cat 966C

hello i need some help with a old 966c the operator was running it when all of a sudden, the starter started spinning over. they pulled the fuel pedal to shut the loader off and the starter was still going so they turned the battery disconnect off and it was still spinning. it finally stopped when they jumped out and disconnected the wires to the starter. so i figured the solenoid on the starter shorted out and grabbed a new starter installed it and now every time you turn the hot start switch to start it kicks the 40 amp breaker, the glow plug side works fine thou or at least the ammeter drops a little to the negative side when you have the switch held im assuming thats normal?. So I lifted the dash up (not much room under there Hehe) and began inspecting wiring etc. i finally figured i would bypass everything and took the signal wire off the hot start switch and disconnected the side of the 40 amp breaker that goes to the ammeter and touch the 2 together and still it kicks the breaker so then i figured maybe theres a bad wire so ran 2 new wires and still it kicks the breaker. so tomorrow i am going to do all new battery cables the old ones have seen better days. any help or ideas i could get would be amazing oh and the batterys had 12.5 and 12.4 readings thankyou. the breaker is a new cat one so i wouldnt think it would be weak but of course new doesnt mean good.