Dell Latitude 7420 - Touchscreen not working since Mainboard replacement

[Mirenia]

Hi everyone,

I've got a little "problem child": my Dell Latitude 7420 2-in-1, which had a faulty mainboard.
I replaced it, but ever since, the touchscreen no longer works — and honestly, I’m running out of ideas…

In Windows Device Manager, I just see an I2C HID Device with error code 10. Sporadically, the touchscreen is recognized correctly for just a few seconds, but I haven’t been able to determine under which circumstances this happens.

The touchscreen also doesn’t work in UEFI/BIOS, although it’s enabled there (and Dell states, that it has to work there)
I’ve updated and downgraded BIOS, disabled and re-enabled the touchscreen there. I replaced the entire screen assembly including the eDP cables twice. I’ve reinstalled Windows, tried Linux, tried different drivers, performed NVRAM resets and factory reset. I checked all connectors related to the screen on the mainboard.

Do you have any ideas on what I should check next? Maybe some components on the motherboard itself I should have a closer look? There are no shorts or blown fuses around the screen-related area.
Could this possibly be an issue with a faulty BIOS?

Original Service Tag: 9SWP2J3
Service Tag of the new mainboard: GJW83J3

Mainboard in both cases: GDC31 LA-K371P Rev:3.0 A02 (original with an i7, replacement with an i5-1145G7)

I've got schematics and boardview, can handle a multimeter, solder station and understand circuits (but currently learning/practicing to fully understand complicated motherboard/laptop schematics like the dell ones)

I am grateful for every tip!

[mon2]

Try this:

https://www.youtube.com/watch?v=a9LN7Tv3HpM

[Mirenia]

Try this:

https://www.youtube.com/watch?v=a9LN7Tv3HpM

I already tried to disable and enable it (multiple times) in device manager.
And tried to untick all power management checkboxes for every single device.

But you remind me of something unusual there:
The power management settings of the i2c host controller (which the touchscreen is connected to) resets with every reboot. So I have to untick the checkbox for power saving there every time.

[mon2]

Which Windows are you on? Dell really needs to step up on supporting their crapware they push out to the masses.

You have a s/w issue and the proper driver is not pacifying this hardware. This hardware is most likely linked to the chipset drivers. Code 10 means the driver cannot start with assorted reasons including the driver is the wrong one.

Another article (not sure if it will work for you):

https://www.dell.com/community/en/co...ffb86bf8700e1b

Have you contacted Dell for help? They should at least be able to guide you to the proper download.

[Mirenia]

I'm on Win11. Tried nearly all relevant drivers on dell support page for the old servive ID and the new one.

Tried with the original Win10 image from dell as well.

Do you think, that it is really only a driver related issue?
Wouldn't the touchscreen work in BIOS without having the correct win driver installed?

The hardware ID of the faulty i2c hid device is strange, too:
ACPI\VEN_CUST&DEV_0000

I expected there some specific ID.

I tried to contact dell via whatsapp support. But never got the chance to write to a human because I have no active service plan.

I'll have a look at your link now

[mcplslg123]

Its not a driver issue as touchscreen should work flawlessly in bios setup.
Seems hardware issue to me.

[Mirenia]

I would assume the same. My guess is that somehow on the motherboard the touchscreen signal is "scrambled"/malformed.

But maybe I need a little guidance from the experts here where to start troubleshooting/diagnosing.
(As I said earlier: Screen lid component and cables are replaced 2 times)

The mainboard has multiple connectors for everything in the screen lid. A "normal" display connector, a touch connector and two more for IR, camera, microphone and gyro stuff. As far as I could read from schematics: there are a few touchscreen related pins on the display connector itself. But there begins my struggle: how can I trace them?
The display connector looks super complicated and I don't know where to start with "counting" pins.

Are only the pins in the middle relevant? Or are there more (in the metal frame around?)
Is there a spec from this kind of connector available?

Or do you have other recommendations, where I should start?



EDIT: I already checked the resistors, which are pulling the signals for touch.

[mon2]

Check if you have a SECURE BOOT option in the bios. Turn it off and save the settings then test again.

[Mirenia]

Maybe I found the culprit(s)...
QV22 seems to have emitted magical smoke. QV23 also looks like it has a burned bump.

In schematics they're responsible for Touchscreen power.

Does anyone know, what TS_5v_Det could means?

Detection if it is a 5V Touchscreen or a 5V rail, which detects if a touchscreen is connected?

[Mirenia]

Good news:
i replaced qv22 and qv23 with help of my other defective board and touchscreen is finally working!!!!

[mcplslg123]

Good job.