there are three components in the starter generator that can fail, the brushes, the field coils and the armature. there are three things that can fail in the armature, the windings can become shorted to ground or one of the windings can have an open circuit or the commutator can become worn out. the winding can be tested for grounding by connecting one lead of an ohm meter to the armature shaft and touch the other lead to each segment of the commutator,the reading should show an open circuit at each segment if any shows any continuity the armature is bad and will need to be replaced, the only way to check for an open circuit is by using a device called a growler, if the commutator is worn down it can be put in a lathe and turned down if it is not already worn to deeply. the brushes are made of a fairly soft carbon and are designed to wear down instead of the commutator, when the brushes are worn down beyond half of there length they need to be replaced as the spring tension on them becomes less as they wear down,worn brushes usually show up as very high starting currant, slow turning over and a loud hum when trying to turn over. the field coils consist of two pole coils one of them, (the armature coil) will have very low resistance about .3 to .5 ohms, the other one, (the field coil) will have about 7.5 to 9.5 ohms, and there should be no continuity to ground on either coil. if the field coil is bad the unit will not generate and if the armature coil is bad the unit will not start. also the insulators where the field and armature studs protrude the shell can crack and deteriorate and cause shorts, if the field terminal becomes shorted to ground the generator will full field and will destroy itself in a very short time (minutes). if the armature terminal shorts to ground the starter solenoid will most likely be ruined also and wiring will get very hot and melt when trying to start. brushes, bearings and field coils are usually readily available.