1972 chevy c10 temp gauge not working. i replaced sending unit and gauge needle does peg when grounded

Did you put in the correct temp sender, (80 - 120 vs 120 - 220) ? It is not uncommon for even "new" senders to be broken from the factory. I would tell you to verify the temp range required and buy another new unit. Did you recheck the sender wire for any damage or chaffing of the wire covering from sender to gauge ?

I have a 1972 C10 using GM 340 aluminum.behieve heads on a 383- 350 vortex.block.. The temp sender is 3/8 npt threads and the factory one is 1/2 npt threads. Bought 1995 sender for a gauge dont know temp range. This issue was going on when i bought this truck. I bought a factory tach assembly from LMC so while i have the gauge cluster assembly out now is the time to fix the issue. I ohm the wire from seder to fire wall and from gauge circut board plug green wire to then end of the sender wire all is good. Installed new gauge started the engine all was good until about 120 degrees then it pegged out again.