Dell Latitude 5490 (LA-F401P) - Boots from USB-C only, not from DC Jack after BIOS flash

[YOUCEF4787]

Hello everyone,
I'm working on a Dell Latitude 5490 with a LA-F401P motherboard that has a very peculiar issue, and I would appreciate your insights.
Initial Problem:
The laptop was completely dead, no signs of life, no power LED.
Action Taken:
I flashed the main BIOS chip with a clean dump. This brought the machine back to life, but with a strange limitation.
Current Situation & The Main Issue:
The laptop now works perfectly when powered through the USB-C port. It charges the battery, powers on, boots into the OS, and functions 100% correctly.
However, it refuses to boot from the traditional DC-in barrel jack.
Detailed Symptoms on DC Jack Power:
* All standby voltages (3.3V and 5V ALW) are present and stable.
* The power button shows 3.3V.
* The AC adapter is detected, as the standby rails are being generated from it.
* To test further, I removed the CMOS battery to trigger Dell's auto-power-on feature.
* On the very first attempt, the board powers on for a second (I can see a brief spike in current draw), but there is no display, and then it immediately shuts down.
* On all subsequent power attempts (either by pressing the power button or re-plugging the adapter), it only pulls a very minimal current (e.g., 0.01A - 0.02A) and does not proceed with the boot sequence. It doesn't complete the typical Dell power cycling that occurs with a disconnected CMOS battery.
Summary:
Power via USB-C: Works perfectly. This confirms the CPU, PCH, RAM, and main power rails are all okay.
Power via DC Jack: Fails to boot, stalling after a brief initial attempt.
Since the machine is fully functional on USB-C power, the issue seems to be isolated to the DC-in circuit path or the power source selection logic managed by the charging IC / KBC.
I have attached the BIOS file I used for flashing. I'm wondering if anyone has encountered a similar issue or could suggest which components to check next (perhaps around the charging IC?).
Board Model: LA-F401P
Laptop Model: DELL LATITUDE 5490
Any help would be greatly appreciated. Thank you!

[mon2]

Probably linked to NB_PSID on the DCin adapter connector. Be sure the power adapter is the proper one for this model. The NB_PSID line is used as a 1-wire communication interface to determine the properties of the power adapter. If this line is defective, the power adapter is assumed to be under rated by default.

Measure the voltage to ground @ PD4 (NB_PSID). Post the measurement.

[YOUCEF4787]

That NB_PSID line was actually one of the first things I checked when the issue started. My measurements are exactly as you suspected they should be:

Voltage to ground on PD4 (with the DC adapter plugged in): 3.3V, stable.
Resistance to ground on the same point (with the board unpowered): 115 kΩ.

Both of these readings seem perfectly normal, which indicates that the adapter is being correctly identified by the EC/KBC.

This makes the problem even more confusing. The board correctly sees the adapter, all standby voltages are present, but it still refuses to complete the power-on sequence from the DC jack, while working flawlessly from USB-C power.

Given that the PSID circuit seems to be fine, do you have any other suggestions on what to check next? Could the issue be with the charging IC's logic for switching power sources, or perhaps an enable signal that isn't being triggered?

Thanks again for your help.

[mon2]

What is the voltage to ground of ACOK (pin #24) on the charger IC? This should not be a logic '0' = low.

Assuming that it is, flux and remove PQ909 and test again without this mosfet.

[YOUCEF4787]

I have just measured pin #24 (ACOK) on the charger IC. The voltage is a solid 3.3V.

Your logic was correct; it makes sense for ACOK to be present since all the main standby voltages (3.3V/5V ALW) are also stable and present.

So, this deepens the mystery. To recap the situation:
- The adapter is being identified correctly (PSID is 3.3V).
- The charger IC confirms the adapter is OK (ACOK is 3.3V).
- All standby rails are present and correct.
- The power button signal is present.

Despite all these "green lights," the board still fails to complete the power-on sequence when using the DC jack, but works perfectly from the USB-C port.

Since the KBC/EC is receiving the ACOK signal, it seems the problem lies further down the power sequence. Do you have any thoughts on what to check next? Could this be an issue with the KBC itself or its communication after receiving the ACOK signal?

Thanks for your continued help.

[YOUCEF4787]

Just to add a more precise detail to my last post, as I believe this is a key symptom.

I want to emphasize the exact behavior when testing with the CMOS battery removed to trigger Dell's auto power-on feature:

1. When I plug in the DC adapter, the board correctly powers on by itself for the first time. The current draw on my bench PSU is normal and variable, peaking around 0.4A - 0.5A, which indicates the main power rails come up and the CPU begins executing code.

2. The failure happens exactly when it is supposed to perform the first automatic reboot as part of its normal boot sequence.

3. Instead of power cycling, the board just shuts down completely. The current draw drops to a negligible level (my PSU shows ~0.0004A). It never attempts to restart or cycle again. It simply dies at that step.

So, the initial power-on seems to work correctly, but the board fails specifically at the command to reboot/power-cycle. This seems to point strongly towards a logic or firmware (EC/KBC) issue rather than a primary power component failure.

I hope this extra detail helps narrow down the possibilities.