28 HP charging system

[Duane w]

My mid 90's 28 horse is over charging the battery. We have checked this with an external digital voltage meter thinking it may have been a problem with the dash gauge. We also tried at different battery. At above 2000 RPM the charging system slowly raises to 18 plus volts and can only be brought back into the normal range by turning on some lights. Under 2000 RPM all seems normal.
What controls/limits the voltage to the battery? is there a voltage regulator similar to an automobile? Is this an easy fix?
Thanks for any info, stumped!

[boats.net]

That model only has a voltage rectifier, check all wire connections and ground wire connections, make sure battery cable connections are clean and tight to battery.

[Duane w]

so where is the voltage rectifier located?

[boats.net]

Number 49 in this diagram, 1990 E28ESLESM Evinrude / Johnson Evinrude IGNITION Diagram and Parts