Voltage from the house or the alternator positive terminal is read. Also note the alternator output switch. An amber led indicates the alternator is in 50% output mode. The multi meter can be shown in the amp mode where it read amps from the alternator shunt. A buzzer alarm is attached to alternator sense power and the negative lead can run be crimped on the red led. I read LED can also be put on the dash at the wheel and wired to the buzzer with the oil pressure sensor.