I am trying to fix my neighbor's Dell Dimension 8400 that is dead in the water. When I push the power button nothing happens. I replaced the power supply with a known working supply, with the same results - nothing. I have been looking for the Front panel button to motherboard ribbon cable that many posts reference, but this desktop does not have one. It has 3 twisted pairs of wires - Turbo LED, HDD Led, and Keylock. When I press the power button, I have verified that the "Keylock" pair is being connected (using ohmmeter). To bypass the power button, I have manually jumpered the motherboard 24-pin connector green wire with the neighboring black wire. Still nothing happens (the power supply fan does not spinup). BUT.... if I disconnect the 4-pin CPU power connector from the motherboard, the power supply fan will come to life. This happens with the old and known-working power supply.
So to summarize, with the 4-pin CPU power connected, the power supply will not start. If I disconnect the 4-pin CPU power, the power supply WILL start.
Any ideas? My thinking is fried CPU or motherboard.
Sounds like the motherboard.
Check the motherboard for any capacitors with bulging tops or are leaking, the tops should be perfectly flat.
Check the sequence of the four diagnostic lights on the back panel of the case.
Are there any 'Beeps'?
If my answer was helpful, please use the 'Did this answer the question' and click: YesForum Member since 2001I am not employed by Dell
"has 3 twisted pairs of wires - Turbo LED, HDD Led, and Keylock. " This is not a Dimension 8400.
The 8400 did not come in desktop model and all 8400's use Skydive Mini Tower Case.
There is no KEYLOCK. 286 systems had these in 1985.
DIMENSION 8400 Documentation
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This is definitely quite odd... The PC I am working on is the same as pictured (same case, etc). My neighbors gave me the original documentation package, which has the manual entitled "Dell Dimension 8400 Series." And from the service tag and express service code on the case, the Dell website brings me to "Product Support for Dimension 8400." There is no confusion that this is a Dell dimension 8400. The only thing I can think of is that they at one time had the original power button replaced?? Not sure why, but from the power button there is no wire harness - just the three twisted pairs mentioned, each with a black 2-female-pin connector.
Removed the motherboard, and the backside has lots of black around several of the cap thru-hole leads. I think I've found the issue. There are also a few surface mount resistors around the caps that have been eaten through. I wouldn't expect caps to leak in such a short time period.... I think this motherboard was around 2007-8ish. Another one for the trash heap I guess.
The Dim 8400 is almost (or is) identical to an Optiplex model. That might be causing the confusion. I have an old 8400 and some of the online info like replacement parts and my packing slip identified it an Optiplex. The Optiplex is a business model so would have a lock. Maybe the Dimension has a diff case?? I agree with Bev--it sounds like the motherboard has failed. Since this model is so old (2003-4) it is not worth trying to fix. Used motherboards should be avoided since many early versions of this model had problems with capacitor failure and had the motherboards replaced under warranty.
Bad caps, have been an ongoing issue with the Dimension 8400, as have the early versions of the six pipe heatsinks and charred or burnt power connectors to the motherboard.
Thanks all for your responses. Today the owner did state this has been in for repair to a local shop a few times over the years. My guess is they replaced the on-off switch at one point in time, and whomever did it knew which motherboard pins to connect the leads to. Either way, I removed the hard drives for them, and told them to start shopping, as this is not to be repaired.
You can get another system for $99. Many Vendors carry these as Off lease.
Dell Optiplex GX620 Desktop Computer Refurbished
Dell Optiplex GX280 Off Lease Refurbished
Optiplex GX280 any model Does not have a keylock. You have to go back 30 years to PC's Limited 8088 before you get TURBO and KEYLOCK functions.
The Keyboard Lock was an IBM PC AND IBM AT CLONE feature.
There were CASE LOCKS but they have nothing to do with the KEYLOCK function or electric wires to the front panel.
Agreed. I did not actually remove the power button assembly, so I didn't see where the leads connected. The KEYLOCK pair was normally open, but when the power button was pushed indicated closed (ohmmeter). For all I know, these may have been old twisted pairs that were lying around in a repair shop that were soldered to the power button assembly. But I assure you, this machine was an 8400, and the 3 pairs of leads had text on the connectors at the MOBO block that said HDD Led, TURBO Led and Keylock. The only conclusion that can be drawn is that the original wiring harness was replaced with some old twisted pairs. The computer worked up until several months ago (witnessed first hand). It is dead now - motherboard is toast, with electrolytic juice all over many of the caps. Thread can be closed. Thanks everyone.