I consider myself a decent mechanic but diagnosing isn't always easy. My 2001 5.9 L truck has been flawless for 235k miles and I've always taken care of it..its been sitting for a while now and it wouldn't start..just clicks..i bought a new battery since it was 4yrs old..same thing..u can jump it off..it would go dead once disconnecting cables..so i test Alt.put new brushes in it..its fine..took starter off..replaced them copper things inside. .its fine..same thing..jump it off..now itll run by itself. .drive fine..but turn key off and click is all i get..Relays i switched out. .same thing...no blown fuses..I'm gona check ground cable on front next and load test new battery..but I'm 99% sure its gona do the same thing..why does it only crank when i jump it off..makes no sense to me at all..i also hooked into OBD..no eng codes at all...no ck eng light even..but all dash lights work fine.

 

Sounds like you may have a bad connection at the battery or a starter that's drawing high current to turn it over. Make sure your battery connections are clean and tight. If that's good, I'd remove the starter and take it in for testing.

 

I already had the starter tested and had new contacts put in it..i watched him bench test it..I'm gona try 2 more things that I've read about..testing voltage at the starter when keys on..and im gona take ground jumper cable only and connect it to battery and truck ground thinking it'll bypass the ground on battery ..after i take loose the ground by water pump and clean it.. I'm sure its something simple
.well..hoping its something simple like that. .because nothing else is making sense at all.

 

Load test the battery and charge it before putting it back in the truck.
THIS!!!

 

Thats what i did..new battery was POS..that was last thing i could think of too...thanks everyone for helping...