Some options:
- Get a fan that doesn't require high voltage to start (ones with variable speeds built into them)
- You can get a fan that has its own temp sensor and plug it into power directly
- Use a resistor connected to +12v to augment SYSFAN_2's output voltage (or a transistor if you want full interpolation)
I use the Antec Tri-Cools that came with my Nine-Hundred Twos and a Twelve Hundred. I have them connected to a 3-position military switch fan controller (UP=12v, MIDDLE=OFF, DOWN=5v). I have the fan's built-in switches to H (high, full voltage) and flip the fan controller to the 5v position when I use the computer (its always on); they start fine. The only exception to that is the 200mm fan on top: I had to modify its correlating switch on the fan controller to drop ~7v to get it to start.
The question to ask is your fan not spinning because the BIOS isn't telling it to because of temps/config or because the fan requires more voltage to start? You can test this by dropping 5v to it instead of 12v: get a Molex/3-pin adapter as shown above and reconnect the 3-pin +12v wire to +5v and see if it reliably starts. Even if it does, SYSFAN_2's output is still in question (you don't want the BIOS feeding it, say, 2.5v most of the time, burning out the fan), hence my third suggestion.