are you sure you have a mechanical gage ? in my lifetime, all the "mechanical" water temperature gages made have had a capillary tube that was filled with a substance that expanded when warm, thus moving the gage pointer. if/when this tube is cut, the gage is useless, and can only be repaired by a restorer that specializes in this field. it almost always is cheaper to just buy another aftermarket water temperature gage than get it restored. all electrical gages use sender units that are calibrated to whatever ohm measurement the gage needs to function with, and the sending unit is connected to the gage with a wire. is this the type of gage you have ? if so, the sending unit could be bad, or maybe just the wrong one for your gage, no matter what the seller stated.