Although it is preferable to have a faulty alternator tested by a dealer or auto electrician, home mechanics with the necessary gauges and testers can assess the feasibility of repair or determine if a new or factory reconditioned unit is a more economical choice, especially if the original alternator has high mileage. To test the charging voltage, first disconnect the battery positive lead and connect an ammeter between the lead and the battery terminal. Then, connect a voltmeter between the 'L' terminal on the alternator and earth; a non-zero reading indicates a defective alternator or wiring. With the ignition on but the engine off, the voltmeter should read significantly lower than 12V; if it reads around 12V, the alternator is likely faulty. After short-circuiting the ammeter terminals and starting the engine, increase the engine speed to between 2000 and 3000 rpm. If the ammeter shows 5A or less, check the voltmeter reading, which should be between 14.1 and 14.7V at 68°F (20°C). If the ammeter exceeds 5A, continue charging the battery until the reading falls below that threshold or replace the battery with a fully charged one, then recheck the charging voltage. For output checks, disconnect the battery negative lead and the lead from the B terminal on the alternator, connecting an ammeter between the B terminal and the disconnected lead. If a tachometer is not already installed, connect one according to the maker's instructions, then reconnect the battery negative lead and connect a voltmeter between the B terminal of the alternator and earth. Start the engine, turn on the headlamps, and increase the engine speed to 1300 rpm; the output current should be between 13 and 16A at 13.5V. If the readings fall outside this range, the alternator is faulty.