97 3.8 Coolant Gauge Pegged Out

FYI. For the 1996-1998 model year there's a temperature sensor for the PCM and a separate sensor for the Dash. The two sensors are NOT compatible to each other and are not interchangeable. Many auto parts stores do not do a good job giving a good way to tell them apart.

There's a table in the Ford service manual that outlines what the resistance and voltage should be at various temperatures. With the motor cold and the electrical sensor wire disconnected what is the resistance measurement and the outside air temperature at the time?

Even better. Do you have a infrared thermometer? If so, what does the coolant temperature read compared to the open circuit resistance measurement at the time?

Repeat the same test by measuring the voltage across the temperature sensor with the key on and the electrical connected attached.

This should give us resistance and voltage readings at two different temperatures.

Where to go next depends upon the results posted.