Starter/ Earth / Battery Master Switch (Rally Car)

[SimonJ 2]

I made the usual mistake of taking something apart and putting it back together weeks apart. In this case, replacing the starter, I'm faced with three wires:

(i) thick live wire to the solenoid - no problem

(ii) thin wire from the alternator to the solenoid - no problem

(iii) thick wire from the battery master switch - this is the one I'm uncertain about but I think it isn't really anything to do with the starter, just uses a bolt between the starter and the gearbox as a good earth point?