Case fan is running way too fast


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.

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CallandorConnect With a Mentor Commented:
A power supply tester should not cause the fan to do that.
Does the fan have an internal temperature sensor, or is it connected to a 3-pin header on the motherboard?  If it's a 3-wire connector, the fan speed may be controlled by the motherboard and the BIOS settings.
semmelbroeselAuthor Commented:
It's a 5 pin plug, but only 4 pins have wires going to them.
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nobusConnect With a Mentor Commented:
try with another fan first, it can be simply bad.
semmelbroeselAuthor Commented:
Sorry, forgot about this one - doing too many things at once, and I haven't had time to go back to the PC.

I think I will try to replace the fan.

Before that, I'd like to ask one more question:

Is it ever normal for the fan to immediately go into screaming noisy speed when you attach a power supply tester to it,  but not when you run the PC normally?
So far we have not heard the problem again after replacing power supply, motherboard and CPU, but it still comes up when we use the power supply tester. So if no one has ever heard of a fan speeding up like that with a power supply tester then I think it should definitely be the fan itself.

did you try the updated bios for a fix - see the dell link i provided.
semmelbroeselAuthor Commented:
I didn't try updating the BIOS since the motherboard was replaced once with one with a lower Revision number. I didn't look at the BIOS version though. I'll ask a co-worker to try it.

Callandor: I didn't think so - thanks for confirming!
semmelbroeselAuthor Commented:
We're pretty busy these days, so I still haven't had a chance to try it. I'll ask my co-workers again who are in the office more often than I am.
semmelbroeselAuthor Commented:
Agreed with the split.

Sorry I couldn't follow-up anymore (should have at least said something) - the PC is no longer with us...

Thanks for everyone posting and helping out!
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