You could try changing the order they connect to the splitter, to see if that changes anything. When you have multiple PWM fans on a single header the 12V, 0V, and PWM signals are common across all of the fans, but the tachometer signal only comes from one fan. Fans will typically spin in the wrong mode, but speed control is likely to have a problem.

A shorted out (to either 12V or 0V) PWM wire could probably also cause them to go to 100%, so check for damage.Ĥ pin fans are PWM, 3 pin fans are DC so check that the header's mode is correct (default is likely to be auto, which usually works ok) in the BIOS config. The current flowing through the PWM wire is negligible, so it could have been hanging on by just a strand of wire for years and finally fallen off just from normal vibrations. I guess a broken tachometer wire might cause it as well, although I'm not sure what the typical response is from the speed controller in that situation (I'm guessing it might ramp up to 100% to try to avoid losing cooling). The wires are usually tiny and can break off at the connectors or fans. Many fans will default to 100% if there's no PWM signal detected. One of the more likely hardware causes is the PWM wire of the 4 pin connection (assuming you have 4 pin PWM fans) has broken somewhere. Check the software thoroughly before going beyond a simple visual inspection of the hardware. It also shouldn't hurt to run the auto fan tuning in both the BIOS and Fan Xpert, in case the configs have somehow become corrupted.

That's based on the AI Suite variant of Fan Xpert if you have the Armory Crate variant, there should be similar settings (but I can't see them, as ASUS don't allow me to get the AC variant on my motherboard). If they do react there, it's a software type problem (which could include the BIOS fan config). In Fan Xpert, you should be able to change from 'Smart Mode' (which uses the fan curve) to 'RPM Fix Mode', and vary the RPM setting manually to see if the fans react to that. The first thing is to confirm there's no software control.

Asus chipset fan control