The original 3B may have had the rotary pump. I dont know about the 3B ,but the other rotary pumps had a sensor on the pump (2L 1HZ 1KZ etc).
The pump type sensor needs it own tacho to recognise the signal.
My point is you may need to use a 13BT tacho to recognise the sensor on the flywheel.
The 12v/24v situation is not an enviable one
