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maury11215
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How to Jump Dell Dim 8400 Motherboard

I'm looking for some detailed specs for a Dell Dimension 8400 motherboard (my friend's system). Specifically, the system for no apparent reason could not be powered on.

The power supply checks out pretty decent, voltages are within specs. So I figure either the power switch or motherboard is bad.

Most of the generic instructions available for jumping motherboards refer to a two-pin momentary switch connection labeled PWR, or PWR_BTN. The problem is, on this board, there's a "front-panel connector" with around 30 or so pins, and a flat cable going to the front panel lights, power switch, etc. The power switch itself is fairly inaccessible, and I don't see any instructions provided showing how to remove it.

The cable can be easily removed from the motherboard. Where can I find out which of the 30 pins are the correct ones for starting the power-on sequence?

Thank you for any ideas!

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12 Replies
fireberd
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Re: How to Jump Dell Dim 8400 Motherboard

The Dell front panel connector is a proprietary and undocumented connector.  Dell has never released any info on these. 

I've seen one or two posts from users that have traced out all the connections however the connections can vary from one model to another so unless you could find an 8400 specific user wiring diagram it may not do you any good.

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speedstep
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Re: How to Jump Dell Dim 8400 Motherboard

The ATX 12V specification has 2 pins that can be used to turn on the power supply with a paper clip.

Grounding PS_On with a paper clip turns it on.

ATX PC Power Supply Pinout and Connectors

Manually turn on ATX power supply without motherboard

However I doubt that this is going to fix your issue.  Most likely issue is that the power supply is bad OR the motherboard has Bad Capacitors and essentially looks like a dead short.

You can use a standard power supply in the Dimension 8400/GX280 Tower System but you need to Hack a hole in the case to get it

to work.   Newer Dells don't have the issue.

 


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maury11215
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Re: How to Jump Dell Dim 8400 Motherboard

This is helpful... I have tried jumpering /PS_ON  to ground, with the power supply disconnected from the system. The fan turns on, and checking output voltages with DMM shows outputs very close to spec. I will check PWR_OK signal again, and make sure it's good.

Is there a way to get to the power switch, so it can be tested independently? It seems pretty inaccessible.

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shesagordie
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Re: How to Jump Dell Dim 8400 Motherboard

This is helpful... I have tried jumpering /PS_ON  to ground, with the power supply disconnected from the system. The fan turns on, and checking output voltages with DMM shows outputs very close to spec. I will check PWR_OK signal again, and make sure it's good.

Is there a way to get to the power switch, so it can be tested independently? It seems pretty inaccessible.

 maury11215

The power switch and circuit board is one unit.

Dell has not published a guide to remove the front bezel of the Dimension 8400, you can use this GUIDE, <ADMIN NOTE: Broken link has been removed from this post by Dell>

it's for an Optiplex GX50 Small Mini Tower, the procedures are the same.

A hint: The Dimension 8400 also, has a Hex headed screw on inside right side of front bezel/panel, just below the front USB ports, holding the front panel to the metal chassis, this MUST be removed and is not mentioned in the guide.

Bev.

 


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speedstep
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Re: How to Jump Dell Dim 8400 Motherboard

Putting a paperclip in the back side of the  PSU connector can FORCE the power supply on.

However if the Caps are bad they may Blow up and cause a fire.

There is a ribbon cable from the front panel to the motherboard and then traces from there over to the

Power supply to turn it on/off.

You could get a normally open Push button and wire it to the PSON and Ground wire.

If that works then the switch is broken .

I have SERIOUS doubts that this is the issue.   I have Many Years Experience with the GX280/8400 and

never recall that being an issue.   Shorted out USB Ports aka key broken off and Bad Capacitors

are the issue I remember the most.

Getting A Used Dimension 8400 is Very Easy for me So it really wouldn't be worth my time to

Fix a broken Motherboard/Front Panel etc.

I assume you have an 8400 Tower with the SKYDIVE chassis.

You can Buy an off the shelf power supply and plug it in and see if it works aka

the Connectors and wiring are standard.  The only non standard part is the Hole in the rear

where the power cord goes in .  You basically cut away metal to make a bigger hole in the rear

and the screw holes are the same as a standard power supply.

Dell Optiplex GX280 Tower Desktop Computer (DHM) - Refurbished ...

Micro Center - Dell Optiplex GX280 Desktop Computer Off Lease ...


 


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speedstep
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Re: How to Jump Dell Dim 8400 Motherboard

A better solution would be to look for a newer USED dell like an Optiplex GX620 and up.

Computer Warehouse Tyson's ...

Micro Center - Dell OptiPlex GX620 Refurbished 555555000149

OptiPlex GX620 Desktop Product Details | Dell

The idea is to migrate parts that are not broken to the new system.



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maury11215
Bronze

Re: How to Jump Dell Dim 8400 Motherboard

With the 24-pin connector disconnected from the MB, and with the PS_ON signal (green) grounded, the PSU's fan turned on, the 24-pin voltages were good, and there was good voltage on the drive power connectors. That had led me to think either the power switch or motherboard had the problem.

But when connected back to the MB, there was no fan, and no voltage registering on the drive power connectors, even with PS_ON grounded.

That didn't seem right to me, so I finally got a hold of a known-good PSU, hooked it up to the system, and everything worked fine, power button and all.

I ordered a new 400W power supply, installed it, and all is working well.

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shesagordie
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Re: How to Jump Dell Dim 8400 Motherboard

With the 24-pin connector disconnected from the MB, and with the PS_ON signal (green) grounded, the PSU's fan turned on, the 24-pin voltages were good, and there was good voltage on the drive power connectors. That had led me to think either the power switch or motherboard had the problem.

But when connected back to the MB, there was no fan, and no voltage registering on the drive power connectors, even with PS_ON grounded.

That didn't seem right to me, so I finally got a hold of a known-good PSU, hooked it up to the system, and everything worked fine, power button and all.

I ordered a new 400W power supply, installed it, and all is working well.

 maury11215

Please to hear that replacing the power supply was the solution, to the issue.

Bev.

 


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fireberd
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Re: How to Jump Dell Dim 8400 Motherboard

Testing the power supply without a load can be misleading.  If you unplug the 24 pin connector and jumper pin 14 to ground and the power supply comes on, that is all that means.  If you measure the voltages with no load they can all be OK, BUT when connected and under load the voltages can be bad and that sounds like what happened with this model.

This is the problem with the $25 Power Supply testers they sell.  It will power on the power supply and it has lights for the various voltages, but that is with no load and can be misleading. 

I am not a Dell Employee

Dell forum member since 2002

Dell Inspiron 15 - 5577 Laptop

Home Built Desktop PC with ASUS Z170, i7 6700K CPU

Windows 10 64 bit Pro and Windows 10 Insider Program Beta Versions. SSD drives.  Studio One 3 Recordng Studio Software.

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Member of Nashville based R.O.P.E.

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