Dell Dimension 4600C cpu fan dead.

My guess would be something that gives a thermal feedback to the fan control.

Look for bulging and leaking electrolytic capacitors around the CPU area.

It's a thermistor. It controls the fan speed and exhaust air temp.

There are quite a few replacment fans available on eBay. For example:

$14 with shroud. Replace.

Exactly, a thermister

On Sun, 20 Jul 2008 16:25:55 -0700, "David Farber" put finger to keyboard and composed:

If it's attached to the heatsink, then I would think it would be an NTC thermistor. Heat it up with a hair dryer and check for a drop in its resistance. I think you'll find that temperature controlled CPU fans are quite common these days.

If the device is a fuse (unlikely), then one end would probably be connected to the supply pin (or to ground).

You could also bypass the device with a low resistance, say 100 ohms, if only for testing purposes.

FWIW, here is a pic from the service manual:

- Franc Zabkar

In message , Franc Zabkar writes

Which could be equally true of a thermistor. AFAIK though, the thermistor on the fans used by Dell are just there for the fan's internal electronics so replacing with a 'standard' fan would be an acceptable way to test as the system doesn't use it to measure temperature.

Hi Franc,

Heating it up drops the resistance slightly. One terminal is hooked up to the red wire marked "V." So it appears it is a thermal fuse. I temporarily bypassed it and now the motor spins up. I ordered a new motor off of ebay for $25 because it's more trouble than it's worth to special order the fuse, pay hefty shipping fees, and still have an old motor sitting in a client's PC.

Thanks for your reply.

Just to clarify a few things, the 6.3k reading was taken in circuit and it is not mounted on a heat sink. For test purposes, I installed the fan with the component in question bypassed. It seemed to operate at too high a speed and it was almost as loud as a gas powered leaf blower. So I shut it down and will wait for the new fan to proceed.

On Mon, 21 Jul 2008 12:09:10 -0700, "David Farber" put finger to keyboard and composed:

If there is no other connection to the red wire, then it would seem that the fan is getting its power via that device, in which case it must be a fuse. If it is a fuse, and if the motor spins up straight away, ie without waiting for the heatsink to warm up, then the fan can't be a temperature controlled type.

- Franc Zabkar

On Tue, 22 Jul 2008 06:22:19 +1000, Franc Zabkar put finger to keyboard and composed:

I should have written, "if the motor spins up straight away to maximum speed" ...

- Franc Zabkar

On Mon, 21 Jul 2008 12:54:35 -0700, "David Farber" put finger to keyboard and composed:

If it is a thermistor, then there must be two connections to the red wire, one for the thermistor and another for the fan motor and the electronics. I presume the white (?) wire is the RPM output.

But if it is a thermistor, or a thermal fuse, then how does it sense the temperature if there is no airflow? Or does the fan always turn at some minimum speed, regardless of temperature?

- Franc Zabkar

Fan speed would be controlled by the mainboard via CPU core temp. Most fans on newer mainboards run full speed when the PC is first switched on. Some wait a few seconds and then throttle the fan speed back to 1850 RPM. Some are much quicker to throttle. This is the behavior on my MSI AMD Athlon X64 and Intel 865 P4 mainboards.

Probably to test the three wire fan, to make sure it works at full speed before booting.

On Mon, 21 Jul 2008 18:30:07 -0400, "Michael A. Terrell" put finger to keyboard and composed:

I believe that the fan is controlled by a hardware monitor chip. I suspect that this chip defaults to full speed when it first powers up. Thereafter the BIOS or software assumes control of it. That could be the reason for the delay.

- Franc Zabkar

On Mon, 21 Jul 2008 17:58:34 -0400, Meat Plow put finger to keyboard and composed:

IMHO, the only proper way to control fan speed is by using a 4-wire fan, ie one where the voltage supply to the fan's motor and electronics is constant and the speed is varied by means of a PWM input signal. Trying to control a 3-wire fan by chopping its supply would result in the RPM output signal being chopped as well. It may also cause the fan's electronic commutation to misbehave, or make strange noises. Smoothing the output voltage with an inductor and capacitor may improve the performance, but it's still not ideal.

A fan that is internally temperature controlled would behave unpredictably. For one thing, reducing the voltage to the potential divider that contains the thermistor would result in incorrect temperature feedback. Here is an example where reducing the voltage to such a fan actually *increases* its speed:

Strange fan behavior:

- Franc Zabkar

Well complain to Dell, Intel, MSI, ASUS etc.. I'm just making an observation.

On Tue, 22 Jul 2008 12:18:56 -0400, Meat Plow put finger to keyboard and composed:

There is nothing to complain about if the speed control is done properly. When it's done incorrectly, however, this is what you get:

What is the influence of PWM on fans' reported RPMs?

SpeedFan's author is talking about a 3-wire fan which is what the OP has.

I think you'll find that Intel is now specifying 4-wire fans for those cases where speed control is external to the fan, and 3-wire fans when internal temperature sensing is used. It's possible that your fans are one of the above types.

- Franc Zabkar

I received the new fan today. The device in this fan reads the same as the old fan, 6.8k in circuit. Looks like it was a thermistor. FWIW, I reinstalled the old fan (before receiving the new fan) with the thermistor shorted (thinking it was a thermal fuse) as I stated previosly. It ran full speed for about an hour and then it quit for good.

Thanks for everyone's input on this matter.

-- David Farber David Farber's Service Center L.A., CA