This page provides a basic description of how the two wires running from the 2G DSM ECU to the alternator (G and FR) are used.

There are four wires running to the alternator. Two wires are G and FR from the ECU. The other two are a voltage sense line (from the headlight circuit) and an “S” terminal tied into the gauges somehow.

If you have the DSM manuals on CD, check out the “generator” section of the 2G technical manual. It provides a really good explanation of how the ECU controls the alternator. I'll briefly summarize below.

The alternator has two “modes” - low voltage/low load and high voltage/high load. The ECU controls which mode the alternator should be in using the “Generator G” pin on the ECU harness (pin 33). When G is grounded, the alternator is put into low-voltage mode (around 12.3v). When G is left floating, the alternator is in high-voltage mode (Around 14.4v).