no start

hello to all. I am working on a komatsu pc100-6 that will not start. it has been sitting for 2 1/2 years and it ran when it was put away. it has new batteries and when you go to the crank position of the key all power goes away {like a major voltage drop**. where the battery positive cable hooks up there is a unit I don't know what it is. 25volts goes in. there are 2 smaller connections with white wires going to it and a bigger connection that feeds the starter. when I put the key in the crank position it goes from 0volts to 2.5volts. I pulled the cover off this and it is rusted inside. first of all what is this and am I safe to say that my issues with loss of power at the console may be due to this part. most of why am asking for help is that I have no trouble shooting charts or books for this excavator