One of my clients brought in a Lenovo (IBM) Thinkpad T420i for an inspection after his client reported a water spill. When you press the power button the ‘Caps Lock’ led would light up. The laptop would power on then shut off after a second or two. Repeated pressing would make it boot, but it was very unpredictable. So I disassembled and inspected the machine. There was some watermarking but no damage on the motherboard so I reassembled it.

My client thought that maybe the keyboard was the problem and I agreed. Since the Thinkpad power button is built into the keyboard if the keyboard is damaged it can keep the notebook from powering on. Standard troubleshooting logic is to verify that the notebook is ok by powering it on without the keyboard. But how?

Using a meter measuring volts clip the black probe to a ground on the laptop and use the red probe to check each keyboard pin one by one looking for 3.3volts. Then once you find it you connect one end of a ten ohm resistor to ground and the other to a probe with which you tap the pin where you found the voltage. If its the right pin the laptop will respond just like you pressed the power button. If it’s the wrong pin the resistor will prevent damage from excessive current flow from occurring. On the T420i its pin 19. Started right up and ran perfectly, so he ordered a keyboard. This is a good example of troubleshooting by elimination.