ROW spec wiring for fuel sender

I’m in the process of rebuilding my 200tdi 110 on a galv chassis that is using a plastic TD5 fuel tank, and TD5 dash/clocks, almost ROW spec. I have WQB100440 as the fuel sender, and “standard” 110 200tdi wiring loom.

I have modified the loom for what I want out of it, but it’s basically adding extra earth wires and moving “tap offs” to better positions.

With the old steel tank, I had a 3 wire sender. The wire colours were black/green, and 2 plain blacks. From what I can gather, the black/green is from the gauge, but the 2 blacks just went to the earth stud at the rear. I had a low fuel warning light, and this worked when the tank was empty.

So the problem is, WQB100440 has 4 pins, 2 are not used, 2 are for the level sensor. In theory, the black/green should go to one pin, but what goes on the other? Is it earth, or 12v feed? I don’t think it should be 12v, as I’m quite sure the black/green will have 12v on it, and then go to earth via the variable resistance of the sender, or is that wrong? How is the low fuel light activated on the ROW spec?

I think I may have worked out how the low fuel light works. On the TD5, the fuel gauge is fed via the speedo head. This has an output for the low fuel warning, so guessing some form of comparator is inside and outputs high when the resistance changes beyond the specified level, caused by low fuel. That’s just a guess though.

Thinking about that, I can assume the sender is black/green to one pin and then earth to the other.