2nd alternator

[basicbajax]

I have a problem I am trying to get fixed. I have 2 new batteries put in within the las 6 months and a new positive lead wire from the batteries to the starter???...Yesterday I was on the way to work with my dash voltmeter going down towards 8volt range. Had a new alternator put in and i thought all was well. Next day did the same thing and after the alternator was checked it was already bad. Possibly a bad alternator???? Had it replaced and during warmup this morning the voltage meter was going towards 8 volt again. I think I may have a dead short in some wiring that obviously is not fused. Any ideas before my 2nd alternator goes out? Thanks

[basicbajax]

Its a 2000 F250 SD diesel with 222,000 miles on it

[KatieRay]

I am on my third alternator...My truck only has 55,000 miles...they said that my second one was bad when I got it...I have had this third for a month its seems to be doing OK...I also had some jacked up wires to a horn under my hood that could have caused a short...hope you figure it out