I have 08 F250 6.4L powerstroke and I have a really weird problem the last month or so I keep going out to dead batteries or really low batteries to where it turns over once then its dead. The truck sits for about 5 days at a time. I was told to unhook both + cables from the batteries and put a test light in line on the + cable to the post.
and behold the test light lights up really well. and to pull relays and fuses one by one tell it goes out well I did that on the BJB under the hood and under the dash passenger side and none of the relays or fuse made the light go out. oh and the trucks locks and anti theft is all gone to sleep after 30 mins.

someone please help sorry for the long page.

 

Has any electrical work been done recently ??? If not check your batteries & then your alt.

 

thanks for the reply the engine was built with studs and all performance last year but the alternator hasn't been touch could the alternator be draw amps while truck is sitting? Batteries are 2 months old.

 

So here's an update I started testing some more and I traced the battery draw to the alternator with the B+ unhooked and the passenger side B+ cable has the alternator cable nutted to the terminal. when I separated all the cables the test light stayed lite up on the cable hooked to the alternator. So my question is does this mean the alternator might have a bad diode?