Curious about a Dell Dimension 4600 cooling fan with thermistor-->pulsing on and off

I have a Dell dimension 4600 desktop on the bench with a JMC-Datech 9232-12hb cooling fan which is attached to the case and draws air through the heatsink by way of a cowling over the heatsink. It has a thermistor set up on the stationary hub of the fan housing which monitors the air temp, and adjusts the fan speed accordingly.
The problem is the fan is running fne until the computer is stressed. While using a burnin test, the fan begins to ramp up in speed and then when it reaches high rpm's it shuts down then ramps up over and over again. Sounds just like a heavy breather over the phone.
I have already changed out the fan and now have a working cooling system.
My question is--> has anybody out there ever seen this type of behaviour in a thermistor controlled fan , and could it be indicative of any other hardware failure on the motherboard itself?
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eudmianConnect With a Mentor Commented:
The system was using thermistor input for control.  The patch was downloaded from the Sony driver site but in trying to resolve the problem I did run across a similar patch for Toshiba systems so it may have been a common hardware driver of some sort.

If the Dell system in question is using, like most relatively recent, an Intel Bios the controls are there but hidden.  With a Bios hack you can get to them; I know Dell realy likes the KISS approach.  The hack is actualy prety safe and easy.  From the Intel site you can download a gui "OEM Bios configuration tool".  I think you have to sign up but it is one of those 'give us your email and we will send you a password to the download' type of sign ups.  When you run the tool you first download a bin image of the curent bios and work on that copy.  I believe you can also download generic bin's from the site as well (make sure to get the right one obviously).  You can turn on a myriad of features (I was somewhat surprised) as well as creating your own splash screen to replace the 'Dell' splash.  

I had a project some time back where I took a Dell PowerEdge system and did this.  I was rather amazed at some of the nifty little things that were hidden by default.  One is an advanced page that shows the voltage to a 2 point precision on all the busses, thermistor readings for all as well as fan rpms.  There was some other stuff but I dont recall exactly what all was there.  Most of it was rather useless to the average neophyte which is why Dell turns it off.  It gives the option to make any page viewable or hidden as well as control over read or read write for each individual field.
I had a similar issue with a  Sony Vaio system.  I decided for various reasons to wipe the drive and reinstall the OS (WinXP Pro) and that is when it started.  Exactly as you described it would ramp up on load and then cycle, VERY annoying on a system that sat on my desk next to me.  

I never did have the time nor inclination to delve further into it but found it interesting that it was an OS patch that remedied the problem in my case.  The fan and sensors are directly controlled by the BIOS with the OS being able to monitor and in some cases pass commands to them so from a hardware standpoint short of a faulty BIOS, which is easy to check, I cant think of anything else that may be contributory.

I might reccomend, if you have not already, run a load test from a live CD like Knoppix.
MarkAuthor Commented:
Thanks for the input.
Did your fan have the thermistor on the fan body( looks like a green nub sticking out of the fan hub) or was the fan speed dependent on  hardware sensor readings from the motherboard?
I have burnin running to stress the cpu making it heat up and the NEW fan is ramping up slowly and not pulsing on and off anymore. There are no  bios settings concerning  temp controls (DELL makes em' lean) so I guess the thermistor is the only control involved. Can't even find a sensor with speedfan to check a temp with.

Concerning that patch, was it specific to the viao or was it an XP patch?
MarkAuthor Commented:
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