1986 pickup 22re automatic

Just put In a new alternator because my casing was grounding itself out. I was checking my wires leading up to alternator the 3 wires. My S wire reads same voltage as battery (12.4v) but my IG (11.98v) and L (11.75v) wires read lower with key on engine off. Does anyone know what to look for with this? I did ohm test on my grounds and they all read no resistance.

Previously I have been blowing the 80a fuse often. While the last alternator lasted 7 months I believe these low reading have something to do with them going bad. I’m just not really sure what to check when tracing down these wires. I haven’t seen and tears or broken wires and all of my grounds are good. Recent did the big 3 upgrade as well.