Sorry for a very geeky question, but google isn't helping.

I'm building up a PC. The case has two 120mm fans on it (intake and outtake). The connectors for these fans are 2wires into 4-pin Molex for power, with a single RPM monitoring wire going to the 3-pin motherboard header.

Like the middle image here:

This works fine, but is it possible to ditch that 4-pin molex, crimp on new terminals and just power the fans straight from the motherboard 3-pin fan headers? Like the right hand image, but with all 3 wires. I can't see why not? Am I missing something?

Reason being is that my motherboard (AN8 Ultra) is capable of adjusting fan speeds to suit temperature and it does this by modifying the voltage at the motherboard header. But that is **** all use if they are being powered directly from the PSU

Without seeing your connections, (you may want to sort your /img /url stuff)yes, my mobo runs 2x120m fans) but don't hold me responsibe if it melts…

I don't see why that would be a problem, just check the fans don't pull more current than the mobo headers can supply.

You can get adapters so you don't need to re-wire. Or just fiddle* the existing fan connectors and run them at a set 7v or 5v to make them quieter.

As above, check the power rating of the fans, and what the mobo can take.

*left vague as if you don't know what you're doing, don't do it!

Yeah adapters are easily available many PSU and Mobos come with them failing that pop into Computer Orbit they're very helpful.

Sorry, they blocked the image. I mean the one about halfway down this page:
http://www.silentpcreview.com/article63-page1.html

The fans say they are 0.3amp, no idea what the motherboard max is – which is a concern. Doesn't say in the mobo manual, but surely it must be more than that??

Fiddling: yeah I could run them of the 5v rail on the molex, or the 7v difference, but I like the idea of them still being able to spin at max on the odd occassion when they need to.

Adapters: might be good, but I've not seen any to split a 3-pin mobo header to 3-pin+4-pin molex. Plus I'd like to shorten the wires while I'm at it as I'm trying to keep the wiring neat to help airflow (and quietness).

Usually the on-board supplies are fairly low-power (80mm case fans operate at aound 150mA) and you risk damaging them by lobbing on high-power (300mA, 3.6W) fans. This is why the fan supplier has provided high-current connectors and seperate mobo header connectors, they've assessed the risks and decided that their fans take too much power to run from most mobos. Why risk it?

Why risk it?

Because otherwise the uGuru stuff in the BIOS that is supposed to adjust fan speeds to match temperature has no effect and is completely redundant.

I see both sides. your point: why only put 4-pin molex connectors on the fans if they can run from mobo headers? and my point: why add an expensive BIOS feature that allows fan control if the mobo can't physically handle powering the fans?

That's why I was unsure. But I went for it.

Got some 0.1 terminal connectors from Maplin, snipped off the 4-pin molex and re-wired to use the mobo header. Not too hard though I had to bodge-crimp them with needle-nose pliers because Maplin didn't sell the right crimping tool 🙄

Result: much neater internal wiring. All fans come on at startup and are spinning at a much slower and less noisy speed. 😀

Unfortunately though it looks like my graphics card is fritzed 😕
(I suspect this is unrelated though, as I'm rebuilding this PC after the PSU went pop, so I suspect it took out the graphics card back then).