86 GMC van 6.2L diesel ignition problem

i have a 1986 gmc van with a 6.2L diesel engine and when you try and start it the engine turns over very slow and the ground wire for the battery get very hot and melts the plastic cover around the wire.

Could be the starter motor drawing to much amperage from wear and overheating the electrical system. Also loose/dirty connections will overheat. Whatever you find make sure you replace the overheated wires, Wiring that has been overheated makes a poor conductor.