The voltage should be between 13.5 and 14.8 volts with the engine running.Connect the black cable of the multimeter to negative terminal of the battery, and the red cable to the positive terminal.The easiest method to check the alternator is by measuring the voltage at the battery terminals when the engine is running. Before replacing the alternator, you must rule out other causes that may stop or degrade flow of current from the alternator, for example: bad electrical connection or damaged wire. Alternators can fail at any time, but generally they last over 100,000 miles.