Customer arrives with IBM Thinkpad T40 stating that it will work from a charged battery. But it will not work with known good AC Adapter.
Works on battery, will not work on AC Adapter
The problem is caused by 2 things in this case. First the Toshiba TPC8014 MOSFET was reading bad when I checked it with a meter. After I replaced it, the unit was only supplying 10.4v to the charge section of the board. Which is not enough to trigger the adjacent mosfet to supply power to the charge section. So then; checking the Fuse [F2] found that it was blown (open circuit), most likely by the MOSFET being shorted. click on picture to see larger view
Replace MOSFET TPC8014, replace fuse.
30V N-Channel MOSFET ~11a max rating
comment: The funniest wording of what happens under constant use.
Using continuously under heavy loads (e.g. the application of high temperature/current/voltage and the significant change in temperature, etc.) may cause this product to decrease in the reliability significantly even if the operating conditions (i.e.operating temperature/current/voltage, etc.) are within the absolute maximum ratings.
To me this translates as. If this MOSFET is used continously under heavy loads it will stop working or properly (like: your computer being ON all day). In technical terms "It Will Break".
Fairchild Semiconductors FDS6690
When replacing the MOSFET, be careful not to use too much solder or you may short exposed pads under the chip (This is a typical practice of the Chinese board manufacturers to keep unskilled techs oblivious and discourage them from repairing at the component level. Computer techs eventually buy another motherboard where electronic techs fix them). Again, this is my opinion because that is not a test point under the chip and you do not have access to it under a normal repair situation. If you are not be skilled enough at micro soldering, you can pre-insulate the holes with a small piece of mylar tape to prevent solder from bridging the holes before replacing the MOSFET.
Since it was an easy solution, I am trying to be as clear as possible with the solution and the diagnosis.