Problem with asus A6T fan or motherboard

Hi, I'm italian, then I'm sorry for my english that probably will be not so good.. However, I got a Asus A6T, bought in 2006, but one year ago the power jack had been broken, and I had to re-weld it with stain.. Then my girlfriend used this laptop and after some mounts it had the same problem.. Until now I've been busy and only yesterday I could open the PC and weld the pin's another time, but, this time, when I finished all the work, the fan doesn't start at all when i press the power button.. the strange thing is that whit external power (5V DC) the fan works, and when I put the tester on the pins it find, when I press the power button, a not zero voltage (about 0,2V) and after 3-4 seconds the laptop shuts down again and the voltage disappears.. i tried to short-circuit the yellow wire (the velocity sensor) with red and with black ones, but nothing changed.. What can I do?

Thank you