Temperature Gauge Issue

I have a 1973 Dodge Dart Sport 340. The only thing not working was the temperature gauge. I tested the gauge by grounding the push on plug and turned the key on. The gauge pinned to hot. Turned the key off and it went back down. I'm thinking that is easy, I need a new temperature gauge sending unit. I bought one at NAPA, installed it and nothing changed. The temperature gauge did not move. I grounded the wire again and it pinned the gauge. The sending unit was identical to the one I removed. I am now stumped. Can anyone help?