87 chevy astro wont read coolant temp?

When you installed the new sensor did you put anything on the threads? the Threads are used as the "unit ground" (Nothing On the Threads!). Are you using 50%-50% Anti-Freeze & Clean Water=Coolant "All Year Round"? 23 Year old cooling system, When was the last time the system was flushed? Is your Radiator Fan Cycling as the engine Heats & Cools?