2000 Freightliner with 3406E

I have had the check engine coming on and the high coolant light also. When they come on it will shut down. I replaced both coolant sensors and That made it worse . It has one sensor on the back of the head That has one wire to it. When it had the old sensor in it if I unhooked That wire The hot coolant light came on with the old sensor it did not stay on but now it does. What should the voltage be to that wire It shows 4.9 volts What might be my problem THANKS Mike