you may well not need a wiring diagram as the charging systems are quite straightforward ,usually it is a regulator problem 2 types. 1 screwed on at the rear of the alternator half the size of a match box 2 metal box usually in the engine bay { bridge the 2 lower terminal,s to by pass the reg} best bet though is to remove the alternator for a bench test at the local auto electricians, and double check the battery if you are still having problem,s you need to look for any voltage drop on thick cable,s in the engine bay 1 red 2 brown