WARNING: This page is about a different car, the 1992 Acura NSX and 1991 Acura NSX. However, it is still accessible from the selected car via links, so may be relevant.
This signals the PGM-FI ECU when the alternator is charging. See Fig 1
and Fig 2
.
Fig 1: Alternator FR Signal Diagnostic Flow Chart (1 Of 2)
Courtesy of AMERICAN HONDA MOTOR CO., INC.
Fig 2: Alternator FR Signal Diagnostic Flow Chart (2 Of 2)