Lenovo T480 Power cycle

[Clarify4082]

Hello,

I need some help troubleshoot my T480. It´s restarting after 10 seconds without showing screen-output.

I´ve already flashed the bios but this doesn´t help.
Now I´ll try to check the board. I´ve already done some measures:

PU201/3 = 3,30
PU201/13 = 5,03
PU201/12 = 0
PU201/6 = 0
PU201/20 = 0
PU201/7 = 0
PJ201 = 10,3
PJ202 = 10,3
PQ202/4 = 16 MV when running
PQ202/5 = 5,13 V when running
PR217 = 0 / 5,13 when running
PR218 = 0 / 5,13 when running

I think the output of the mosfet PQ202 / Pin 4 is not working, but I´m not sure.

Please find the schematic attached

[mcplslg123]

Re: Lenovo T480 Power cycle

I think your measurement on pin12 is wrong.It cant be 0V as PJ201/202 is 10V and you confirm presence of 3V LDO/5V LDO.

Problem is missing M_ON

[Clarify4082]

Re: Lenovo T480 Power cycle

You´re right, the measurement is wrong, here we go again:
PU201/1 = 0
PU201/2 = 0
PU201/3 = 3,30
PU201/4 = 10 MV
PU201/5 = 0
PU201/6 = 0
PU201/7 = 0
PU201/8 = 16,9
PU201/9 = 4,8V
PU201/10 = 16,9 MV
PU201/11 = 0
PU201/12 = 10,3 V
PU201/13 = 4,2 V
PU201/14 = 0
PU201/15 = 0
PU201/16 = 0
PU201/17 = 4,8 V
PU201/18 = 0
PU201/19 = 0
PU201/20 = 0

[Clarify4082]

Re: Lenovo T480 Power cycle

Measuring on PIN 13 was also wrong:
PU201/1 = 0
PU201/2 = 0
PU201/3 = 3,30
PU201/4 = 10 MV
PU201/5 = 0
PU201/6 = 0
PU201/7 = 0
PU201/8 = 16,9
PU201/9 = 4,8V
PU201/10 = 16,9 MV
PU201/11 = 0
PU201/12 = 10,3 V
PU201/13 = 5,03 V
PU201/14 = 0
PU201/15 = 0
PU201/16 = 0
PU201/17 = 4,8 V
PU201/18 = 0
PU201/19 = 0
PU201/20 = 0

Please see attached more measurings. What can I check next for missing M_ON?

[mcplslg123]

Re: Lenovo T480 Power cycle

M_ON is generated by SIO.So you need to check the requirement of SIO. Link the schematic so that i can be precise what to check.

[Clarify4082]

Re: Lenovo T480 Power cycle

Hi, thank you for your help! I´ve attached the schematics.

I´m not sure if it is really a hardware issue. Because I get alle measurements when I press power and the laptop is running in the power cycle without screen output.

I´ve now started the laptop without RAM. The audio signal for missing RAM comes and than the laptop stays on, so no bootloop without RAM. I think this is a good sign?
Otherwise, when I unplug all batteries and also the CMOS-Battery, ther should also be a beep code on next startup. But there is no.

I´ve no idea what to check next , so really apprechiate your help!

MOD Edit ,link to schematic - https://www.badcaps.net/forum/showthread.php?t=85624
Reminder: rules and organization of this section - https://www.badcaps.net/forum/showthread.php?t=117483
.

[Techtiger]

Re: Lenovo T480 Power cycle

Measuring on PIN 13 was also wrong:
PU201/1 = 0
PU201/2 = 0
PU201/3 = 3,30
PU201/4 = 10 MV
PU201/5 = 0
PU201/6 = 0
PU201/7 = 0
PU201/8 = 16,9
PU201/9 = 4,8V
PU201/10 = 16,9 MV
PU201/11 = 0
PU201/12 = 10,3 V
PU201/13 = 5,03 V
PU201/14 = 0
PU201/15 = 0
PU201/16 = 0
PU201/17 = 4,8 V
PU201/18 = 0
PU201/19 = 0
PU201/20 = 0

Please see attached more measurings. What can I check next for missing M_ON?

The TPS51285B is not generating the output voltage for PL201/202 +5V/+3V. The M_ON signal is enabled to this IC, so you need to trace it back

[Clarify4082]

Re: Lenovo T480 Power cycle

Okay, I think I messed it up completely

I was taking the measurements again, because I´ve noticed some difference:

The old measuring was without the MB in the case and without CMOS Battery. When I plugged the charger, the boot loop started direct after I plugged the charger. So I hold down the power button to turn it off and took the mesasurings.

Today the MB is in the case and CMOS Battery is plugged. When I plug the charger, the MB is not starting directly, but the power button is blinking 3 times. So I think, that´s the "normal" behavior.

While taking the measuring from PU201 I shorted the pins (I thing 1 and 2 ) with the multimeter

Now the board is dead and nothing happens when I plug the charger and I have no more voltages besides 19V.

Now I first have to figure out, how I get this running again.

[mcplslg123]

Re: Lenovo T480 Power cycle

We're on a wrong track. M_ON is generated after power on only-so nothing wrong there. Why it was in a loop needs to be investigated but now it has gone completely dead.

Suggest to remove PU201 and check resistance to GND on all pins without the IC soldered and no power attached.

PS: Dont put any power(both adapter & battery) while PU201 is de soldered. Else mb will go to trash.

[Clarify4082]

Re: Lenovo T480 Power cycle

Hi, okay, I´ve desoldered the chip. I´ve set the multimeter to beeping mode.

Beeps on:
PIN: 8
PIN: 12
PIN: 14
PIN: 18

It´s gonna be a nightmare to solder the chip back on with my soldering skills

[mcplslg123]

Re: Lenovo T480 Power cycle

PIN 12 is your VIN aka 19V main rail-so its shorted to GND. Locate PR58 which is your CLR and de-solder it. Measure resistance to GND on both pads of PR58. The side which is shorted to GND needs voltage injection from DC supply to find out what is shorted. The voltage should not be more then 1V. Do you have a dc bench power supply?

[Clarify4082]

Re: Lenovo T480 Power cycle

Unfortunately I don´t have a bench power supply, but I could get one if there is no way around!?

I cant´find PR58 in the schematics, I´ll have a look at the board.

[Clarify4082]

Re: Lenovo T480 Power cycle

So, just ordered a power supply. I Found PR158, do you mean this?

I´ve desoldered it and the bottom pad goes down to around 2 MΩ. Upper Pad goes down to 2 MΩ and then to 0.3 MΩ.
No beeps when I put the multimeter in beep mode.

[Clarify4082]

Re: Lenovo T480 Power cycle

I´ve soldered PU21 back on and checked PIN12. It´s 1.8Ω. I´ve checked for other shorts, but can´t find any. Every reading is > 1Ω.
The only thing that is getting hot when I plug in the power cable is the GPU.

[mcplslg123]

Re: Lenovo T480 Power cycle

Yes,i meant PR158