There are two types of alternator repair: rebuilding the alternator (usually replacing brushes) or alternator replacement. Both are projects that can be done at home by the competent mechanic. Replacement is the easier option, requiring fewer tools and less time, but more money for the parts.
Alternators break down with time and may need repair. They are essentially electrical generators (motors in reverse) and the light friction and the power conduction through the unit, as well as the shaking and rattling of the vehicle around it, can eventually cause internal components to fail. The most common culprits for failure are loose wiring, failed electronic controls (shorts), or worn brushes that cause power production to drop.
How do I know if the alternator is going bad?
Since alternators fail in stages, it's often hard to tell at first if an alternator is beginning to fail, especially if you aren't using high-current using accessories such as headlights or air conditioning very frequently.
An inexpensive voltmeter can be used to check the voltage to determine if the alternator is working properly. Although a loud grinding noise can be caused by other problems, there's a good chance it means the alternator needs to be replaced.
Most auto parts stores provide free testing for alternators and batteries. It can be done in just a couple of minutes and will tell you whether or not the alternator is really to blame and needs repair.
At home, you can test it with an ohm meter. First, disconnect the battery and put the meter on both leads (positive-negative). Record the reading, which should be close to 12-14 volts. Reconnect the battery and start the engine. Test the battery again. If the voltage has gone up, then your alternator is supplying power to the battery. If it diminishes or stays the same, then your alternator is likely not working and repair may be needed.
Before disassembling the alternator, shut the engine off and disconnect the battery's negative lead. Check the wiring on the alternator to be sure it's secure. Often the problem is the harness or ground coming loose. That can be fixed with a small wrench or a soldering gun. Also check the tension on the belt to be sure it's tight. A loose belt will slip across the pulley rather than turning it, causing the alternator to not spin and produce power.
Click here to watch a video on testing an alternator
photo of alternator by Florian Schäffer