Question xps 8920 and H80i v2 cooler upgrade: cpu fan error because of missing pin

Sep 10, 2019
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After installing a Corsair h80i v2 cooler into an XPS 8920 special edition i7-7700k, I get a cpu fan error on boot up, despite the fans and pump working perfectly.
It looks like it is related to the fact that the 3-pin pump connector connects into a 4-pin CPU_FAN connector on the motherboard (the 4th unused pin is usually for the fan tachymeter wire). The pump does not have a tachymeter and the motherboard freaks out.

Facts:
-The XPS 8920 BIOS doesn't allow deactivation of the CPU_FAN check (i.e. missing signal from 4th wire).
-4 wires comes out of the two fans, which are hooked up in the water cooler.
-The water cooler pump has a 3-pin fan connector which connects into the CPU_FAN header of the motherboard as per the manual.

Is there an easy way to split the outgoing tachymeter signal from one of the two fans (which both have each 4-wire connectors) and send that single wire signal into the unused CPU_FAN header ?

Is there another easy way to circumvent this problem ?

Thank you so much for your help.
 
You have misunderstood the wires and signals, so your problem is not quite what you suspect. For the CPU_FAN 4-pin male header, the pin functions are:
Pin #1 - Ground
Pin #2 - +12 VDC supply, constant voltage
Pin #3 - fan (pump) speed signal generated in the fan and sent to mobo here
Pin #4 - PWM signal.

When a 4-pin fan is plugged in there, the speed of the fan is adjusted by a small chip in the fan housing that uses the PWM signal from Pin #4 to modify the flow of current from the +12 VDC supply (pin #2) through the motor windings. The motor also generates a speed signal (2 pulses per revolution) that is sent back to the header on Pin #3. This is used by the header both to display that speed for user info and to monitor that fan for FAILURE, which is what can generate a warning message.

When that header is used in an AIO system the functions are used differently. In the Corsair system the PUMP is wired like a 3-pin fan would be, and it does not have any special speed control chip because it is intended to run full speed all the time. So it does NOT need or use the signal on Pin #4. BUT it DOES need the power supply on Pin #2 to be a constant full12 VDC. So, IF your BIOS configuration for that CPU_FAN header allows you to set its control MODE, it needs to be set to PWM Mode - NOT Voltage Control Mode (aka DC Mode), and not Automatic fan type detection. IF you do not have such a choice, then set it to use constant full speed of its "fan" (really, its pump) rather than the normal automatic speed control.

Don't forget to connect the signal cable from the socket on the side of the pump to a mobo USB2 header, AND to download and install the Corsair iCUE software utility that actually takes over CPU cooling control.

If that does not solve your problem, there is something wrong with the feed of the pump speed signal to the CPU_FAN header's Pin #3. Check whether there may be a lose wire connection. Also see if you can tell by feel or small sounds whether or not the PUMP unit really is running and pumping water around the loop. Also feel the hoses between the pump and the rad. One hose should get warm, and one a bit cooler, if the system is operating properly.

If you decide the system is operating properly but it still generates the CPU Fan Fail message, contact Corsair and discuss with them. Their pump unit may have a flaw that is failing to send the signal. If you have iCUE operating, check there also for what it says is the PUMP speed. That reading is taken by iCUE from the header, so if you have a reasonable speed reading there for the PUMP (not the rad fans), the header is getting a signal. Just check, then, that the pump speed really is as expected - certainly over 1000 RPM, probably closer to 2000 RPM, but ask Corsair Tech Support.
 
Sep 10, 2019
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Wow. Thank you so much for writing such a complete and clear answer. I don't have access to the computer for a week but I'll try that right after.
 
Sep 10, 2019
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To solve the issue, I plugged the Corsair h80i v2 connector in the TOP_FAN connector of the motherboard.
I've put the original Dell cpu fan into an empty hdd slot and plugged it into the CPU_FAN connector, where it was originally connected.
Seems everything is working fine. No heating / cooling problem with the Corsair device. I've not heard the original fan spinning at high speed.
P.S. adding the h80i v2 cooler to the XPS 8920 is not for the faint of heart: the case needs to be Dremeled a lot to fit the fan outside. Having done it, I would not recommand this.
 

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