Originally Posted by F35008
Wich one does what?? are they just in parallel or separate?? because I tryed to boost my truck with the passenger side one and it barely cranked and then I tried the driver side one and it started right away??
They're in parallel. Most diesels I've ever owned or driven do this for the extra cranking amps. If you've ever had only one battery go bad, you'd notice the difference in cranking speed. I found that out quite by chance when my one good battery failed. They tested both and found them both bad, and when both were replaced, you wouldn't believe the difference in cranking speed. It wouldn't make any sense to have them independent of each other either, because then you would get into the "which one does what" scenario. Obviously they both have to be tied to the alternator for recharging. Chances are you just didn't have a good connection on the first try, or perhaps you put just enough charge in it before you changed to the other side that it cranked okay. Batteries and jumper cables are strange animals sometimes.