Two things could cause the ammeter to work wrong. First would be that the ammeter would show charge when the lights are on and the engine not running, in which case either the ammeter is hooked up wrong or the battery is the wrong polarity, S/B negative ground on Chevrolet. The other would be that the ammeter shows discharge with lights on and engine off and goes farther to discharge when the engine is running, which would mean that the generator is polarised wrong and would cause a twelve volt differential between the generator and battery and fry wires.
