Classic/Antique Car Repair: Generator Idiot Light, chevy el camino, voltage readings

Question
I have a 1969 Chevy el Camino with the generator idiot light illuminating.  I have replaced the alternator and voltage regulator, but the lamp is still lit.  Voltage readings at battery terminals are as follows:  engine off - 12+ Vdc, engine running at idle - 13+ Vdc, engine running at 2000RPM - 15+ vdc. The accessory belt is tight and battery does not run down overnight; voltage pretty constant.  Any ideas?  Your input would be greatly appreciated.
drew

Answer
There a a diode in the alternator that controls this light and the charge rate for the alternator. It is called the diode trio and most of the time when the idiot light stays on but the alternator is charging it is the diode trio that is defective. Also make sure that the ground between the alternator and the regulator is in good contact.
Brad