Steve, here's where I was headed with my questions in your other thread about this:
Both the fuel and temp gauges are basically 0-10 VDC voltmeters, calibrated to show either gallons or degrees. If you have half a tank of gas, for example, hooking up a temp gauge to the fuel gauge's wires will show 85deg temp -- the mid-scale reading. Likewise, if your car runs at the nominal temperatures, it's "half-a-tank" hot when using the fuel gauge. So, you can determine if your temp gauge is truly faulty by making that substitution after noting how much gas you have. If the temp gauges shows the same needle deflection, the gauge is not at fault. (Be sure to verify ground as part of the substitution.) There are other tests you can do to determine if it's the sensor, wiring, grounding, or the regulated 10-ish volt 'reference voltage' that's being used to begin the measurement task. But it's worth a check of your meter (by substitution) before you go thru tracking down another one.
Now, if there's a screwdriver jammed thru the face of the meter, or it's been on fire recently, then yes, it's probably bad...
