Power Supply comes on Momentarily in New System ASUS P8B75-M USB Won't Boot

[Barry Marshall]

Hi, I read the various posts about "power supply comes on momentarily then goes off". The new problem system was an ASUS P8B75-M motherboard. I had an old dead system which I fixed with the above new MOBO then a new Corsair CX500 Power Supply. The old PS was dead but MOBO was old so the new ASUS seemed a good idea.
SYMPTOMS NEW SYSTEM:
1. The fans would flick then stop in 1-3 seconds.
The green light on the MOBO was on. There was no POST beep from the speaker. I checked the PS with an ANTEC "Digital PC Power Tester". The tester beeped and said my new Corsair CX500 had a low voltage "LL" on the 12V line. But working system showed the same error so I decided the tester was a bit oversensitive. Then I jumpered the green and black wires in the 24 pin power cable of the Corsair. When I did this the fan came on fine. So it seemed to work. Then I studied all the cables to the MOBO connectors more carefully and realised that the Corsair has an 8 pin "CPU" cable which I had not used! I accidentally had pinned the "PCI-E" cable into the CPU 4 pin extra power socket labeled "EATX12V". Similarly, if I left the cable disconnected, the same issue was present. Then I noticed that if I installed the "CPU" cable properly (it breaks into two identical 4 pin plugs) then the system worked.
2. The system would not boot from an external USB stick or CD-ROM. I suspect that the device needs to be plugged in to the motherboard USB2 plugs at the back of the PC, not the USB extension plugs which go via cables to the front panels. I may be wrong as I have not double checked this and I found a loose cable a little later after I started this post.

THIS IS NOT JUST A QUESTION - IT IS AN ANSWER! The various posts I saw didn't actually cover this fault although the post linked here by JSC the system builder expert was where I started. But I think that thread is closed now.

 

[Praz-1]

Sounds like you were lucky with the EPS +12V connection. The CPU and PCIe cables both use +12V but the pinout is different. Connecting the PCIe cable to the EPS +12V connector usually results in a damaged motherboard. But as the connectors themselves are also different considerable force is required to make this connection. Perhaps the PCIe plug was not fully inserted and not making contact with the board so no damage occurred.