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Need -a- ClueFlag for United States of America

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2 of 5 fans on desktop run at full speed with no reason to.

This is a real doozie! Dell T7500 workstation. Began with both front fans ramping up slowly from normal slow speed to full on. The computer seemed to operate normally. POST said the fan was defective. I replaced every fan in the unit except or the one in the power supply. No change. I replaced the video card with an unused Nvidia Quadra 4000. No change. I unplugged the hard drive. No change. At this point I am a bit desperate. I spoke to a technician that said the motherboard was the trouble. I then replaced the MB with a tested used one. Again...same problem. The ONLY components left is the power supply and the ram memory. So, I had an unused DEll identical part number PS available. After install? You guessed it. Same problem. Then the last thing I tried was a tested cpu that had identical numbers and specs. Again, a waste of time and money. ONLY the ram is left. I tested it using the long test and it shows as OK. Where do I turn at this point?
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BillDL
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Start the laptop in Safe Mode and see what happens:

https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

https://support.microsoft.com/en-gb/help/12376/windows-10-start-your-pc-in-safe-mode

My thoughts are that software that controls the fan speed is misreading one or more of the heat sensing chips on the motherboard.  That software shouldn't run in Safe Mode.
See if Dell has new BIOS and Chipset drivers for this workstation. That may help.  That was a Lenovo fix for fans running at full speed.
I've seen this happen before and the motherboard is usually the culprit. Isn't this model desktop several years ago? There's a good chance that spare motherboard you had might have the same problem. Visually inspect them both for any blown capacitors.
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Experts: I will post as I complete each troubleshooting step.
I booted into safe mode. Prior to rebooting the front fans were at full speed. As soon as the reboot began the fans went back to normal slow (cool environment) speed. Then, while rebooting the fans ramped up once again over a period of a minute or two. I am going to update the bios now.
Updated BIOS from A08 to A18. No change. Fans still ramp up.
I did check both boards for any toasted components. I could not locate anything that looked burnt, bulged or broken.
Boot to BIOS, or better a bootable DVD (no hard drive, no software). Done this way do the fans still ramp up.

If no, then it may be some software you have loaded that is running full tilt.
John, I have two bootable usb sticks. I went into bios and configured the boot sequence to show  only the usb. It is not a super fast computer and takes a while to boot up. It refused to boot from either thumbdrive. However, when it was trying the fans did their ramping up performance.
Early on I disconnected all drives and optical drives. I pulled video, audio and bluetooth cards. NO cards were on motherboard. STILL ramped up.
I went into bios and configured the boot sequence to show  only the usb.

If it was only trying to boot from the USB and none of your software, AND if you updated the BIOS successfully, then it is a hardware problem.
I found a third bootable dell thumbdrive with win10. It does boot up, but still the fans are at top speed. I am sort of hoping that the rpm will get so high that a blade will fly off and destroy this computer. (Kiddin, sort of.....)
Good, so it is a hardware issue of some kind. CPU, both motherboards (one was used). I assume you moved the CPU from one board to the other.
No John, I bought a used and  "tested" mb from ebay. It did not come with a microprocessor. When it did not help the ONLY items remaining were the memory cards and the cpu. So I ordered another cpu. It did not help. Unbelievable.
So:

You removed video, audio and Bluetooth cards. You said "NO cards were on motherboard. STILL ramped up"
You tried two motherboards.
You replaced the CPU.
You upgraded the BIOS and Chipset.
You booted with a USB key (NO software running, just BIOS)

And the fans still run HIGH.

CPU  / Motherboard conflict with BIOS?   I don't know what else it could possibly be.

NOTE:  There could be a fault in the power supply. One of the voltages gone wrong.

Time to replace it?   Looks like it.
John, this is almost funny huh?    I actually had a brand new never installed power supply. I installed it with no change. I hate to give up on a problem. I found a board that supposedly has never been used. I will order and hope for the best. I appreciate you John!!
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John
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John, thanks Sir for your Expert advice. I will order a "new" MB today and hope.
Thanks all!!
Thanks for the update and good luck with a new board.