Chonic Alternator Failure

^^ Amen on that. We try to only get alternator/starters and associated parts from NAPA or a local rebuilder.

Just a bit more on this problem: Blowing alternator diodes can be caused by increased resistance in the wiring to the battery. Whenever you have any sort of surge load (like turning on headlights, which have low resistance until the filaments are glowing, or the heater blower motor), the surge of current has to come from 1 of 2 sources: the battery or the alternator. If the wiring to the battery through the ammeter has higher resistance than normal, like corroded connections to the ammeter, the battery, through the firewall, or a fusible link that is getting marginal, then the surge will have to come almost solely from the alternator. In a properly functioning system, the surge will be shared by the battery.

With the excess voltage in this case, the turn-on surges will be even higher, and with the marginal parts put in the Duralast units, the didoes will blow just that more easily. So, I suspect this system needs to be gone through thorougly to:
1) find the reason for the improper regulation (high alternator voltage)
2) check the whole wiring from battery to alternator to find any high resistance points

This may be happening without any high surge parts with the voltage regulation problem. Case in point: I have raced 2 rally cars that blew diodes in good GM internal-regulator type alternators; it would ususally happen in night racing when I would be turning on the 6 high powered halogen racing lights. I finally figured out that the remote mounted battery in the back with the cut-off switch had enough added resistance (even with a 00 ga battery cable) in the battery circuit that the racing turn-on surge of the racing lights would 99% be sourced from the alternator rather than being shared with the battery. BTW I never fixed the battery resistance in either car; I just kept a lot of alternators on hand! Using 135A didoes helped some.