Re: Solution for ANTMINER S9 FANS SPEED Up & Down every 20 Seconds.
I just found another post here on the forums about cutting the blue wire... and it worked :p thanks for the earlier reply with the information though, if you hadn't told me I might have tried the splicing.
Glad I could be of some help 
The blue wire tells the fan how fast you want it to spin, and the yellow one tells you how fast it is spinning. I had assumed "the signal" to mean the yellow wire.
Generally if you are using a 3-pin fan or the blue wire has been removed, the fan will spin as fast as possible.