T420 won't power on, stuck on blinking green light (solved)

[excal32]

I followed the manual at http://download.lenovo.com/ibmdl/pub/pc ... 0i_hmm.pdf to a "t".

This is not my first time doing a full disassembly/reassembly of a laptop.

However after putting it back together, when inserting battery or AC, the green power light on the keyboard comes on and seems to "pulse" in 1 second intervals.

I've gone mad trying to double check all my work and can't find anything I messed up. Why would it be doing this?

I can't find an answer searching here or Google at all. I did make sure to get an intel graphics motherboard so that should all be fine...

No fan spinup or anything, the power button does not respond to being pushed either, just goes green and "blinks", or it starts up just long enough for me to see the "Thinkpad" logo before it shuts off again.

[TPFanatic]

I've done full reassembly of Z60m and T500.

I've found that after removing the CMOS battery from a motherboard of the Lenovo era, the board will automatically turn on with AC power and power cycle twice. After power cycling twice, it will work as normal.

....I used this method to boot a machine without a keyboard, actually, to test the screen. Remove and reseated CMOS battery, then plugged it in and it booted by itself.

Perhaps try letting it pulse its light, indicative of power cycling, a few times and it might begin working properly

[excal32]

At this point the screen will come on for just a second and then it dies again. New motherboard bad or did i screw something up?

cmos is plugged back in BTW.

If I press the power button, a couple lights come on, it goes dead for a few seconds, tries to start again, and then same thing. never seen this before. It's actually the same behavior as if i have no battery (normal or CMOS) in and plug in power, same thing happens.

[TPFanatic]

Well... I don't know if that's normal behavior. I don't recall if the screens ever flashed during my machines' power cycling. I can pull my CMOS battery and analyze in better detail the details of revival.

My bad. Okay, now I read your posts in better detail and I think, it showing the Thinkpad logo, then shutting off doesn't sound right at all.

[excal32]

Well... I don't know if that's normal behavior. I don't recall if the screens ever flashed during my machines' power cycling. I can pull my CMOS battery and analyze in better detail the details of revival.

My bad. Okay, now I read your posts in better detail and I think, it showing the Thinkpad logo, then shutting off doesn't sound right at all.

Even weirder. Everything works... if they keyboard isn't plugged in.

Is it really actually possible the T420 keyboard is screwing it all up?!

I will make a new thread for this...

[jaspen-meyer]

Did you put the screws in the right holes? Long screws in the wrong holes can cause trouble.

[excal32]

Did you put the screws in the right holes? Long screws in the wrong holes can cause trouble.

It turned out that my keyboard was the culprit this whole time. I swapped in a replacement motherboard as one was only $20, and a brand new keyboard was $35, so for $55 my T420 works perfectly again

Not even sure if I needed the motherboard replacement? It could have been the keyboard this whole time. Either way, the feeling of a fresh new keyboard on my old T420 is great if nothing else!

[t20user]

I had a very similar issue after a recent tear-down and rebuild. My solution was to connect the track pad connector I neglected to reconnect when re-attaching the palm-rest. Hope this helps someone out there who may have the same problem.

[pattmayne]

I'm having this issue right now. I took a 1600x900 screen from one laptop (with an M2540 CPU) and put it in another with a lower-res screen and a M2520 CPU. I tested it, it started up! I was very happy. But then I put it all back together and it gave me that flashing green light as soon as I plugged it back in.

I removed the bios battery for a few minutes, then put it back in and it gave me a "date and time error." I went into the setup and saw the date was 2001. So I set it to the current date/time and restarted. And it worked! So I put it back together, and it's flashing again.

Not sure what I did wrong. So now I'm putting all the good parts back into the laptop with the M2540 CPU.

If I learn anything new I'll paste it here.

Also, here's a related issue:
https://www.thinkpads.com/forum/viewtopic.php?t=131707

EDIT:

I don't know exactly why it's working, but it's working again now. I didn't need to replace anything, although there might have been a hardware compatibility problem. I was switching around keyboards and screens.

If I had to bet, I'd say it was the advice I got from one of these threads: unplug the bios battery and leave it for a good while (I left it for more than an hour, but the suggestion had been "overnight"). When I turned it back on it made me reset the time in the bios settings. I did that, and rebooted, and it rebooted fine. It's all back together now and operating like normal.