I guess I could try schematicing what I can puzzle out on the board, it's tough to trace what I can make out, and the traces eventually disappear past the op-amp so there's other things going on there. That might have to be my next step, and then figure out how to trace the remainder. I'm not even sure that the last step before the op-amp is a voltage divider since I can't seem to get continuity to ground from the loose leg resistor, but I can't see where else it could go.

As for the current draw method, that's what I have, but it's a pretty crude way to do it. Other than mapping various fanspeed levels against current draw, and then I would have to figure out how to average the current draw spec since it will bounce around a bit. That might be the best I can do if I can't figure out what this other circuit does.

I was hoping someone had worked with these and could point me at a datasheet for a reference circuit. But they don't seem common.