I have a 93 xp with the same problem.  Just got to working on it the other day and found the VTS works when I jump it straight to the battery and switch leads to change direction. Also noticed there is a steady draw of around 4 volts on the plug coming from the box, the was a nice day so i stopped wrench'n and went riding. Going to work on it again after the 4th.
Hope this might help you a little