This one has me stumped. My temperature reading was always reading low on the gauge but finding the engine (6cyl) is running fine but the gauge is off. I replaced the sending unit and it read right for a day, dead center on the gauge when hot. Then next time I drove it shows cold, even lower the the original unit. I bought another sending unit but it shows the same thing. After extensive research and testing I find everything seems to work right, the sending unit, wiring and the gauge itself but it still reads almost at cold.
Here is what I have done so far.
1. I checked the gauge by grounding the wire to it and it swung up to high as it supposed to do.
2. With key on i put a test light on the wire and it pulses, meaning the wiring and the resistor is working.
3. I put an ohm meter on the sending unit cold and it read around 310 ohm and when I boiled it in water it went to about 30 before water boiled, (I found some info it should read 49 ohm at 180 degree F) so it seems to work properly. I even tried 2 sending units.
Still it reads almost at zero and moves very little. So I don't know where to go from here. Any ideas? Can this be a ground issue at the sending unit?