It's been quite a while, but I had the same problem with my A1150 & I was working on it one day, but forgot to disconnect the battery & when I unplugged both fans, it magically started. I tried to figure out every different thing that could possibly be happening & even switched out the i/o board, but the only way it will start is if I unplug both fans & then plug back in as soon as the optical drive starts up. I couldn't explain it if I had to but I'm on it right now & it works as if nothing is wrong once it starts, I just can't get it to start without unplugging the fans first?? Completely Random
Jess