Repeat after me: "I am not as smart as I think I am."
These are words to live by, especially when buying surplus electronics without a spec sheet...from Active Surplus. Case in point --> this pair of 80x25mm Panaflo FBA08A12U 12v DC fans. These are server grade 80mm fans, but I incorrectly assumed I could "just" solder on a 3-pin fan connector and "just" punch down the RPM signal wire. The fans look like they SHOULD be wired up for RPM and Matsushita Electric simply left the cable out, but that's not the case.
After wiring up an RPM signal wire to the Sense terminal behind the motor - nuthin. Bah! On motherboards with fan speed sensing, this means the fan constantly spins up and down because the computer thinks the impeller is bound. Aaargh!
Naturally, wiring the fan up to the 12v side of a full-size 4-pin molex connector (yellow+black wires) is the solution.
I am not as smart as I think I am.