ok....the power to alternator is a constant battery voltage power, if you pull the green plug out, you short take a test light, an check the power, you should have one power 12.00 volts, an one is a 5.0volt reference, an a ground, then a signal wire where your ELD sends a signal out of the computer to the alternator to tell it to charge, hope this helps, most causes like yours its the alternator, an gas milage?, check your timing, since you put a used engine in with miles on it, its due for one, an if you put a timing belt on, then you might have it off a tooth, check your plugs, the gap,tune up, O2 sensor,fuel filter, Bad fuel preasure regulator, etc. hope that help you to go in a direction,