4 Pin ATX12V In Backwards

[kenwood850]

I think I have a pulled a real bonehead maneuver. I was moving a complete previously working PC from one case to another including mother board, drives, PSU, cards, everything. When I powered it back up the green LED on the mother board lit but the fans did not run, there was no noise from the hard drives, and there was no POST. I unplugged everything and started over by plugging in just the motherboard power connections. When I tried starting it again the CPU fan ran but there was no post and nothing on the monitor. Although I cannot be sure, I think the 4 pin ATX12V had been plugged in 180 degrees from what it should have been when I first started it. When I went to plug it the second time it seemed to plug in differently but I am only going on memory. I know what you thinking. That would be impossible because of the way it is keyed and that is what I thought too. Unfortunately I tried both positions with the power off and it does go in either way.

My question is as follows. If it was plugged in backwards what ended up being destroyed? Just the MB, or the MB and CPU. What about drives including a brand new never even formatted SDD, the cards in the slots, or the PSU?

If it was plugged in backwards I would have thought the 12V pins would have been just connected to ground and creating a short with no damage to the board and only possible damage to the PSU but I am no expert on the internal configuration of the motherboard. Is there anyone with real knowledge on this that could advise me?

 

[clutchc]

Does the board work with it plugged in the "correct" way now?

 

[kenwood850]

Does the board work with it plugged in the "correct" way now?

The first time I fired it up in what I think was the backward position only the green LED on the motherboard lit. When I did it the second time in the known correct position the CPU fan ran but that was all. There was no POST, no single beep, no beep codes, and nothing was shown on the monitor screen.
The fact that the fan did not run the first time and did the second time is indicative that something changed and that adds to the probability that I was indeed an idiot and plugged it in backwards.

 

[clutchc]

If you really did insert the CPU power cable backwards, having the polarity reversed may have damaged the CPU or the MB's voltage regulators. Normally, I would expect a modern MB/CPU to have a fail safe for such a mistake. But I don't know that for sure. Have you tried removing the CPU from the socket and disconnecting all power cables from the MB and other devices for awhile? Maybe allow time for resets to occur.

Do you have a PSU tester or a voltmeter to check and see if the CPU power cable from the PSU is still supplying +12V between yellow and black wires?

 

[kenwood850]

If you really did insert the CPU power cable backwards, having the polarity reversed may have damaged the CPU or the MB's voltage regulators. Normally, I would expect a modern MB/CPU to have a fail safe for such a mistake. But I don't know that for sure. Have you tried removing the CPU from the socket and disconnecting all power cables from the MB and other devices for awhile? Maybe allow time for resets to occur.

Do you have a PSU tester or a voltmeter to check and see if the CPU power cable from the PSU is still supplying +12V between yellow and black wires?

Finding the DVM was easy. Finding probes that would fit down into the connector was not so easy but I was able to jury rig a probe that would fit. The result was no 12 V voltage on that connector, which I guess is not all that surprising. If I had sat and thought a minute I would have noted that even with the green LED on, the PSU fan was not running. That should have been a clue since it is probably 12V. I guess the next step is to pull cover on the PSU and investigate.

Update: It turns out there was nothing wrong with the PSU. I made the mistake of trying to measure the voltage with the PSU turned on but I had not turned on the motherboard. Once I turned it on I could measure the 12V on the ATX12V connector.

I also tried the MB with a known good PSU with the same results.

I will try to bring it up again as you suggest after it has set for a day but to be honest I am not very hopeful.

As for removing the CPU and trying the motherboard alone I am not going to bother. If either the MB or CPU is fried I would not bother replacing the other. They are both rather long in the tooth. (Asus M2NBP-VM CSM) so it is no great loss. What really bothers me is that after working on PCs since the early 90s I finally really messed one up being stupid. At least it belonged to me and not someone else. I only hope none of the supporting hardware including the new SDD drive went with it.

Thanks for your suggestions and taking the time to reply.

 

[clutchc]

I have my fingers crossed for you as to the rest of the components. We've all done something like that, though. What really surprises me is that the keyed connectors go in either way. Something I'll have to watch for in the future. That CPU power connection is always one of the hardest to get my fingers on to plug in. It usually sits up in the corner with all sorts of stuff in the way to get your hand around it and see the header at the same time.

 

[kenwood850]

I have my fingers crossed for you as to the rest of the components. We've all done something like that, though. What really surprises me is that the keyed connectors go in either way. Something I'll have to watch for in the future. That CPU power connection is always one of the hardest to get my fingers on to plug in. It usually sits up in the corner with all sorts of stuff in the way to get your hand around it and see the header at the same time.

There must be magic in your crossed fingers. All the drives survived and came up fine in another machine. That leaves only a 100/1000 LAN card and a USB 3.0 card to determine if they work but that is low priority.

Just to clarify on the connector. The connector appears to be keyed to go in only one way but with only four pins it didn't offer much resistance when plugged it in the wrong way. The bottom line is that it was something I had done enough times before that I was comfortable doing it , it was late, I wanted to get done and I was simply not paying attention so I paid the price. In the grand scheme of things, no one was killed or injured and it was just a piece of hardware that can be replaced. I just feel rather stupid at the moment for doing it.

Thanks again for your help.