with the truck runnin take off your neg. cable on your battery, if the truck stays running then your alt. prolly isnt the problem, i had this same problem with my big f-250 7.3L ford diesel and i cant stress enough your battery cables have to be the sh*t trust me, i had to spend the money and get all new battery cables for the ford (has 2 batteries 130 bucks for the main cable) and i put like 2 brand new grounds off each battery, and the thing would turn over like all the cylinders were shot it was amazing, i would also agree to take your battery to a parts store they can test it for you and tell you whats up, plus its free so why not <img src="/forums/images/graemlins/smile.gif" alt="" />