Duster temp gauge problems

I always had a stock temp gauge issue with my '69, new sensor. We scrapped a '68 Cuda that had sat in the woods for 28 years. I grabbed the orginal sensor off of the 318, my gauge works just fine now. The new sending unit or sensor was junk. Checked the sending unit with a circuit tester, clipped circuit tester to pos battery terminal, sending unit to neg ground and circuit tester lead to terminal on sender. Wait for a dim light, no light, sender is bad.