[rotorwrench]

If the wiring from the gauge to the senders is broken somewhere, it will always read hot since the gauge goes to full H when there is no current flow. The two terminal sender unit is a bi-metallic switch that opens the circuit if it reaches a set temperature like just above boiling point. All it can do is give a Hot indication when it opens the circuit. The single terminal sender is the calibrated sender that will give you a C or cold indication when you turn on the key with good connections in the circuit. It has a bi-metallic switch that works in unison with a ground arm and is normally grounded when the unit is first turned on which gives a full C indication (cold) until the engine warms up and allows the sender bi-metallic arm to bend farther away from the ground arm to give a warmer indication on the gauge. Current flow decreases in these units to give a hotter indication on the gauge. It starts cold with high current flow and gradually gets less current flow the warmer the coolant gets.

This is different that the pressure and quantity units that King Seeley made. They have higher current flow with higher pressure or higher quantity. Sort of backward to the temperature reading instruments they made.

The reproduction senders can be problematic. I prefer to find good used or NOS original King Seeley units when I can. Gauges are generally marked for connections on the back. If it works backwards then it either doesn't have the correct gauge or it is turned upside down or something. They always read Hot when Off. When you ground the sender wire it should go to Cold since it will be at full current flow. I wouldn't do this test for long though or it might burn out the heating coil in the gauge.