Vostro 3558, two motherboards, two displays, same problem

Hi,

At my wits end with this laptop, I plug in ac adaptor, has sense pin, no battery connected.

No power button light but white battery light, fan spins for a second then turns off and white light then flashes off then on and sits solid white forever. unplugging everything, ram, lcd etc makes no difference. Power switch tested, same symptoms on two motherboards and now two displays. Getting voltage at sense pin.