I had 2 dynamos - neither worked - turned out to be just dirty brushes - quick emery-up and I now have 2 fully functioning dynamos !! -- Unbelievable it was as simple as that .
I had a similar experience, I got the dynamo working and a few hundred miles later it stopped working, cleaned the brushes and the same happened again, despite it passing the usual “join wires” test (on a drill). The solution was new brushes, the old brushes looked ok but must have gone soft or something and they caused enough resistance after relatively few miles to stop the dynamo’s “self start” capability at low revs, but otherwise the dynamo resistances checked out ok. Had me
for a while.
