12-14-2018 10:41 AM
To add some more discoveries to the ongoing problem, I have two new bits of information. First, I noticed that the screen doesn't go completely black until it gets past the bios and goes into windows (I can faintly see the splash screen and bios menus, better if I hold a light up to it--but it goes out completely if I go into windows). I decided to test another OS to see if Windows might be the problem, so I created an Unbunto boot stick and booted off that. It had the same symptoms--I could faintly see the Unbuntu spalsh screen, then also the boot menu asking if I wanted to "try" or "install". When I selected "try", as it started to boot Unbuntu the screen went completly black. As with windows, I plugged in an external monitor and it worked fine. On this same point, neither Windows nor Unbuntu can recognize the internal monitor, only the external (both OS' only see one monitor attached to the computer). This last part is weird to me since the laptop's screen is (faintly) displaying at startup.
The second thing I observed today, was that if I move the lid back (further open) it causes the screen to flicker and cut out. I can specifically replicate it my flexing the lower left hinge area. I am perplexed by this, as the main LED cable is nowhere close to this and I thought it was just some antennas running through that part.
Does anybody have any ideas? I know the LED cable isn't being pinched or pulled and I know it is seated as I just replaced it again.
09-17-2019 10:04 AM
Found the solution for anyone else with a T400 that has the dim screen issue. Despite the advice given here, the display panel was fine, the cable was fine, the inverter/LCD control panel was fine. And the motherboards were fine until they got installed in my system and had a fuse blown--BY THE BLUETOOTH CARD. The bluetooth cards in these systems are crap and die early, then they short out the fuse on the motherboard that powers the display backlight. I got another motherboard, removed the bluetooth card, and put everything back together with the original screen and cable and everything is working like new.
Again, for anyone with a T400 with a dim (almost blacked out) display issue, do not waste your time or money following the advice others have given here by chasing screens and cables. Just remove the bluetooth and replace the motherboard (or fix the blown fuse). If you leave the crappy bluetooth in it will just immediately short out the new motherboard.
09-17-2019 11:28 AM
More possibilities. Do you have an LED backlight or CCFL? T400 had both options. If CCFL, it could be the inverter. It could be the Hall sensor (lid-closed switch). If you changed the motherboard before the screen and the old screen was shorted, you could have blown another fuse.
09-18-2019 09:14 AM