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?
 

mnewxcv

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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.
 

Furious_Styles

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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.
 

mnewxcv

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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?
 

Furious_Styles

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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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Dan_D

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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.
 

Nebell

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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.
 

mnewxcv

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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.
 

lopoetve

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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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