Dell Dimension 4600C cpu fan dead.

[David Farber]

I'm servicing a Dell Dimension 4600 which is shutting down after about 5
minutes. The next time the PC boots, it gives a warning that the previous
time it shut down was due to a thermal problem. That led me to notice
that the CPU fan wasn't turning. The part number of the fan is 9G180. Inside
the fan housing is a small two terminal device. It measures about 6.3k ohms.
Is this an open fusible link,
a thermistor, or something else?

Thanks for your reply.

 

[Franc Zabkar]

I'm servicing a Dell Dimension 4600 which is shutting down after about 5
minutes. The next time the PC boots, it gives a warning that the previous
time it shut down was due to a thermal problem. That led me to notice
that the CPU fan wasn't turning. The part number of the fan is 9G180. Inside
the fan housing is a small two terminal device. It measures about 6.3k ohms.
Is this an open fusible link,
a thermistor, or something else?

Thanks for your reply.

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:
http://www.users.on.net/~fzabkar/sffprcs5.jpg

- Franc Zabkar

 

[Clint Sharp]

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

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.

 

[David Farber]

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:
http://www.users.on.net/~fzabkar/sffprcs5.jpg

- Franc Zabkar

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.

 

[David Farber]

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.

 

[Franc Zabkar]

Franc Zabkar wrote:

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.

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

 

[Franc Zabkar]

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.

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

- Franc Zabkar

 

[Franc Zabkar]

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.

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

 

[Franc Zabkar]

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

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

 

[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.

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:
http://groups.google.com/group/comp.sys.ibm.pc.hardware.chips/msg/c8619c11421cd5cc

- Franc Zabkar

 

[Franc Zabkar]

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

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?
http://www.almico.com/sfarticle.php?id=1

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

 

[David Farber]

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

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.