T420s thinks it's docked when it's not / AC adapter doesn't work

[Thinkso]

Hello,

I own a T420s which suddenly stopped seeing the AC adapter when plugged in: no battery charging, not powering on without batteries. It just seemed like a common problem, a worn-out dc in board, so I changed it. But that didn't fix the issue.

While trying to figure out what was wrong, i noticed that when I slid out the keyboard (without disconnecting it from the motherboard), the AC adapter would be seen by the motherboard and work fine, although being sometimes intermittent.
I tried to locate some place where it could make some kind of short-circuit but have a hard time finding it out. If I slide the keyboard back in, the ac adapter stops being "seen". Again, this can be kind of intermittent too.

As a Linux user, I noticed (from dmesg) that when it wouldn't see the AC adapter, the laptop would think it was docked, which explains why it won't use the 20V DC input.

How does the laptop detect a dock/port replicator? I don't use a dock and wouldn't mind disabling dock support altogether, but unfortunately, there is no BIOS option for that. (My T420s has the latest 1.41 BIOS version.)

Has anyone encountered this issue and has any idea how I can locate/fix it? Any hint welcome!

PS: some history: I recently upgraded to a full hd ips panel with a kit from rmsmajestic. (It works beautifully.) I took advantage of this to change the palmrest which was a bit worn out.
The AC adapter problem started about one week later. I protected the kit board with black tape.

[bit_twiddler]

You've probably thought of this, but did you have to file down any metal pieces to do the IPS conversion?
You could have a piece of metal anywhere on the motherboard causing an intermittent short.

Or, you could have a trace which is intermittently making contact, and when you install the kybd,
the motherboard flexes, causing it to open up (or vice versa).

[Thinkso]

No, I didn't have to file down anything, the new kit is very slim and easy to install.

There is most likely some signal which is intermittently open/or short-circuited, and yes, pressure from the keyboard may aggravate it; right now, after a lot of testing, the keyboard is not pushed in and the DC input doesn't work... so I'm still trying things... but earlier, it seemed to be located more on the left side of the motherboard.

[bit_twiddler]

I'd look for a nicked trace on the motherboard. Something that tells the machine that it's docked.
If all else fails, you could try another motherboard, they're inexpensive for that machine.

[RealBlackStuff]

How about trying another keyboard (from any T/W/Xx10 or T/W/Xx20)?

[Thinkso]

Thanks for your suggestions.

It turned out to be rather simple... the flat cable from the lcd upgrade kit was lying right on top of the soldered pins of the docking connector. You can take a look at the motherboard... I just didn't think it would be a problem because this side of the flat cable is not supposed to be conducting! But the connector pins underneath are rather pointy and some pin ended up getting through the cable plastics!

So for everyone installing this great kit, I strongly suggest protecting this area of the motherboard with two or more kapton tape layers (the whole docking connector area).

Everything is fine again.