Well as far as temp gauges it sounds like you have one mechanical and one electrical. One with a tube and one with a wire. They are best tested buy putting the "sender" part in water over a heat source and watch gauge compared to a thermometer in the water container. Take it to boil and back down while observing.

The electric is more trouble: You can use a battery or a battery charger for the electric one while in the water. (submerge only the threaded portion).
Read this:
http://www.madelectrical.com/workshop/water-temp-gauge.shtml


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