Heat gauge

I have a 95 Monte Carlo with a 3.1 engne.It blew a head gasket then had
a rod knocking.I  had the engine rebuilt.Before,the heat gauge worked
perfect staying at just under the half mark.After I got my car back,the
gauge reads 3/4 before the fan comes on.I took it back to the mechanic
and he checked everything out.The engine is NOT getting hot and all the
grounds in the wiring are good.He replaced the sending unit AND the temp
senser...which doesn't have anything to do with the gauge,and it still
reads wrong.I replaced the whole gauge cluster with a known good one and
it still reads hot.I unplugged the sending unit and grounded the wire
and the gauge pegged hot,so I know the gauge is good.What made the
original sending unit go bad? And could the new one be bad too?