Surface Book 2 15" motherboard fan pwm pin does not work. Need help

[Ramen_LadyHKG]

Hello there,

I need some tips to diagnose the pwm circuit.
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I've used an Arduino and tested the 5V, PWM fan is functional.


However, I don't think the motherboard is generating any signal out of the pwm pin.

I don't have an oscilloscope with me ATM but the voltage meter says 0V.

I can't seem to capture any signal through Arduino either.

I've also run continuity test cannot find any short circuit either.

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The motherboard schematic and boardview can be found in this post
https://www.badcaps.net/forum/troubl...surface-book-2

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I'm the author of this post, I made some modifications to my Surface Book 2 15", added extra fan and copper heatpipe.

https://www.reddit.com/r/Surface/com...ing_a_fan_and/


Last week, I disassemble it to replace Battery and Thermal Paste.

Out of curiosity and laziness , I want to test the (5V,0.5A) maximum speed. I connect a Lithium Battery (18650) directly to the motherboard 5V and GND without any resistor protection.
The Fan did spin at max speed at the time.


When I assembled everything back together, the fan doesn't spin even CPU hit 100C degree.


I wasn't sure if the battery was the root cause.

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[Ramen_LadyHKG]

If I short 5V to the PWM pin on the motherboard, the fan spin at full speed.
The TACH pin sits at around 3V.


So, maybe the PWM cannot get enough voltage to reach High polarity because the diode between the 5V and pwm is broken?


I don't have heat gun and the diode is extremely tiny.

What's the best way to get PWM signal back?

[FriedFred]

If this diode is just a TVS diode, for protection, if it's just blown, all is fine. If it's shorted, you need to cut it. Test the resistance over the diode. Is it nearly Zero Ohm?