There is no capability provided in BIOS for selecting support for 3-wire fans - and indeed none is necessary. With the exception of the Processor (CPU) fan header -- which supports only a 4-pin, PWM-controlled fan -- all other fan headers on the 7 and 8 Series Intel® Desktop Boards utilize a hardware circuit that automatically detects, at power-on, whether a 4-pin PWM-controlled fan is present or not. If a 4-pin PWM-controlled fan is detected, a full 12V output is supplied to the fan and it implements its own fan speed control based upon the PWM control signal. If a 2- or 3-pin fan is detected (or a non-compliant 4-pin fan; alas there are some out there), on the other hand, a voltage-variance control circuit is automatically enabled and the fan speed is controlled by varying the voltage between 0 and 12V.
OK, so your next question is "Why isn't the fan speed being reported?" There are a number of possibilities:
- If you are seeing the fan speed being displayed properly within BIOS Setup (or Visual BIOS) but not within Intel® Desktop Utilities (IDU), then the monitoring of this fan has been disabled within IDU. To re-enable monitoring, go into IDU's Options Menu, select entry "Set Sensor Threshold", click on the "Fans Redetect" button, close all IDU windows and then reboot the system.
- If the fan's speed is not being displayed properly within BIOS Setup (Visual BIOS), there may be something wrong with the tachometer output being generated (or not) by the fan or with the detection of this tachometer signal by the board. There are three possibilities that come to mind:
- The minimum duty cycle for the fan is set too low and the voltage is being taken below 5V. In this case, the fan may continue to spin but the fan speed will not be detected. The circuit requires that the tachometer pulses reach the 5V threshold to be detected. To see whether this is the problem, raise the minimum duty cycle to 30% (or higher) and see if the fan speed is then detected.
- If raising the minimum duty cycle doesn't help, there may be a problem with the tachometer signal being output by the fan. Your best bet at this point is to try replacing the fan and see if the problem fixes itself.
- If replacing the fan doesn't make a difference, the tachometer detection circuit on the board may have been damaged (you did follow ESD protocols while working on it, didn't you?). There is no recourse for this except board replacement.
Hope this helps you get things working properly...