Need Help With Fuel Gauge and Sender Unit

[DeadMansOhms]

Hi. New here to this forum. Found it by trying to find answers to my problem. Any help is greatly appreciated.

I have a '69 Chevy C20 and everything was fine with my fuel Guage until I went to fill up my tank when it was empty and noticed when it was full my gauge only went to half. Now I have half a tank and it says empty. So I checked the wiring and it looks good. Did a resistance check on the sender unit. It seems to be correct with what's in the tank. So I'm guessing it's my gauge. I guess there is a resistor on the back of the gauge. Could that be the problem? If so can I purchase that only or would I have to get the whole gauge.

One other thing I noticed is, when I checked the resistance at the sending unit, it only showed me a value with the key off and it was nothing when the key was on. Don't know if that's normal.

Thank you for reading and I hope someone can help.