To fix a dynamo, you must first remove the distributor drive gear on the end of the armature shaft and remove the back plate. Wear on the carbon brushes and commutator is the most common cause of dynamo failure, according to How a Car Works.Continue Reading
After the back plate of the dynamo is removed, the commutator and the brushes are visible. Remove the two wires that go to two of the brushes, and slide off the body of the dynamo from the brushes. Check for broken wires using electrical test meters, batteries and bulbs wired with two probes or a proprietary continuity tester, which usually either buzzes or glows when there is continuity in a circuit.
According to the Cornwell Austin Seven club, there must be continuity between each adjacent segment all around the commutator and no continuity between the shaft of the armature and any place on the commutator. Check if there is continuity between the wire that went to the brushes and the terminal on the dynamo body.
Check for insulation on the brush assemblies to make sure the insulation at the arm pivot is good. The last step involves putting the back plate onto the dynamo and testing the assembled dynamo by connecting a short wire between the two terminals on the outside, then connecting the body of the dynamo to a battery.Learn more about Car Parts & Maintenance