Corsair H115i RGB Platinum fanspeed question

I have a new build using a Corsair H115i RGB Platinum cooler. Everything seems to be working as expected through software, reported RPM. fan speed control, LED control etc. The thing that is odd is the fan speed reported to the BIOS.

The MB is a new Asus Crosshair 8 wifil. The Corsair block has a ribbon cable coming off it with all but 1 conductor going to the SATA power and the last conductor goes to a 3 pin connector that plugs into the motherboard fan connector.

The software reports the fan speeds to be around 450 RPM and the pump is running about 2000 RPM all is well.

The BIOS on the motherboard is reporting about 4500 RPM.

My questions are:

1) what should be reported to the MB, fan 1, fan 2 or the pump RPM or something else?

2) shouldn't there be a ground wire going to the fan connector that goes to the motherboard?

Does this FAQ answer my first question?

Thanks,

DB

A11. I have my cooler plugged into the CPU_FAN header and I’m seeing some really crazy fan speed readings for CPU_FAN. What’s up with that?

Most Corsair coolers report RPMs to the CPU_FAN header equal to approximately ½ of the fan speed. This prevents a CPU Fan warning from your BIOS when booting but and can give you an approximate speed of your pump if you aren’t running Link but otherwise it’s nothing to be concerned about. Note that the new Pro series coolers (H150i Pro and H115i Pro) report actual pump speed on the CPU_FAN header.

You don't have either of those coolers. It is most likely the BIOS is reporting twice the pump speed. A pump does not move like a fan and there is usually a multiplier/divider to turn it into the number we recognize as RPM. The 1 wire connection from pump to MB is just a tachometer wire to report that speed. I don't know why your specific motherboard interprets it this way, but it has no affect on function. The pump is only controllable through the software. You could unplug the MB lead and it would not change its operation. All power is through the SATA. All control through the USB.

However, inaccurate value or not, that motherboard connection does provide one other service. As long as it is on CPU fan, the BIOS will automatically trigger an alert and prevent you from booting to the OS if the cooler does not start at power on. You will get the classic "CPU Fan Error" warning and that is a strong indicator there is a serious problem. You will know in 3-5 seconds after pressing power vs the 30-90 seconds it would take for you to boot up and hit the thermal limit.

Is the RPM usually only supplied to the motherboard with 1 wire? I would think it would also have a ground but I think regardless of the 1 vs 2 wire, everything sounds like it's correct.

Thanks,

DB.

1 wire is enough. the motherboard ground is the power supply ground so no need to run a second wire.

I did make a custom extension with just one wire to report my D5 pump speed to the motherboard from the connexion to a commander pro, works like a charm :)