Fan running in full speed

I got this imac from a friend, the original problem was that not power at all, so i made all the diagnostic suggested in the service manual and i concluded that i have to change the logic board, and i did it, now the status is the following with the good logic board:

When i plug the ac cable…

1- Led #1 is on

2- When i press the power button Led #2 flicker for less than 1 second and off

3- The fans start to work at full speed all of them and stay on forever jeje.

And that’s all…

I press the power button, stop for a second, and all the fans start to work again.

I change the RAM memory too for a good one and nothing happens :(

If anybody can help me with this i’ll appreciate it. Thanks.