Ford Fiesta Charging fault

Hi ive got a mk6 1.25 ford fiesta 2012 that im working on, it had an issue where the battery light would come on and the battery would die while driving and cut out, ive put an alternator on and it still does the same thing and ive put a new battery on it, it has the big wire going to the alternator and a plug with a single wire that goes to the ecu do you have any idea what it could be also if i unplug that little plug it it charge at about 13.7 volts and when plugged in its showing as 14.2 volts also everytime its died if you use a jump pack to start it, it will start up and show as charging

Any help would be appreciated