Hi, maybe you could try a dry test - buy a 0 to 500 ohm potentiometer and connect it to the wire going to the sensor. You probably have to calibrate the potentiometer first - find where 20 and 270 ohms are, set it for say 270 ohms, connect it up and see what the instrument cluster reads - then dial it down to 20 ohms, if it reads correctly, then you know it's your sending unit.
-Jerry