Temperture gauge is pegged on high

[scotts72heavyhalf]

So to begin with this is a project truck that has been sitting for 4 years and I have been chasing down poor grounds and corroded connections with great success so far, we got the engine running and noticed no temp reading and when I checked the sending unit it was broken so today I replaced it with a new one a felker brand and as soon as I turned the key on it went stright to the far right pegged at high, it comes down a 1/4 when its cold then right back up. The engine seems to be running a bit warm but no overheating at idle, new thermostat installed and changed the coolent too, the upper hoses are hot so fluid is moving through the system. Bad gauge? or ? it appears that everything is correctly connected but seems like a ground short. Any ideas??