Noisy cpu fan on Dell

[Hodsocks]

I have been looking at a Dell Dimension 8400 which won't boot up, all that happens is the cpu fan/case fan spins up and just gets steadily faster until it is running flat out and making a hell of a noise. There is nothing on the screen and the 4 diagnostic lights at the back are all off.
I blew out the heatsink and reseated the memory and the machine booted up but when I restarted it the same thing happened ie noisy fan, its obviously a hardware problem but as it won't boot into the BIOS I an restricted as to what I can do.
Has anyone any experience of this or have an answer to the problem?

 

[kimsland]

How to Bench Test Your System

 

[Tmagic650]

This could be caused by a failing power supply. Most of these Dells have proprietary power supplies, so you must purchase a power supply for Dell systems specifically

 

[Hodsocks]

The problem is obviously a hardware problem as its not booting into the BIOS, so far I have tried the following:-
Removed the memory and reseated it, tried just one module in one slot then the other, swapped modules and slots.
Removed all cards, then put back in the graphics card.
Cleaned out the heatsink but don't think its heat related as it spins up in a matter of seconds.`
Reseated all cables and swapped for others, tried the SATA hard drive in a different mobo connection.
Tried new PSU, a normal psu works with this mobo as its a recent one, I think its only older Dell mobo's that had a proprietory psu. I checked the wiring in the 24 pin plug before I fired up the machine of course.

Whichever of the above I tried sometimes it would fire up OK but when I switched it off and then back on again I had the same problem with the noisy fan. I am begininng to wonder if it maybe a dodgy mobo or processor.

At the moment it is working and the guy is going to leave it on until I come up with a fix (maybe).

The Dell forums confirm that several people are having similar problems with this model and have fixed it in any of the above methods, there doesn't seem to be any one cause of failure.