I have a Dell Dimension 8400 here. It runs normally, but after a while the case fan in the back (not the power supply fan) will increase speed to what must be its maximum (which is very loud), even though the CPU is very unlikely to be overheating.
Since this only occurs randomly, but once it does it prevails, I tested the power supply using a power supply tester, leaving the 4 pin connector on the mainboard. If I do this, the fan will immediately go to max speed.
We replaced the power supply and the mainboard, and it still does the same thing.
Is it possible that the fan will go to maximum only when the system is on a power supply tester?
What I am trying to find out is if the system will run normally in its normal configuration and only go to max fan speed if put on a power supply tester. Is that possible or likely?
Any other advice to get the fan to stop acting up?
The fan appears to run normally, just fast - it can blow half-way across the room. Once it runs, it stays at a constant speed. It seems like it is only doing what it is "told" by some other component, but since I already replaced everything else...
Could it be the fan itself? I don't know if they have an electronic speed control inside or something or if it's just the motor.