The tach signal is rather simplistic. It is designed to count the number of times a motor pole passes a certain point. Most fans use 4-pole motors. When the liquid cooler pump is plugged in, it reports the number of times the pump motors poles pass the same point, but the pump only uses a 2-pole motor. Because the BIOS is expecting a fan, it divides its tach signal count by 4 and only reports half the speed of the pump. Note: ChillControl is aware of this difference and reports the correct pump speed.