This truck in its past had been converted to 12 volts but I did discover that the alternator exciter wire had been cut off near the alternator and that wire had no in-line diode, also they had incorporated a "idiot light" that was also cut off below the dash. I've installed a new excite wire(with diode) / sense wire harness and wired in the alternator "idiot light." The excite wire is wired to the ignition switch and coil.
