Thinkpad T400 Backlight Issue

[weethomas]

Hi,

I've got a thinkpad t400 with CCFL backlight. I recently replaced a bad LCD (the backlight was working at the time) with a new one. However, while the new screen works, it's backlight did not. I ordered a new inverter and plugged that in - still didn't work. I grabbed the old screen's backlight and tried it with both inverters - did not come on. I then grabbed another laptop I have and plugged in the old screen's backlight - it came on. Plugged in the new screen's backlight (while lcd was still plugged into the thinkpad), it's backlight came on.

So, I know both backlights are fine, and suspected I just had a crappy inverter. I got a replacement new inverter and it still doesn't work. Tried new lcd and old backlight with all three again, no dice. Backlights still turn on when connected to spare laptop.

Now, I suspect that the inverters are fine and I'm wondering about the LCD switch or LCD cable. I suppose I could just buy a cable, but before that, I wanted to take a step back and try to figure out if I'm overlooking anything. I've not been able to find a picture or location of the t400 lcd switch in either the lenovo hardware manual for the t400 or elsewhere online.

I currently have the t400 disassembled down to the base and don't see anything that stands out as a microswitch. I'm looking at a component on the LCD cable near the connector for the bluetooth radio and I'm thinking perhaps it's a magnetic switch of some sort. However, I don't know. So I figured I'd ask here. Anyone know of the switch's location? Am I correct that it's on the cable? Should I just go ahead and replace the cable?

[Harryc]

Which LCD did you have in the machine, and what did you replace it with?

[Colonel O'Neill]

That little thing next to the Bluetooth is the microphone.

I've had this happen to me. Turns out I blew something on the motherboard.

[weethomas]

Which LCD did you have in the machine, and what did you replace it with?

14.1" WXGA - FRU 42T0593. Had a warning sticker on the back saying it has a fluorescent backlight. I replaced it with one advertised for thinkpad t400s with the exact same specs. And like I said, I've verified using another laptop that the backlight works (and is CCFL because both the old backlight and backlight in the laptop that I used to test are CCFL)

The inverters were 41W1479. All three are virtually identical with the exception of what I guess is a serial number and you can tell based on the coloring of the screw hole which one is the original that was on the laptop.

* We're getting somewhere. In windows I changed the power setting to maximum performance a few days before I got the new lcd. I now modified the "what to do when the lcd is closed" from "Do Nothing" to "sleep". I closed the lcd waited, opened it fully, closed again, waited, a few times - nothing happened. Laptop doesn't goto sleep. I'll try again after a reboot but I'm really starting to think this is connected to a faulty lcd switch or something very related.

[Harryc]

14.1" WXGA - FRU 42T0593... I replaced it with one advertised for thinkpad t400s with the exact same specs.

Link to the LCD Ad or for sale item please? EBay?

[weethomas]

Link to the LCD Ad or for sale item please? EBay?

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0514204583

Though this can't be the problem. Otherwise the original backlight would still work in the t400.

[Harryc]

You bought a 'compatible' LCD and not an original FRU. The compatible LCD's on EBay and elsewhere have been know to cause issues in some cases. I asked for the link so that I could read the feedback on the same LCD. I found a couple of positive feedbacks, so others have gotten it to work. As you suggested look at the LCD switch or possibly a fuse that blew on the systemboard. If it's a fuse, you have to question why it blew to begin with.....the new LCD as being one possibility? Even though others have gotten it to work, this particular one could be defective.

[weethomas]

You bought a 'compatible' LCD and not an original FRU. The compatible LCD's on EBay and elsewhere have been know to cause issues in some cases. I asked for the link so that I could read the feedback on the same LCD. I found a couple of positive feedbacks, so others have gotten it to work. As you suggested look at the LCD switch or possibly a fuse that blew on the systemboard. If it's a fuse, you have to question why it blew to begin with.....the new LCD as being one possibility? Even though others have gotten it to work, this particular one could be defective.

You are forgetting something. The LCD works fine. The backlight is not turning ON. The LCD and backlight are separate devices that are packaged together. If I wanted, I could just replace the backlight in the new LCD with the working one from the old LCD.

Here is the problem: I plug the new LCD's backlight into a different laptop - it turns on. I plug my OLD backlight into a different laptop, it turns on. So - both backlights - new and old are fine and are not destroying inverters or any other circuitry.

Do you know where the LCD lid switch is located? This is the main problem I have now. Without that knowledge, I can't test any further. Just stuck changing out components.

[Harryc]

If I were to guess the lid switch would be on the inverter.

[weethomas]

If I were to guess the lid switch would be on the inverter.

A magnetic switch couldn't be placed on the inverter and I don't see a mechanical one. The closest I can find online seems to suggest a switch in the keyboard bezel.

[Harryc]

There are several models where that is exactly where it is ... see this thread as an example. It's a tiny reed switch. We're talking maybe 2 mm's here.
http://forum.thinkpads.com/viewtopic.ph ... 63&start=0

[weethomas]

There are several models where that is exactly where it is ... see this thread as an example. It's a tiny reed switch. We're talking maybe 2 mm's here.
http://forum.thinkpads.com/viewtopic.ph ... 63&start=0

Hmm. Ok, I'll order another inverter - from a different seller and this time try it without the LCD plugged in with the old backlight. If it works, I'll just go ahead and swap the backlights and call it a day.