Burnt commutator

Battery discharged

Worn brushes

Worn pinion tip

Provide grip over armature shaft

Avoid the side way movement of armature shaft

Prevent pinion striking fly wheel

Resist wear on the fly wheel

Faulty regulator

Loose mountings

Wornout bearing

Loose drive pully

Open rectifier circuit

Open field current

Voltage regulator setting too low

Voltage regulator setting too high

Charges at high rate

Low voltage output from alternator

No change when engine running

Alternator noisy