PWM Fan Only Idles

[Dezzimal]

I noticed that my CPU fan no longer spins quicker when under load. Fairly certain it is a problem with the motherboard, the fan properly winds up to full speed on boot for a few seconds but then will just idle at low speed.

Can I connect a 3 pin fan to it that'll just run at full bore all the time, or can I modify the fan to always spin at full speed? BIOS has no fan speed options (dell).

[Binkie]

Probably just the colder weather making there no need for it to run full speed.

[Rockstaru]

If your Dell is anything like mine, they use proprietary connectors for everything--I bought a new processor heat sink + fan hoping to shave a few degrees off my temperatures, only to discover that the motherboard used a proprietary 5 pin connector (with the 5th pin not actually connected to a wire) rather than the 4 pin standard.

[xerodok]

Have you tried SpeedFan?

[Dezzimal]

Probably just the colder weather making there no need for it to run full speed.

No, get out of the thread.

If your Dell is anything like mine, they use proprietary connectors for everything--I bought a new processor heat sink + fan hoping to shave a few degrees off my temperatures, only to discover that the motherboard used a proprietary 5 pin connector (with the 5th pin not actually connected to a wire) rather than the 4 pin standard.

It looks proper, theres a bit of plastic that makes it so you can't insert the plug backwards but things like that can always get lopped off.

Have you tried SpeedFan?

Yes, it is what I used to monitor the temp of the CPU. Fan controls aren't software accessible on dells.

[xerodok]

Are your temperatures going way up in your CPU? It's possible that the temperature gauge isn't working properly and isn't triggering your fan to go into high performance.

Since you've noticed this problem, have your average temps sky rocketed where it's unhealthy to put a heavy load your CPU?

[Dezzimal]

Temp gauge is working, it'll read 50C and the fan won't wind up at all.

[Senji]

Can I connect a 3 pin fan to it that'll just run at full bore all the time,

yeah

[Dezzimal]

I think the PSU might be failing then? When I connected an old 3 pin fan to it, it didn't turn at full speed. I should be seeing 12 volts across the circuit there right?

Edit: Somewhat resolved. I've come up with a temporary solution that involves powering the fan using an old cellphone charger that provides 3.6W at 12V, which makes it turn /really/ fast, possibly too fast. So I'm rooting around for a 1 ohm resistor at the moment.

[Dezzimal]

http://i.min.us/immT0o.jpg

There, I fixed it.

Boring description of PWM:

Spoiler: show

A PWM fan normally requires an analog pulse signal on the 4th wire in order to regulate the speed at which the fan turns. It pulses at 22.5kHz and the length of the pulses determines how fast the fan will go, longer pulses (always at a rate of 22.5kHz) mean the drive goes faster. Anyway, that signal is broken on my motherboard and the fan always idles, this might be because the signal is always telling the fan to idle, or the signal is not present and the voltage across wires 1 and 2 is low. When the signal isn't present the fan fails over and turns at full speed.

So what I did was I found an old AC Adapter and plugged that in to the wall, and plugged the two power wires directly into the fan plug. WHOOSH, it turned way too quick. What did we need? Less current by putting a resistor in the circuit. Most volume controls are forms of variable resistors called potentiometers or pots for short. I tried using the volume control on the old CD player but it smelled like burning because it was only meant to handle a few milliwatts and I'm putting around 1-2W on it. Also the range on the dial was 4Ω to 16,000Ω, the fan completely stops turning at around 150Ω by my estimate. That meant that the fan would be either full on, or full off, it was very difficult to find the spot on the dial where it was somewhere in-between.

So then I went out and grabbed a 100Ω potentiometer from the local industrial electric supply company. Flawless victory. At full resistance (100Ω) the fan idles quietly, and I can gradually speed it up to full blast ever so smoothly.