[C320-list] alternator question

[dmcnamara3 at woh.rr.com]

Hello,
THanks to all that responded to my first post.  For some reason I do not have permission to respond to posts.  I am on the digest version, do I have to change that to be able to post?  Anyway, the batteries are 1 year old (2-1100cca batteries)  I set up the first new alternator and indeed it appears that the diodes are blown.  That being discovered I am even more perplexed as to what I am doing to blow the diodes.  As I stated before both alternators worked briefly and I did not turn the key off before shutting off the Perkins m30 engine.  I don't know what to check next.
Any help would be greatly appreciated.
thanks
Dan
(064)