Understanding 3 & 4 pin fan headers on motherboard.

Please help me if I am incorrect. My motherboard (GA-MA770-UD3) has four fan headers:
4-pin cpu_fan
3-pin pwr_fan
4-pin sys_fan 1
3-pin sys_fan 2

AFAIK, the 3 pin headers offer zero variable fan control via motherboard. Any fan plugged into this slot will run at full rpm speed.

The 4-pin headers do offer variable rpm speed via motherboard. These fans are controlled by PWM, this is what the 4th pin is.

1. So, in the mobo's BIOS settings, there is a setting for CPU smart fan control (on, off). Setting this to on will allow to the motherboard to control the fan speed of the cpu fan. I assume this is based off of temperature? I further assume that you can use software to adjust the stock setting.

2. The only other related setting is "system smart fan control" (Auto, voltage, PWM). This is what confuses me. Is this setting ONLY for that 4-pin sys_fan1 header? And what would the Auto or voltage setting do?

Thanks. BTW, i am not discussing anything about fan controlling software. Just the mobo BIOS.

It no doubt varies from company to company and mobo to mobo, so you might want to ask in a dedicated forum. Here's an interesting thread on Asus fan headers:

ASUS Z87-Deluxe fake 4-pin headers & other fan control info

I can say that my Gryphon Z87 controls my PWM CPU fan perfectly as well as my 3 pin chassis fans. The BIOS itself offers some control, but the Thermal Radar software offers even more. My CPU fan idles at 200 RPM, and the two chassis fans at 480 RPM. The fanless GPU and mostly fanless PSU add up to a very, very quiet i5-4670 system.

Oh, i didn't think that it would vary with different manufacturers. Interesting. Well, then I really don't like how Gigabyte do theirs!

Gigabyte uses the Easy Tune app for controlling the PWM fans - off the top of my head you have three options - Silent, Normal & something else (Cool?) and these set a base line for the fan speeds which increase or decrease according to temperature.

However the Easy Tune app will have no effect on the 3 pin as you rightly point out - that will run at full speed unless you put in a fan controller to adjust the speed and it is then set to that speed unless you change it - it is not temperature controlled.

As for question 2 - sorry but I have no idea.

Well, I wanted to keep apps and fan controllers out of this discussion
Perhaps the Auto & voltage setting in the BIOS is so you could go into an app like EasyTune and then adjust it there.

When I select Auto or voltage (IIRC) the fan slows down to around the PWM rpm. There is a slight difference, but it is being controlled.