Why is my exciter wire fluxuating on my alternator

i've replaced my alternator an battery an starter on my k-2500 an my battery wires from starter to battery, an battery to alternator an y charging system wont charge the battery . I've votl tested all the wires in the system that i can find an every one of them read 13.20 except the exciter wire it reads 10.29-12.40 it stays in a constant state of fluxuation