If you find your car battery is not charging or the battery warning light on your dashboard stays on once you've started your car, it's possible that your alternator brushes are bad.
Alternator brushes last substantially longer than brushes on generators, as the brushes only need enough electricity to power the rotor. Additionally, the slip rings on the rotor that the brushes rub against are completely smooth so wear and tear is minimal. Alternator brushes are made of graphite held in place by tensioned brush-holders. Expect brushes on an alternator to last for in excess of 120,000 miles before they go bad.
Alternator brushes are usually positioned at the rear of the alternator and are difficult, if not impossible, to see. The best way to tell if alternator brushes are bad is by doing voltage checks.
Check that you have no loose clothing or jewellery that could become caught up in the engine before checking to tell if your alternator brushes are bad. Many serious accidents occur as a result of clothing becoming caught in engines while they are running.
Open your car bonnet and secure in place. Start your car and let the engine idle for a while.
Use your voltmeter to check if your brushes are bad. Connect the negative wire from the voltmeter to the negative terminal of the battery using the spring clip on the end of the wire to secure in place.
Connect the positive wire from your voltmeter to the positive terminal of your battery. Secure in place using the spring clip on the end of the wire.
Check the readout from the dial on your voltmeter. With the engine idling and electrical equipment turned off, it should read 14.2-volts. If the volts are less than 13 you may have bad brushes.
Turn on electrical equipment such as lights and other devices that use a reasonable amount of electricity. Check the voltage on the voltmeter. It should remain at around 13-volts. If it falls below 12-volts your, brushes may be bad.
Get a friend to sit in the car and increase the engine speed to just above 2,000 RPM. Look at the voltmeter to see if it displays an increase in volts. Your voltmeter should display about 14 volts--maybe a little more if the battery is low. If there is little or no increase in voltage your brushes may be bad.
If the voltmeter indicates that little or no charge is being supplied to your car battery it is wise to check the voltage regulator. If the regulator is bad, it shows similar symptoms. The only definitive way to tell if you have bad brushes is to completely remove the alternator and have it independently tested. Only do this once you have completed the other checks as the process can be expensive.