hate to beat the dead horse....charging system problems

guys, first off it's been a year or 2 since i had my mustang running. so decided to get it working.

replaced starter..cleaned the wires/terminals...all good.

threw in a used battery, started right up. however, using a midco systems checker, i have a charging problem. however my AMP battery warning light is off with the engine running, and on with the ignition on engine off.

i spent the past couple hours reading countless posts and no avail. guys i am lost...


o by the way, this is a different alternator.

thanks guys and sorry for beatuing the dead horse.

i installed a re manufactured alternator. when running, my voltage is around 12.30 i have continuity in the wires. but the only thing thats puzzling me is why the amp light is off if there is a problem?