Temperature Sensor...Wrong one???

My parts dealer gave me a Niehoff TS25381 temperature sensor with two wires on it to replace the original temperature sensor on the left hand side of my 350 motor that runs the temperature gauge(1994 GMC Sierra 4X4). My original had only one wire, and so I connected one lead to ground with another wire. The gauge started working....BUT....it went up to about 125C, then the check gauges light came on, and then it went back down to 120C. This is like 250F. I put a thermometer on the gauge, and it read correctly, about 80C, about 165 F.

Do I have the wrong sensor??? If I reverse the leads...might it work correctly? I think it's an ohmic reading no matter which way it's connected....true? Can anybody help?

I dont know if you have the wrong sending unit or not, but it may be the guage itself messed up, espeically since you said the actual temp. is 165 F. My 1992 Silverado did the same thing (it pegged the needle passed 260 degrees), so I changed the sending unit (I believe it just had one wire too, but I'm nto sure), and the temp. guage fluctauted from abotu 210 to 240 degrees (truck has a 190 degree thermostat). I replaced the water pump and the thermostat and still the truck showed it was running hot. So my dad plugged an anilyzer to it, and it never got above 188 degrees. I bought a manual temp. guage and my truck never gets above 190 degrees.