Why won't my z390 mobo work with the 4-pin CPU connected?

Nebell

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I was swapping out 750w PSU to 850w and connected both 8-pin and 4-pin CPU connectors and my mobo did not want to power up.
Nothing at all, completely dead. I did not have the 4-pin connected before so I guess that's why I never encountered this issue.

If I disconnect the 4-pin, the mobo is still dead.
If I disconnect everything (even the motherboard cable) and connect everything except the 4-pin, it works normally.

I tried both 750w and 850w and neither want to start the motherboard with the 4-pin connected.
The motherboard in question is MSI MPG Z390m Gaming Edge AC. The CPU I'm using is 9900k oced to 5ghz.

Now, everything works fine without the 4-pin, but I'm still curious why this is happening. Is it on its deathbed? Or perhaps not stable enough considering it's mATX?
 
works normally? I have a hard time believing the computer posts without the CPU 4-pin. Weird..... OH it's one of those boards with 8 pin cpu AND 4 pin cpu. Kind of a weird problem, but really is fine with just the 8 pin.
 
Just to make sure you're using 2x CPU cables from the PSU and not PCI-e cables right? It honestly should not affect the board in any way if you have the 4 pin connected or not. It's mostly just there for extreme OCs where you need extra voltage to the CPU.
 
OP, you said you swapped your PSU. Are you using all the cables that came with the new PSU, and NONE of the old cables?
 
OP, you said you swapped your PSU. Are you using all the cables that came with the new PSU, and NONE of the old cables?

Yes good point. NEVER re-use cables unless the PSUs are the exact same model, or you researched first to make sure they are swappable, because in most cases, they are NOT.
 
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works normally? I have a hard time believing the computer posts without the CPU 4-pin. Weird..... OH it's one of those boards with 8 pin cpu AND 4 pin cpu. Kind of a weird problem, but really is fine with just the 8 pin.

It doesn't need the extra 4-pin cable. The 8-pin can supply up to 384w of power. The 9900K, even when overclocked consumes no where near that.
 
I'm not sure if I used the old cables or not. But they are two different PSU (Corsair and EVGA) so I'm not sure if they would fit.
Besides, both have issues with the 4-pin cable so if I used the wrong cables and they fit, the old PSU should have powered up the motherboard.
It's just a strange problem.
 
I'm not sure if I used the old cables or not. But they are two different PSU (Corsair and EVGA) so I'm not sure if they would fit.
Besides, both have issues with the 4-pin cable so if I used the wrong cables and they fit, the old PSU should have powered up the motherboard.
It's just a strange problem.
potentially not if you used an old 4 pin/new 8 pin or new 4 pin/old 8 pin. There is definitely a chance the plugs would fit, yet be wired differently, and if either the 8 or 4 pin was wired differently, it would be a direct short to ground and the psu would trip overcurrent immediately or just not turn on.
 
potentially not if you used an old 4 pin/new 8 pin or new 4 pin/old 8 pin. There is definitely a chance the plugs would fit, yet be wired differently, and if either the 8 or 4 pin was wired differently, it would be a direct short to ground and the psu would trip overcurrent immediately or just not turn on.

This. Never. Ever. Reuse. Cables. Killed 2 SSDs by accident recently this way - two different makers, cables looked the same.
 
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