how do i determine what causes the cpu fan to spin really fast and hard on a motherboard?

It could be that the sensor is telling the cpu that the temperature is very hot, or a BIOS setting says run at 100% all the time.  You can check what the sensor thinks it is reading with a software program like SpeedFan http://www.almico.com/speedfan.php.

You can also check if the input voltage of the fan is too high. Can be as trivial as this.

how can i get to see that on the pc, when  the pc won't boot or post. as soon as you push the power button, you get solid amber light and then after a few seconds, the cpu fan starts to spin for its life as if trying to cool something down. i have 2 gx280 systems doing that right now. i replaced the caps around the fan, and even swapped the cpu to see if it was bad.

how can i check if the input voltage is too high?

I'd suspect a faulty PSU if the symptoms include the system is not bootable and the fans spinning too fast. You can try with a different PSU or just measure the 5V and 12V voltage outputs with a volt-meter. Since you are changing capacitors, I assume you have electronics experience.
Hope it helps!
Delyan

lol...yes actually i do have experience in electronics. that is my field career, but as engineers and techs, we always run into problems where we need our fellow tech supporters. and thanks by the way for getting back to my question. when i unplug the yellow and black psu plug from the board, the fan stops spinning so fast, but still won't boot.

Attach probes to the cpu fan leads and see what voltage you get.  Repairing a motherboard, beyond replacing caps and anything obviously broken off requires a schematic, since these are multilayer and have so many components.

yellow and black psu plug
What do you mean? You unplugged the 4-pin ATX +12V connector?
In this case you may have a problem with this 12V line on the PSU outputting higher voltage.
If you haven't already fried your mobo, I'd suggest taking out every single card, HDD, CD and basically leaving the mobo, CPU and the PSU. Plug back the 4-pin +12V connector. Powering up like this check:
- fan speed. If normal, you may be overloading your PSU by some peripherals.
- beeping for no RAM (should be constant beeeep-beeeep-beeeep). If beeping, the dead system may be due to faulty RAM/video/etc. Plug the RAM only and listen for the "no video" beep (should be constant long beeeeeeeeeeeeeeeeep). This way you can work up until you find the culprit. (I still vote for the PSU which may have toasted your hardware).
BTW the high-voltage-from-PSU problem also has the burning-silicon-smell syndrom ;-)
Hope it helps!
Delyan

YES I KNOW THEY ARE ALL OUT OF WARRANTY. I AM A REGIONAL TECHNOLOGY ENGINEER, AND AS MUCH AS I GET THINGS FIGURED OUT, I CAN'T PLACE A FINGER ON THIS ONE. I DID REPLACE ALL THE CAPS THAT ARE LOCATED NEAR THE FANS AND EVEN SOME OTHER ONES, BUT STILL NO FIX. I SHOULD JUST THROW THEM AWAY...LOL

Hmm what is wrong with high spin? If the computer works, I'll take the high spin, keeps extra cool. Other than that, could be just the fan itself. Replace the fan with one that fits your taste, a higher rpm or lower rpm. And of course you can buy the fan that either has a switch for 3 settings  (low, medium, cool) or one that has a knob and you adjust to taste.

A update bios flash might help. Once I had a abit board, the rear fans kicked it at high, the manufacturer released a bios to auto adjust which worked like a charm.

Temp are factors. But it sounds like you took all steps for proper cooling. Bios has an option on how often you want the fan to throttle at what temps, could be set too low. Then again if you have a dell board, you wont see this option. Another reason why is great to buy a good board, control over all features.

Voltage regulator perhaps not operating correctly, if you love soldering and good with micro soldering, replace that.

Thanks Bro!   :)