CISCO SG250-26P Noctua Fan replacement problems - Fans spin up, then stop after a few seconds

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New Member
Jan 14, 2024
Hey there dear STH-Community,
a few months ago I was on the look out for a 19" rack mountable manage switch with PoE+ and came across a dirt cheap CISCO SG250-26P which I scored for 100€. It was (and still is) in perfect condition, but upon powering it up, what I've not taken into consideration were the ultra loud jet-engine type fans which are not near acceptable for my environment (though I also know that this switch wasn't made for homelabs, either).

So I put the switch aside and researched on how to replace the fans and quickly came to the conclusion that it's not easy as swapping them out with Noctuas.
What I've came to learn is that the 12V and ground pins are swapped, as well as the tacho pin not being compatible with the signal of the PC's Noctua PWM wire.
I followed the few guides, YouTube videos and forum/ reddit posts and memorized the pinouts, and tried to replace the fans.
I bought two of each the PWM (4-pin) and the FLX (3-pin) 40mm Noctua fans, because I couldn't find a definite answer on which to use (I ended up using both, without luck, more on that shortly).
Using the included Noctua extension cables and Scotchlok adapters I spliced the wires to match the pinout on the Switch's mainboard. I had different configurations:
First I tried the FLX fans. I wired the 12V (red) pin on Switch to 12V Noctua wire and Ground pin (black) to ground Noctua wire, and left the blue wire (tacho?) out at first.
I know that there were complications with Fan errors in the Web panel or PoE disabled, but people seemed to find a way around that by grounding the blue tacho wire to ground (or something like that?).
I got the Noctua fans to spin up, but they only spun for like 5 seconds, then stopped forever. The Switch seemed to boot up however, since all lights on the ports lighted up.
I also tried to use one Noctua fan with my modified wire-switching cable as well as an original fan to see if maybe the firmware expects both fans to be present during startup, but it didn't change anything.
I also tried to wire the blue wire to ground shorting it on the mobo, but it also didn't make a difference.
Using the exact same configuration, but with the PWM version of the Noctua fans, also with and without shorting tacho to ground, but the fans only spun up again for 5 seconds (or similar), then stopped, so I tuned off the unit.
I should add that I measured the fan header pins for 12V and ground on the Switch's with the Noctua fans attached with a multimeter, and they only seem to deliver 6-7 volts.

It seems like some people have got it working, though I only seem to find guides for SG200 or SG300 or higher.

I don't care about the warranty since I already bought it used, and I don't expect a high security risk regarding overheating because I'm not planning to nearly use the 195W PoE budget of the switch (max. powering 3 to 5 Pis over PoE, if I even can get it working).

Do you have any experience, suggestions or recommendations?
Thank you in advance and stay safe.