So after stupidly fiddling with SpeedFan for a good half hour wondering why I couldn't change my fan speed, I realised I had a 3 pin connector to the mobo, so the PWM pin was free. As a solution, I've decided to disconnect the Ground and the 12v pin from the mobo and connect them up to my front-panel fan controller, whilst leaving the Fan Speed Sensor wire connected to the mobo.
My question is this: if, for some reason or another, the fan-controller fails and the CPU fans stop spinning, will the motherboard sense this through the Speed Sensor connection and shut off the CPU/computer?
My logic tells me the answer is yes, and that this is a (relatively) safe way of being able to control my CPU fan-speed without having a 4-pin connector, but I just want to make 100% sure, so I don't fry my CPU. I've seen how quickly that happens on the AMD chips (the one I happen to have!)
Yes and no, depending on wether or not you have the bios set to perform a function in the case of overheating.
Though i cannot think of a product with such a "fail safe" built in, i know that you can set the parameters for which the bios will shut off or throttle (depending on the MB - some have different options) the CPU
If i were in your shoes, i would closely monitor the temps and be sure, under load, what your absolute maximum is (with heater on, on a hot day - something like that) and set the bios temp monitoring safety features to just over this maximum. Not much more, but just enough so that if you test (by holding the fan or something, NOT THAT I WOULD EVER RECOMMEND THIS) and it proceeds to shut off the computer when that temp condition is met, then you will have a somewhat fail safe scenario precaution.
-Oh, and yes, most motherboards will tell you if the CPU fan has been disconnected or stopped, but unfortunately will not do this for other fans (unless otherwise built into the bios of a particular board).
I also just noticed you said you have a front panel fan monitor - if it is digital, you can usually configure it to perform the same function as a fan alarm going off when a fan has stopped or slowed to a certain speed.
There is no problem in connecting the fans 12v and ground to a fan controller or even the molex pin of the PSU and connecting the sense wire to the mobo for monitoring speed... lot of mobos have CPU & chasis fan failure warning function and they will beep when no rpm is detected inspite of a connection... i have 2 cpu fans and 3 case fans connected to mobo with only their sense connection and powered by the PSU and works like a charm...
I checked the BIOS, and there's an option "CPU Thermal Throttle" which is currently set to Auto (the other options are 12.5%, 25%, etc up to 85% I think). I assume this is the one you're talking about? Unfortunately there's not much info in the manual for the Asrock 890GX Extreme3, and I don't know how to set it so it has a temperature-too-high condition...