HP Proliant MicroServer Gen8

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Using a Standard 4-Pin PWM Fan in the HP Microserver Gen8

The big flaw in the Gen 8 Microserver is the choice of sole system cooling fan, a 120mm Delta fan rated 3400 RPM (as far as we know) at 0.8A at 12VDC. This fan is impossibly noisy at full speed (rated at 46.5 dB by Delta) and even though its speed is thermally controlled, it keeps the new Microserver always a few decibels noisier than the earlier Athlon II Neo N36L based machine. Because the fan connects via 5-wires to an odd 6-pin header on the motherboard, we could not replace the stock fan with a quieter one.

Some time after the review was posted, I received an email from a HP Microserver Gen 8 user who had come up with a way to use a quieter 4-pin PWM fan in place of the noisy stock 6-pin fan. Mihai Tianu sent me this link on "Faking the Fan Signal" at the forums of homeserver.com where he first posted his findings.

When I asked him to write up his method as a postscript for our review, Mihai Tianu asked that I distill his forum postings for SPCR audiences, as English is not his native tongue. Here's then, is my distillation.

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The fan in the HP Microserver Gen8 has a Rotational Detect (RD) tachometer signal. What that means is that it doesn't output pulses like ordinary PWM fans, but just a low voltage (0.12V measured) when the fan spins. Shown below is a technical description of this type of fan.


Technical details of RD tachometer signal fan


HP Microserver Gen 8 fan pinout key

You can trick the Microserver into "seeing" an ordinary 4-pin PWM fan by connecting the Rotation Detect/Tachometer pin to the ground, and SHORT CIRCUIT THE GROUND pins from the fan connector. See the diagram below.


How to connect a 4-pin WPM fan to the HP MS Gen 8 6-pin header

Personally I'd go replacing the FAN with a silent one which has ~75% of the original fan's airflow. I don't use the server for heavy tasks and I think it will be enough. Whatever you do, pay attention and leave the thermal protection enabled in the BIOS.

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Our HP Microserver Gen 8 system had long been dismantled and stored away. We didn't feel confident about posting the above fan swap mod until we tried it ourselves. That was finally done in the past 48 hours. The biggest challenges are finding a connector to fit the tight 6-pin fan header on the Microserver motherboard, and a PWM fan of the appropriate rated speed.

You may recall from the notes on page 8 of the original review that I used an old 4-pin cable for audio from a CD drive to the motherboard. The cable was cut in two and both ends used with pins reconfigured so they made contact with the relevant five pins on the motherboard fan header. This time, the cut ends of the wire were bared and soldered to a male 4-pin fan header.

I reached for the first reasonably suitable PWN fan on our shelf, which was the stock sleeve bearing PWM fan of the Silverstone Argon AR01/AR03 heatsinks. This fan is rated for 2200 RPM and 0.23A. That's about 1/3 lower RPM than the stock fan.

I can report that this method of hooking up a 4-pin PWM fan to the Microserver Gen 8 definitely works with Silverstone's much nicer sounding fan. The minimum noise level of the system dropped to about 20 [email protected], with a smoother, less tonal sound quality. Many other PWM fans could have been used; any PWM with good sound quality and 2000~2300 RPM rated speed should work well.

Even slower rated fans will work without triggering the MS Gen8's draconian fail-safe system. A Scythe SL1225SL 12LM-P fan , which we've measured at 1300 RPM top speed ran at just 700 RPM hooked up this way in the MS Gen8, making almost no noise at all (around 12 [email protected]). This is not recommended, however, as your are likely risking the thermal safety of the components with such low airflow. Besides, the little fan in the power supply makes considerably more noise, so there's no aural advantage in running the main fan this slow.

So there you have it: A way to run your HP Microserver Gen 8 much quieter than stock without having to run in RAID mode, with a much nicer sound fan. The mod is relatively simple, the cost just the price of a good medium speed PWM fan. Thank you, Mihai Tianu!

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