The dash sending unit is a single wire sending unit that should between 2 & 4 plugs on the head. The ECM is by the water inlet on the intake manifold and should be 2 wire plug. Try disconnecting the plug at the water inlet and the dash gage should peg high or low I do not remember if open circuit is high or low. Someone could have changed things around. Check the side of the cylinder head. There is also a plugged port between plugs 1 & 3 someone might have utilized for a sending unit. Find a single wire sending unit and that is your dash gage/light. The 2 wire sending unit is the ECM Temp Sending Unit. It is possible someone eliminated the ECM sending unit. The wire for the Temp Gage is close to the exhaust and is easily burned. If someone burned up the wire they may have plugged the head and re-located the sending unit to the water inlet and removed the ECM tem sending unit. Look for a plug that by the water inlet that is not connected.

And that would cut the truck off and cause it to not crank?

I do not know. The ECM uses input from the sending unit to adjust fuel mixture.

Ohm your coil out