I have all my gauges working but the temp gauge is doing some funny stuff. I checked the gauge with a 1.5v battery and the needle moves to both the cold and hot side. So I'm assuming the gauge is functional, not sure if it can be calibrated. The temp sender is new and I have an older sender that I checked against the new and I get the same reading with both. This issue: Every time when I turn the ignition on the needed fluctuates briefly, then settles at about half reading on the gauge when the engine is cold. When I start driving the needle creeps up another 1/8" or about 190. I'm expecting the needle to start out at 100 (the lowest reading) but it never goes below the half or 175 mark. Any ideas?
I'm not sure but I imagine the temperature gage works on resistance through the sending unit. From what you've said the one variable you didn't mention is the wiring. Have you thought about bypassing the original wire to see if the trouble could be there? Just a thought.
Just because you think the wiring is good, or has continuity does not mean it will pass a load test, one strand of wire will give a good continuity reading, but will create high resistance. Corrosion or bad grounding will do the same thing.