GA-P55-US3L: operating a PWM fan from a 3-pin arrangement

[forumjoe]

Can a 4-pin PWM fan be operated from a 3-pin connection? Or is the fourth control pin essential to use in order to get the fan to spin?

I powered up my new US3L machine for the very first time today, with a view to just checking the system supply voltages and setting one or two essential things in the BIOS. But the PWM fan that's used to cool the CPU and which I'd wired (via a Fanmate2 speed controller) as a 3-pin arrangement wouldn't run. I couldn't afford much investigation in that scenario, as the CPU got hot very quickly, and I had only around a minute to dive into the BIOS, disabling CPU Smart Fan Control and looking at the main voltages.

In order to test the fan, I've since disconnected the fan from the mobo and the Fanmate and have connected it directly to a small 12v lead-acid battery that I have. The fan still wouldn't run, though. That's more-or-less equivalent to a 3-pin setup.

Before I decide that the CPU fan is dead and needs returning to the retailer, I thought I'd check with others in this forum as to whether you can only ever run a PWM 4-pin fan from a 4-pin arrangement? (The US3L mobo provides for a 4-pin PWM CPU fan). Or perhaps it depends on the particular fan? This one's an Arctic Freezer 7 Pro Revn 2 fan.

Incidentally, I decided to wire it 3-pin and use a Fanmate rather than just plugging it into the 4-pin connection on the mobo and allowing the BIOS to control its speed because of the recent report in these forums that there's a serious bug in the BIOS of P55 boards wherein the CPU core temperature is completely wrongly reported and this results in the fan almost stopping.

[Dark Mantis]

I am fairly sure that a 4 pin fan won't work off a 3 pin header but a 3 pin fan will work off a 4 pin header.

[forumjoe]

If that's the case, then, that would explain all.

I'll try plugging the CPU fan into the 4-pin header on the mobo, then, and using the BIOS to control the fan. Under "CPU Smart Fan Mode" in the BIOS, there's a choice of fan operating mode. I'll use the "Voltage" mode. I'd intended using "Voltage" from the outset but that worrying report about the CPU temperature and also the possibility that although the fan's speed could be set by voltage, the fan might still increase or decrease in speed with temperature put me off using the BIOS for the control.

Pin no.4 on the CPU fan's header is described as "Speed Control" and appears to be the same as pin no.2, which is "+12v/Speed Control".

[Dark Mantis]

Pin no.4 on the CPU fan's header is described as "Speed Control" and appears to be the same as pin no.2, which is "+12v/Speed Control".

That's because it's a sort of tacho system that controls the speed.

It is better anyway to run the larger third party heatsink/fan combinations straight off the PSU rather than the motherboard headers as there have been instances of the tracks burning out under the heavy load of some fans.

[forumjoe]

Yes, the need for a tacho would possibly fit if you're using the BIOS to control the fan speed, but if you disable the BIOS control and you instead connect the fan to (in effect) a 3-pin source of 12v (effectively leaving the fourth pin output on the mobo's header open), surely the fan should still run?

As for tracks on the mobo burning out, I can only summise that in the examples brought to mind the tracks must have been exceedingly thin. Most CPU fans these days draw a maximum current of around 200mA or less (0.2A), so should be perfectly safe to use on any half-decent motherboard. The CPU fan I'm using draws 0.15A. It's peanuts.

[Lsdmeasap]

You should be able to use them in either, 3 pin in 4 pin header, or 4 pin in 3 pin header

[forumjoe]

DM,

Lsdmeasap may well be correct but only for some 4-pin fans. It could depend, I think, on the particular fan.

Yes, I've googled the issue and it seems that, in normal PWM mode, the fourth connection carries a pulsewidth modulating signal. The speed of the fan is controlled by the duty cycle of that modulating signal.

Earlier today, I completely disconnected the fan from the 3-pin setup I had and connected it instead straight into the 4-pin PWM header on the mobo. I then powered on and quickly dived into the BIOS and made sure that CPU Smart Fan Control was enabled. However, the fan still wouldn't run. It appears to be as dead as a dodo. I've e-mailed the retailer, QuietPC, to get their view. If it really is dead, I'm sure they'll send me a replacement.

[Dark Mantis]

Lsdmeasap is an extremely clever guy but that doesn't always mean we see eye to eye and I would still stand by my original statement

As far as I am concerned I would agree with what you found out on google and that without the fourth pin connection the fan would stay idle. As you say maybe there are exceptions.

[forumjoe]

I remembered last night that the Intel CPU for this mobo came with a 4-pin PWM cooler/fan, so this morning I connected that instead into the 4-pin header. It ran. The Arctic fan assembly is therefore definitely duff.

QuietPC have been in touch and are expediting a replacement fan assembly to me today.

[Dark Mantis]

Oh well at least you have got to the bottom of it.

[forumjoe]

Yes, but earlier I was fearing that perhaps my realigning of the mobo on its standoffs had resulted in a piece of tracking on the board being shorted. Before elongating those mounting holes, though, I'd carefully assessed whether I would still be keeping within the bounds of the fixings and that anything I did would still be well away from any tracking (except for perhaps any part of the ground plane on the board). It seems I did gauge it right.

I must say that I'm very disenchanted with the Lian-Li case I'm using, indeed angry that any case manufacturer should market what's turned out to be such a poor example of design and quality. It's continuing to cause all sorts of problems and I'm literally having to use sticking plaster and glue all over the case in order to stop metal-to-metal rattling and to fix other problems. The alignment with the mobo was only one such problem. If it weren't now for the gross amount of work, and risk, involved in dismantling everything, I'd dispense with the Lian-Li case and get a much better one. Never ever get a Lian-Li case!

[Dark Mantis]

As they say "we live and learn" and it is very true. Maybe your testemony will save others from making the same mistake.

[forumjoe]

I hope so.