I fixed a friend's JD that wasn't charging. Took a while to find the problem. Turns out that the connector to the regulator had developed a poor connection which in turn cause the connector to overheat which caused no or poor connection.
The regulator is a sob to get out but in order to get at the connector to make sure it has a good connection, remove it. Remember to disconnect the battery first.
Look at the connector, it should be a white opaque color. If it has brown or black color around one or more of the connectors, there is the problem. I was able to repair the unit I worked on by removing the terminals from the connector housing, pinching the terminal back into it's original shape and the put it back into the connector. Also cleaned all the terminals on the regulator. Get them nice and shinny.
Good Luck
Richard