below is a extract from a article i wrote years ag
hope this help
One trick worth trying first is repolarising the Dynamo simply disconnect the leads and ensure the battery is charged and connected. Now connect a wire to the battery live side (NOT THE LIVE TERMINAL) and quickly flash the “F” terminal. Redo this if you strip the Dynamo.
The first thing although obvious to most is to remove the end cover and kick the bike over. Does the Dynamo turn? If not check your chain on twins or the clutch on the end of the mag on singles. If it dose turn the next step is to connect the “F” and “D” terminal together and lead a wire from it to a 12 volt bulb, take a wire from earth to the other side of the bulb and start the bike if the bulb lights and is bright the problem is in the regulator, If you have a Multimeter you need over 13 volts even on a six volt system. However magdyno’s can suffer from a slipping clutch which means the dynamo is not turning fast enough to check for this remove the Dynamo and try and turn the large fibre wheel in the Magneto case with your fingers if you can turn it your problem is the clutch needs to be tightened. The next step is to remove the Dynamo and carefully strip it down.
Once striped look at the copper/brass part of the armature this is called the commutator very gently clean it with a 600 grade or higher piece of wet and dry. Next step is to clean the gaps between the segments on the commutator with a slim blunt instrument do not press, very carefully scrape out the black carbon, now clean the brushes the next step is to check for circuit as well as lack of it wire a bulb in series with a battery so that you have two wire which when connected together light the bulb, put one wire on the commutator and one in the shaft of the armature if the bulb lights you need a rewind. Now place one wire on a segment of the commutator and touch each segment in turn. The bulb must light on each segment if not rewind once more. THESE MEASURES CAN NOT GURANTEE A GOOD DYNAMO BUT WILL DEFINETLY IDENTIFY A FAULTY ONE. If all is well so far, we will now check the field coils this is the winding inside the dynamo case. Using our bulb connect the wires direct to the battery and note how bright it is, now connect the wire to one side of the field coil and the other to the battery if the bulb is noticeably dimmer the Filed coil is okay if you use your Multimeter you are looking for a resistance of around 2 amps.
A excellent tool for checking a Dynamo can be made from a extension bar off a socket set. If you know someone with a Lathe ask them to turn the end which normally attaches to the wrench enabling it to fit into the chuck of your electric drill (battery drills are not fast enough). Using this tool join the “F” and “D” together and attach to a bulb take a wire from the Dynamo body to the other terminal of the bulb, spin it up ensuring you observe the correct rotation for your Dynamo as “A” & “B” group spin different ways again it should light the bulb or with a Multimeter show over 13 volt’s. If it does not light the bulb try spinning the Dynamo the “wrong way” if it lights simply swap the field coil wire over and try again the correct way it should now work. If you do not have a reversible drill try swapping the Field coil wire and spin again.
Now the bad news if all the above fails you need a rewind.
