Temperature gauge not working

What happens if you create another ground at the sender. And which sensor have you changed. The two wire sensor for the ECM, or the one wire sender near the #1 spark plug?

Open circuit would give you full cold, and shorting the sender wire should send the gauge full hot. See if you can get a resistance value of the sender when at operating temperature, and we can translate that to a temperature, to see what the sender is actually reading. Compare that to a thermometer reading if possible, for cross checking. If the senders resistance value corresponds to the correct temperature, then the problem would have to be in the wire or the cluster. It is a simple circuit, I don't see what else would influence it. It would be interesting to know as well, what temp the ECT sensor for ECM is registering on the data stream. If you have a scan tool, you may be able to view it.