Weird problem with keyboard backlight in Thinkpad T470

[szogun]

I`ve bought two T470, both of them has a backlight keyboard, and in one of them (lets name it computer A) it doesn`t work.
There is no "welcome light up" and no reaction to Fn+Space combination. My first suspect was the keyboard itself, so after checking the connectors, cleaning them na ribbons which didn`t help, i`ve swapped the keyboard as i have second computer (lets name it B)

From now, things are getting weird.
Both of keyboards started to work as it should. Welcome light up, switching on and off, changing brightness etc.
Computer B with keyboard from A works flawlessly.
Computer A doesn`t. After few cycles of getting into sleep mode, backlight stopped to work. When i pressed Fn+Space, the onscreen notification singalized changing state of backlight but keyboard itself didn`t respond (it was off).
After reboot it`s back to square one. No backlight, no notification.
Swapping keyboards didn`t help this time, however both of them works fine in computer B.
BIOS version in both of them is 1.67.
A i running on Fedora, B is running W10.
After all, i think that there is problem with KBD_BL_DTCT signal, but VCC3M is ok, so maybe MEC1653 is faulty?
Have anyone have similiar problem?
How is the detection of backlight works? Can i override this?

[mon2]

Re: Weird problem with keyboard backlight in Thinkpad T470

Since they are the same system, can you take the HD/SSD with Win10 from Computer B and test on Computer A ?

Only to confirm the issue is not s/w related.

Otherwise:

1) Monitor the voltage of -KBD_BL_DTCT on the working Computer B. Believe the '-' prefix = Active LOW ? But confirm this logic level.

Under the same condition, what is the value on Computer A ?

When the backlight is not working, what are the values for this rail on Computer A ?

2) Monitor the voltage of KBD_BL_PWM which is responsible for the backlight intensity as you increase / decrease with the keyboard action.

What are the voltage values on Computer B ? Computer A ?

3) Investigate the voltage on -LID_CLOSE for the working computer and compare the reading on the non-working Computer A. Is the opening / closing of the LID a factor ?

4) Be sure that the keyboard mating connector and cable are clean and firmly seated onto the logic board.


Since the MEC1653 controls many other functions like BACKLIGHT_ON (LCD Display) and others, believe this component is working.

[szogun]

Re: Weird problem with keyboard backlight in Thinkpad T470

Since they are the same system, can you take the HD/SSD with Win10 from Computer B and test on Computer A ?

Regretfully, no luck.

1) Monitor the voltage of -KBD_BL_DTCT on the working Computer B. Believe the '-' prefix = Active LOW ? But confirm this logic level.

Under the same condition, what is the value on Computer A ?

When the backlight is not working, what are the values for this rail on Computer A ?

Yes, it`s active low.
I`ve done that before, and at first everything looks the same, BUT what i didn`t check, is voltage on that rail when computer is turned off.
Computer A hits 3,3V, when in computer B it`s 0V.

2) Monitor the voltage of KBD_BL_PWM which is responsible for the backlight intensity as you increase / decrease with the keyboard action.

What are the voltage values on Computer B ? Computer A ?

Computer A: 0V, Computer B: 0V, 1V, 3,3V dependent from brightness of backlight

3) Investigate the voltage on -LID_CLOSE for the working computer and compare the reading on the non-working Computer A. Is the opening / closing of the LID a factor ?

That led me, to measure voltage on when computer is turned off.

4) Be sure that the keyboard mating connector and cable are clean and firmly seated onto the logic board.

checked,no problems here

Since the MEC1653 controls many other functions like BACKLIGHT_ON (LCD Display) and others, believe this component is working.

It`s true, but isn`t it also responsible for turning VCC3M on and off? If it failing to do that, maybe this is the source of problems.

[neth524]

Re: Weird problem with keyboard backlight in Thinkpad T470

I know this is an older thread, so sorry in advance.

I am having a very similar issue. I have a two T470, one is I5-6300 and the other is I5-7300. They both now have a backlight keyboard, but the 6300 is the only one that lights up and that Lenovo Vantage recognizes as a backlit. I have swapped the boards around, so I know that both boards do light up correctly. Both are fully BIOS updated and win10 updated. I don't know hardly anything about Linux, but I did make a usb boot for ubuntu, and found the right drivers for the light but didn't work there either. It also doesn't light up in the BIOS. I am not sure what the issue could be. I checked the fuse for the keyboard, and it works. I am now leaning towards the MEC1653, but I have no idea if 1, that is it replaceable, or 2, if it is even worth doing.

szogun, were you able to find a fix for your issue?

[clincher]

Re: Weird problem with keyboard backlight in Thinkpad T470

Apologies again for digging up an old thread, but I just ran into this issue as well. Everything checked out with voltages, however after a some serious banging my head against the wall , I inspected the JTP1 connector that contains KBD_BL_PWM and -KBD_BL_DTCT.

On inspection under a microscope, I could see some of the pins on the top layer had become bent up ever so slightly (probably from removing the keyboard and not releasing the clips instead of sliding the connector out). I gently bent them back down to give some more pressure to the connection and BOOM! that was it. Powered the machine back on and the backlight is working agin.

I doubt either of you have your boards still, but hopefully this helps someone else who comes along in searching.

Cheers!