Temp sending units

[dr09911]

I have a '77 C10, 250 6 cylinder with gauge option, 3 speed manual. I am having temp gauge problems and I am starting with the sending unit to narrow the problem. There is one at the the thermostat housing that has a "nail type" head on it. There is also another in the head that has a "spade type" head. The wire from the harness (dark green) goes to the spade connector, then over to the "nail" head type unit. I am being told the "nail" head unit is for the gauge option. The "spade" unit is for just an idiot light. I have both. Anybody else encountered this problem??