To check an alternator, link the leads from a voltmeter to the terminals on the battery, and check the reading. Any reading between 13.8 and 15.3 volts means the alternator is working correctly.
- Prepare the car
Turn on the engine and headlights, but leave off accessories such as radio and air conditioning. Unplug any device attached to a charger outlet.
- Connect the voltmeter
Line up the positive voltmeter lead to the positive terminal on the battery, and line up the negative lead to the negative terminal. Red plus symbols indicate the connection that is positive.
- Look at the gauge or display
Check the digital readout or positioning of the needle on the voltmeter gauge. If the reading falls between 13.8 and 15.3 volts, the alternator is sending the proper current. Don't unhook the negative battery cable while the car is running to test the alternator, since this method may damage the electrical devices in the car.
- Charge the battery if necessary
If the voltmeter reading is too low, turn off the engine, and charge the battery with a battery charger. Turn on the engine, and read the gauge. If the reading is still too low, hire a professional to repair the alternator.