Mercury - Alternator Charging System 8M0057693. Overview. When the alternator on a Mercury FourStroke starts to get weak, you usually notice it long before it quits-electronics flicker, the battery never seems fully charged, and the engine just doesn't feel as "settled" as it should. This alternator is built for the 135-150HP EFI models and is designed to keep up with everything these motors run: ignition, pumps, lights, and whatever gear you've added over the years. It holds steady output wheth