A technician is assembling a desktop PC with a standard ATX motherboard. After connecting a 4-pin PWM cooling fan to a 4-pin fan header on the motherboard, the fan runs continuously at 100% maximum speed regardless of system temperature. Pin layout inspection confirms: Pin 1 (Ground), Pin 2 (+12V Power), Pin 3 (Tachometer Signal), and Pin 4 (PWM Control Signal). Which of the following is the most likely cause of this issue?
- The fan header in BIOS/UEFI is set to DC (voltage control) mode instead of PWM mode, delivering a constant 12V on Pin 2 without modulating the signal on Pin 4.Cevap
- BThe motherboard system memory sticks were populated in adjacent RAM slots rather than alternating dual-channel slots, causing the chipset to disable fan modulation.
- CThe system power supply unit wattage was undercalculated, triggering an automatic motherboard safeguard that locks system cooling fans at maximum power.
- DThe assembly process bypassed the troubleshooting methodology by connecting peripheral fan headers before completing the POST beep code verification.
Cevap
The fan header control mode in BIOS/UEFI is configured for DC (voltage) operation rather than PWM mode.
Motherboard 4-pin fan headers support two control modes in BIOS/UEFI: PWM and DC (voltage). In PWM mode, Pin 2 provides a constant +12V supply while Pin 4 transmits a control signal to vary fan speed. If the BIOS header is set to DC mode, the motherboard attempts to adjust speed by varying voltage on Pin 2 and leaves Pin 4 undriven. Because Pin 2 provides continuous full 12V, a 4-pin PWM fan will operate continuously at 100% speed.
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Motherboard 4-pin fan header pinouts and BIOS/UEFI PWM vs. DC fan control modes
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