I am having a similar problem with a Dell 5150. The power light blinks slowly amber when I try to turn it on. I checked the power supply, out of the machine, and when I grounded the green wire which is pin 4 sometimes labeled P_ON everything worked as it should. When I reinstalled the power supply and shouted the P_ON wire the machine started up fine. I had to put the machine back into service so I didn't have time to do anymore testing, but the machine would not start when I brought it back out to the production floor so I had to jump the green wire again to start it.