try disconnecting it from the battery side of the starter switch and see if it still sparks when you put the cable back on and if not, replace your starter switch and if it does still, check the battery wire to the disconnect switch for shorting out, then the same thing at the generator and keep trying til you isolate the problem. hope to be of some help. It is ground cable to -, (1st battery), + to -, (second battery), + to starter switch and disconnect switch, isnt it, for 24 volts systems.
67 M725, 67 M715, 68 M715