Case fan always running

[madbeggar]

I've got an HP Pavilion a6142n. It's a few years old, and a year or two ago it started making a loud fan noise, cycling on and off, but I was too lazy to haul it out from beneath my desk and research it. Yesterday I rearranged my "office", so I took the side cover off, blew the case out with lots of compressed air. Upon restart I now have the following: the three-wire case fan, which is the source of the noise (the bearings are obviously shot), is always on now. It appears the CPU fan is always on as well, despite the CPU being cool at startup. Are they both supposed to be on all the time? Or is it supposed to be temp-related? I'll replace the case fan ASAP but I'm concerned about the logic behind the on/off and whether I did a bad thing with the compressed air.

 

[madbeggar]

BTW, the case fan is wired off the mobo. Also, if I unplug that fan while the computer is on it shuts it down. Which I find interesting.

 

[Rugger]

They'll always be on, but the mobo will ramp up speed for the CPU fan based on temperature. If the case fan is a 4-pin PWM (doubtful), then the mobo will also control it's speed based on temps.

 

[Rugger]

First, I wouldn't unplug anything with the PC actually running...quick way to shock yourself or damage components.

Which mobo header is the case fan plugged into? I know that the PC will shut down if there is no fan plugged in to the CPU fan header, but it's unusual for that to happen for a case fan header.

 

[madbeggar]

First, I wouldn't unplug anything with the PC actually running...quick way to shock yourself or damage components.

Which mobo header is the case fan plugged into? I know that the PC will shut down if there is no fan plugged in to the CPU fan header, but it's unusual for that to happen for a case fan header.

Well, it's only 12v max for the fan, so I'm not too concerned about shocks. As for damage to the mobo, you may be correct.
The fan is plugged into the sys_fan header. Also, I just unplugged it again, started it up and got a boot up error re the fan being undetected. I was able to f2 around it, and it is now running w/o the case fan.

Based on yr answer (re the fan being always on) I guess the cycling I heard in the past was simply the bearings in the case fan cycling in-and-out of bad positions, or perhaps seizing up. Thanks.

 

[Rugger]

You're exactly correct on the cycling...fans usually die slow horrrible deaths like that.