Playing with an Alternator

The Mitsubishi alternator is a three lead variety. The main current supply is output through the large bolt-like terminal, with the chassis as the return. The two remaining leads are spade connectors, labelled L and S - these correspond to the Lamp and Sense. The sense lead is typically connected to the cars main loom, where it is used to sense the voltage and regulate the alternator output. The Lamp output is used to feed the dashboard indicator light, but it serves a dual purpose. The Lamp connector, when the alternator is fully operational, outputs +12V, and is connected to an indicator lamp, but the second terminal of the indicator is also connected to +12V, but to the battery. Hence, when the alternator is functioning properly, the indicator bulb has +12V on both sides and so the indicator lamp is not lit. Before the alternator is generating an output, the field coils of the alternator require a small current to establish the magnetic field - and this is the dual purpose of the Lamp connector - before the alternator is spinning, the alternator Lamp connector is at 0V and so some current flows from the battery, through the indicator bulb (causing it to illuminate) and into the alternator field coils via the Lamp connector. This provides the initial magnetic field against which the alternator acts.