T420 LED backlight

[richk]

Before I pull out what little hair I have left, I hope that someone knows more about this than I do. I have a T420 that was dropped. The base was obviously broken, but beyond that, there was no information. When I tried it, the machine would work with an external monitor, but the backlight would not come on. I tried the screen on a T410 (same connector) and it works fine with the primary LED cable plugged in. The cable on the right (that controls the status LEDs) was not plugged in because the motherboard connector was different. The strange thing was that when I tried the screen back on the T420, the backlight works if I do not plug in the right-hand cable. I believe that the lid-closed switch is on the same little card as the status LEDs. In a separate, unrelated, transaction, I acquired another T420 motherboard. When I tried the screen with the other MB, it works the same way - no backlight if the right-hand cable is plugged in. At that point, I was sure that the lid-closed switch had failed, so I bought the LED sub card, which is only available from Lenovo and expensive, even though it is tiny and there is almost nothing on it. In the spirit of full-disclosure I will say that before I got the 2nd motherboard, I assumed one of the 2 fuses at the connector had blown. They are much smaller than the ones on a T400 and are hard to test. I moved ones from a dead T410 board and it made no difference. Now, I am thinking it must be the cable, but that is another expensive, hard-to-find part and before I change it, I will post this hoping someone knows more than I do about the backlight switch.

[mikemex]

I'm sorry, I don't know much about T420 internals. I've got one but I've never opened it beyond adding some RAM and such. But I'm with you in that it sounds like it's the lid switch the one preventing the screen from turning on.

[rssb]

Faced a similar experience recently. Where a screen was replaced. The new screen would not turn on. There would be a image on the screen as if the backlight was gone, with a light shining on the screen could make out what was happening on it.

With external monitor it worked fine.

Then changed, the cable, motherboard and screen, but the inverter was the same. Then the screen started working. Should have replaced one part at a time to see what made the difference, but the tech was in a hurry he swapped all at once.