Yesterday I purchased an Asus M4N78 Pro motherboard and tried everything on it. The heart of the problem is that after powering it on, the PSU fan turns on and off continously and doesn't proceed to the boot screen. This occurs even as the motherboard LED is a constant green and as I have only the bare minimum hardware attached to the motherboard:
--Athlon X2 5600
--Evergreen PSU 450 watts
--1 stick of DDR2 800 ram
--Case connecter: "Power switch" connecter attached only
Even if I attach a monitor, a hard drive, CPU heatsing/fan and such, same problem occurs-- the PSU fan (as well as the CPU fan) comes on and off continously at 2 second intervals. However if I remove the CPU, the power supply fan would act normally. BTW, there are no bent pins on the CPU. Though I suspect there might be something wrong internally with the CPU. However I have no spare AM2 CPU around to test with.
With the same 450 watt PSU after I performed the above test, I was able to run my old system easily that consists of: a socket 939 ASUS board, AMD sempron 3500 cpu, a huge GeForce 6800 videocard, a Soundblaster soundcard, 3 hard drives, and 1 dvd-r drive.
Any thoughts as to why the PSU fan comes on and off at boot up with my new M4N78 system?
With the CPU cooler off and your finger on the CPU turn the system on and see if it begins to warm.
If it doesnt start to warm up then it is a decent indication that the problem is near there.
Possibly bad contact between the CPU and the board or one of the power connectors.
Ps I think the cpu fan must still be plugged in when doing this.
I have also heard that on some boards that moving the ram to slot 3 can make a difference.