My s 300 bobcat starts up runs 30 seconds and blows fuse, I changed the fuse and it blew the new one. Can't find any shorts but I did notice the solenoid wire was not tight.
I think you need to check that mainframe harness closely. I don't have a manual on an S300, but most Bobcat machines that 100 amp fuse feeds circuits going straight to the fuse box. Once it gets to the fuse box pretty much everything in the main power circuits are fuse protected with much smaller amp fuses with the exception of power supplied to the starter relay and glow plug relay (if used). Not to mention all wires from fuse box and onward are much smaller wires, 100 amp would melt them before it blew that 100 amp fuse. You have a short to ground somewhere in that main supply circuit.