (negative) wire, which is typically black. Then, disconnect the positive wire. Pop the battery out of the engine bay and connect it to a charger. Once the charger indicates the battery is fully charged, leave it be for about 12 hours. Then, measure the voltage of the battery with a voltmeter. If the readout is 13 volts or lower, the battery is likely the cause of the problem, as it is no longer able to hold a charge. If the reading is close to 14 volts or more, then the alternator is likely the reason the battery hasn't been charging.
Inspect all the wiring and connections
After you've determined the battery is not the problem, you should check all the wiring that connects to the alternator for signs of damage and replace anything that is frayed, torn, rusted or otherwise appears worn. It is far easier and less expensive to replace a faulty wire than it is to replace the alternator. You will also want to inspect the drivebelt for signs of damage. If the belt has slipped off the pulley, become too loose or broken entirely, the alternator will not be able to send the energy output along to other areas of the vehicle.
However, if you encounter no issues with these areas, you're likely going to need to replace the alternator. Before you do this, it can still be a good idea to benchtest the part to check once and for all that the alternator is actually malfunctioning. If it does work, then you may have misdiagnosed a problem earlier in your investigation. If not, then go ahead and get a new alternator and get ready to install it in your vehicle. It is important to find an alternator that has the same capacity as the old one. Otherwise, the new part can produce too much energy and overcharge the battery.