I am confused as to why "cutting the blue wire" would alter the RPM of a 3-pin fan. My understanding of a 3-pin is as follows:
- Ground: The black wire in all cases I have seen.
- +12V: Usually yellow, maybe Red on some fans?
- Tachometer: Blue wire? This is a signal FROM the fan TO the controller. It allows the controller to "sense" what what the speed of the fan is. Originally used to detect a stalled/broken fan. This is how you can get the RPM on the GUI.
Usually most variable speed fans these days have a 4th wire, which carries pulses TO the fan to control it's speed. Prior to the PWM (aka 4-wire) fans, the controller would have to adjust downward the actual voltage on the +12V line to slow it down.
I'd swear the S5 had 4-wire PWM control logic and connectors for it's fans.
It seems to me that if you want a 3-wire fan to always at full speed, you connect the +12 and Ground wires directly to the power supply, and call it a day. No need to have the controller board get involved at all. This seems particularly attractive if you add a 2nd fan to the S5.
I've read that the S5 can in some cases lower the fan speed and still run the hashing hardware at near full speed. A fan directly wired to the power supply will continue to run at full speed even in this case.
Your misconception seems to be the "Tachometer" wire. This is the wire that lets the controller control the fan speed. Cut it, and it should be 100% all the time. You probably already know that, but I'm confirming for you.