We bought a GM AC Delco sending unit and tested 3 gauges all do the same thing. The oil gauge is ok but....
When you turn the key on the temp gauge slowly starts going up and pegs at 260 or would keep going until it completely pegs out at the hot side of the gauge , but the engine isn’t even hot yet. Then when we turn it off the needle don’t go back unless you manually do it.
the grounds are good!