T400 no screen light

[ajg]

I have a T400 with a CCFL backlight that used to stay dark occasionally, but closing and reopening the lid would cause it to come back on. Then the screen went abruptly dark, except for a flicker while the bios loaded (no pinkish color associated with bad bulbs ever occurred). The lcd is working, it is just the bulbs that do not turn on. Also, the VGA output is working normally. I have replaced the inverter, cable, and screen, but the problem remains. The screen no longer flickers when the bios is loading, it is just dark 100% of the time. The only component left I can think of is the fuses on the mother board, but if those were bad the screen shouldn't have been flickering right after boot. Any help that can be given to diagnose the bad component is appreciated.

Thanks,
Andrew

[rkawakami]

Welcome to thinkpads.com!

It seems like you've changed out everything that could be the cause of the problem, except for two items. I'm assuming that you used a known good LCD panel as a replacement so that the CCFL in that panel has been removed from the list of possible faults. If so, then the fuse on the motherboard could still be the issue, along with the "switch" that senses when the lid is closed. It is fairly easy to test the fuses if you have a multimeter (ohmmeter). I believe that the power to the inverter goes through fuse F7 but you should check F3, F8 and F16 as well. Unfortunately I cannot tell you exactly where those fuses are located on the motherboard but previous experience says to start looking around the area of the LCD connector and move outward. I don't think I've ever seen a Thinkpad motherboard that had the fuses protecting the LCD and associated circuits mounted on the bottom of the motherboard. With power to laptop removed, connect the ends of the multimeter to the ends of the fuse and if it reads anything above 0.1 ohms, then that fuse is bad. Once you get to that point, then we'll have to figure out exactly what the rating of the fuse is in order to specify a replacement. Searching the archives, I ran into this, although I cannot recommend you doing the same thing: http://forum.thinkpads.com/viewtopic.ph ... 93#p731793

As to the lid switch, it should be a switch that is activated by a magnet. Again, I don't know exactly where the switch and magnet are located on the T400 but if you dangle a steel needle around the edges of the LCD and keyboard bezels, you should be able to find where the magnet is. The switch would then be located opposite of that position when the lid is closed. With most of the recent designs, I believe that they have the magnet in the base somewhere near the front edge and touchpad and the switch up in the lid along the top edge. I think that it's rare that the switch gets stuck in the " lid closed" position as that normally requires that a magnet be located very close to the switch but I suppose it's possible.