The Corsair H45 cooling system is designed to have the pump unit always run at full speed using a fixed 12 VDC power supply and reporting its speed to the mobo header. The FAN on the radiator should be connected to a 4-pin mobo header that will vary the fan's speed according to the internal CPU temperature, using the new PWM Mode of control. The exact instructions for this are not spelled out well in its manual. Further, the exact abilities of the fan headers on you mobo are not clear it its manual. either. But I can see how to do this properly by making a few assumptions that certain things are "standard".
As background, I'm going to make use of a quirk of fan design that is widely used in these systems. If you plug a 3-pin fan into a mobo header that actually is using the new PWM Mode to control it's fan's speed, that fan will NOT have its speed controlled. That is because, in the new PWM system, the header puts out on Pin #2 a constant +12 VDC supply, and then sends the fan the new PWM signal on Pin #4. A 4-pin fan will use the PWM signal and a special chip inside it to modify the flow of current from that power supply though the windings of the motor to change its speed. But a 3-pin fan will never receive that PWM signal, and it cannot use it anyway because it does not have that chip, so it runs full speed always. That is exactly what we want the PUMP unit to do.
So, OP, what you need for this is a simple 4-pin fan SPLITTER with two output arms, like this
https://www.newegg.com/p/N82E168124..._re=Coboc_fan_splitter-_-12-423-160-_-Product
If you look closely at its two outputs, you will find that one of them is missing its Pin #3. A mobo header can handle only ONE fan's speed signal sent back to it, so this Splitter ignores the speed signal of the second fan by NOT having a Pin #3 to connect to it. We are going to use the Splitter to connect both the pump and the fan units of the H45 system to the mobo CPU_FAN header, which DOES use PWM Mode and also depends on a temperature sensor inside the CPU chip to guide its actions. You plug that Splitter into the CPU+FAN header. Then find the output that HAS all four pins in it and plug into that output the PUMP's 3-pin connector. Plug the FAN's 4-pin connector into the other Splitter output that is missing Pin #3. When you do this, the PUMP acts like it is connected to a normal CPU_FAN 4-pin header using PWM Mode (and that IS the connection here) and it runs full speed all the time. It also sends back to that header the PUMP's speed signal. Besides controlling fan speed, the header monitors the speed of its fan for FAILURE and will warn you and take protective action if its "fan" (really in this connection system, the pump) does fail. On an AIO system like this, the critical component is the PUMP, and any failure of it will result in no CPU cooling and rapid temperature rise, so that is the item that needs to be monitored for failure. Now the FAN, on the other hand, is also connected to that same header using PWM Mode for speed control, and this 4-pin fans IS designed for that and WILL have its speed controlled that way, according to the cooling needs of the CPU chip. The one disadvantage of this is that there is no way for the mobo to know or display the actual FAN speed, so you will never "see" that info AND the mobo header can NOT monitor the FAN for failure. IF it does fail, the cooling of the CPU will be poor and its temperature will rise until a different system detects a CPU internal temperature above a pre-set limit, and slows the system down to prevent overheating. This is a slow process and the automatic failure response system can handle it well, so it does not require rapid response. But that also means that, from time to time YOU should check the FAN on the radiator and verify that it is still operating properly.
When you do things this way the CPU cooling system gets the power AND the right control signals to do its job properly using only the CPU_FAN header. This leaves the two mobo SYS_FAN headers free for use for case ventilation fans.
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