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Over 15V is on the high side and getting onto overcharging of the Battery range...I suggest you check the chassis connections of the Alternator (it gets its chassis connections through the paint-free areas on the bracket, connected to paintfree areas on engine, and also the engine to chassis strap, and finally the chassis to Battery Strap), any poor chassis connection in-line with the internal VReg can result in a voltage drop (which gets added to its internal reference), resulting in a higher ref V than it should be, with your symptoms. Also have the state of Battery checked...an open (or high resistance cell) might allow buss V to soar also... If your Battery checks OK and you assure all low-side connections (external to Alt) are shiny, clean and tight, and you still see these voltages, I would have the Alt itself (did you get one of the recommended Delco rebuilt quality units, or a $40 Pep Pills Boys special?), checked by a qualified automotive electrical shop.
Please let us know what you find, so that any newly learned items can be fed back into the documentation.
Cheers