what would cause an alternator to overcharge? while at a cruise i max out my volt gauge at about 18v. idle is about 12v. and free rev the motor and it pegs about 18v?

any ideas?

 

That means the regulator isn't switching the alternator properly. Either the regulator is bad, or the regulator isn't properly grounded. To test the latter, run a jumper wire from one of the regulator mounting bolts to engine ground. If that doesn't improve the problem, inspect the wiring between the regulator and alternator. If neither of those items are at fault, replace the regulator.