Thinkpad X390 (motherboard NM-B891 shared with other models) : battery sensor reporting wrong temperature -> sudden power off

[ErParucca]

I have a thinkpad X390 that would work fine if not for a problem: sensor temp5 (battery as far as I know) shows variable values over 100° C. When it reaches higher temps (I guess 110° as I was able to list temperatures up to 109°) it simply powers off. All other sensors show 0° except for the CPU which seems to report correct temperatures.

In a wishful thinking I hoped replacing the battery (as far as I know the temperature sensor is in the battery) would fix the problem, but it didn't.

Given this situation I can't even upgrade the firmware as the machine powers off in the process (had to reflash the BIOS with CH341).

Schematics are already in other posts and I found no traces of the temp sensors.

Any suggestion? Thx!

[mon2]

Remove all power. Disconnect the battery. Meter in resistance mode. What is the resistance to ground of pin # 4 (M_TEMP) on the battery connector?

This pin mates with the ADC (analog to digital converter) on the EC to gauge the temperature. Be sure there is a pull-up on this line. Disconnect the battery and power up. Measure the voltage to ground of M_TEMP. You should have a voltage here of ~3v due to the PR0101 pull-up resistor. Confirm it. This resistor with the capacitor @ PC0102 will build a RC circuit much like this:

https://www.teaching.eng.ed.ac.uk/op...elay-use-timer

Confirm that the series resistor @ R8510 = 1k ohms. With no power to the unit, measure the resistance across R8510. Be sure it is ~1k ohms.

[ErParucca]

first of all, thank you for taking the time to help.
I have no readings between ground and pin4.

I have an AMD version (Thinkpad X13 Gen1) of the same laptop (same chassis, same battery) which behaves exactly the same.

Went through this video to grasp what a pull-up/pull-down is but still have to understand what I should test with my multi-meter (I am expert at IT but not at all at electronics).

Tried to power up the laptop and it won't power on anymore. USB Power voltage now reads 5.2v instead of 20v and I can hear some coil whine when the AC adapter is plugged. The laptop powers up if the battery is plugged. Could I have physically broken something while testing with the multimeter? I doubt something could have electronically fried as no power source was connected while testing).

GND-M_TEMP reading when running on battery was of 2v.

R8510 : no readings (tried also on other nearby resistors): I fear my toy-multimeter's tips are too thick to be used on components with such a small footprint will get new thinner ones and try again. In the meanwhile: can you help understand what did I messed up to prevent USB to negotiate input at 20v?

thanks again!

PS: photo of the MB attached in case you can identify something off.