X1C3: Fan error - PMIC broken?


I'm having some trouble with my X1C3 (20BS): While using I noticed the system getting considerably hot and throttling down. On closer inspection I noticed the fan didn't spin at all, which shouldn't happen at these temperatures. Since the system looked clean I suspected a power management bug and rebooted, now the system gets into POST but shuts itself off before accessing the bootloader due to "fan error". Resetting using the "hidden" button or cycling power (including battery) didn't show any change.

After opening the system I noticed corrosion on the fan connector - not quite unusual over here, probably due to wet climate since it's only around the air intake. Sadly the connector itself measures fine after some cleaning - I get 5V at the fan and the fan motor works fine in another 20BS. All digital lines (PWM control, speed sense and one I couldn't identify) are however shorted to ground at the mainboard side. The working 20BS shows a .4-.6V diode drop on these lines.

Whatever IC controls/monitors the fan seems to be broken. I couldn't find and schematic so far and following traces doesn't quite work on multi layer boards. So while I try to identify allICs and hope for something useful: Has anyone an idea which IC controls the fan or if there is a way to bypass the forced shutdown?

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For those searching along: I checked the board and looked in all corresponding datasheets I found for fan related content. I'm pretty sure the fan is driven by U23 (bottom side, middle), a SMSC MEC1633L-AUE embedded system controller. Sadly this one is a rather high pincount BGA and additionally features an internal flash, so while the IC itself would be available for ~5$ I would probably have to read the program from the old chip and reprogram the new one if I would try to replace it. Another possibility would be to intercept the system bus and manipulate the fan status messages, however not that easy without a proper schematic.

tl:dr: Doesn't seem like there is a viable solution for this problem. A shame since the device itself works and only monitoring seems to be broken. If only lenovo would have offered a possibility to ignore those fan errors instead of shutting the device off :(.


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