seem like fan setting problem in bmminer.
with my S9 13.5 other firmware's plus bmminers and tricks all over the internet the custom fan speed not work my miner always follow auto fan. But custom fan speed only work with LPM firmware and the bmminer inside LPM.
my controller board is c41 v1.20
I will double check the compiled version. Possibility fan speed change not allowed but pretty sure I changed that. I know it is set so you can unplug the fans and submerge it too