Hi guys,bought a cheap x reg focus 1.6 today,but its got a wierd problem

Ignition on,charging light on,start it up,light goes out,but after 20 seconds it comes back on again??? Have had the charging checked and its charging perfectly well at 14v ish constant,so alternator is fine. Stripped back about a feet of wiring from the plug on alternator and its all good

Any ideas?? Kinda want to sort it quickly as i have someone wanting to buy it already

Cheers