No Start 6.5 Diesel Hummer H1

[mpwr99]

I have a 2002 hummer h1 with the GM 6.5 deisel engine and yesterday pulling out of the garage the damn thing died. After 5 minutes of looking through fuses i found a fuse in the engine fuse box labeled ALT IGN. I have no idea what this fuse is or does or controls but its a 20 amp fuse and i replaced it 2 times and ones i turn the key the thing blows. Does anyone have any idea what this fuse is for and have had this happed before?? thanks JDMDC@AOL.COM