Can't control fan speed CPU_OPT

[IlMarkuz]

I just bought 4 akasa viper and an akasa pwm splitter with sata power. I'm trying to connect them on my Maximus VI Hero but if I plug it into a Chassis header it runs always at 100% regardless of BIOS an AI Suite settings.

I tried to plug it into the CPU_OPT header as suggested on the akasa pwm splitter package, but if I do that it runs always at 600 rpm. I can't find , both in bios and in Fan Fan expert 2, a way to control the the speed of the CPU_OPT header, any suggestion?

Intel i5 4670k 3.8ghz

8 GB Corsair vengeance pro red Cl9 @2133mhz

Corsair h80i

Maximus VI hero

MSI 760 OC

Corsair CX750

Corsair Carbide 400r

Seagate barracuda 7200rpm 1TB

Samsung 840 evo 250GB

[LogicalDrm]

Have you tried under stress or is that just idle?