MacBook air 13.3" 2012 motherboard shorted out

I made a stupid mistake. Back cover was off with laptop on and and charging. I pulled out the battery cable out wanting to run off power supply. The screen went completely white, I shut the laptop off and since then when I pawer the laptop on, there is a black screen , I tested with another motherboard and the screen is working. When I power up the laptop, the fan starts slowly than speed up to maximum and after about 40 seconds the keyboard light up and goes off after 20 seconds , but when you press the power button light up again. When you plug it into the power supply the fan would go on without pressing the start/on button. I did reset SMC Shift+Control+Option+Power

and a PRAM reset Command+Option+P+R without success.

I assume motherboard has shorted

I am not a expert in motherboard reapair and can replace components, bought a multimeter but do not know where to check for fault.

Thank you