Temperature Gauge Issue

The sending unit is a controlled ground. Make sure the threads are clean and you're NOT using teflon tape on the threads. Make sure the port that the sender screws into is clear and not plugged up with rust and other garbage. Make sure your coolant level is up and there's no air pockets in the motor. Fire the engine and bring it up to temp. Is the gauge working now? If not, your new sending unit could be bad, too. Welcome to the wonderful world of aftermarket parts.