To determine which part of the starter circuit is at fault, turn on the light switch and again operate the starter. Should the lights go out or become dim, the trouble is either in the battery, its connections or cables. A hydrometer test of the battery should indicate better than 1.250 specific gravity, while a voltmeter, placed across the positive and negative posts, should indicate about 6 volts for a 6-volt battery and 12 volts for a 12-volt system. If either of these tests prove okay, clean and tighten the battery connections and cable terminals or replace any cable which seems doubtful.
If the lights remain bright when the starter is operated, the trouble is between the battery and the starter, or the starter switch is at fault, since it is evident that there is no electrical connection between these points. If these connections are clean and tight, it is safe to assume that the starter or starter switch is defective.