What exactly does the voltometer measure? It's the 12V flow of which circuit: battery - alternator - battery? My 83 Jimmy, after a man-made MudFest (had to excavate and pour water tankers since we live in the friggin desert!) voltometer has dipped from its usual full 13 down to around 11, and my horns quit. Other than that all checks out fine. Where would you start troubleshooting a lower-than usual Voltometer reading?