Temp guage keeps going to "H" but truck not overheating

What is your question?

I had recently took out my instrament cluster trying to get my gas guage to work again replaced the lights and the voltage regulator but still having rhe same problem gas guage barely working but now a new problem the temp guage keeps saying that my 90 nissan d21 base model is overheating and just turning it on to let it warm up for about 5-10 mins. I cant figure out what it is. Ive also replaced one of the ect.'s ive test the thermostat (tested fine) ive added new coolant. Im at a fork in the road now dont know what else to do. Can anyone help me with this problem . 1989 nissan d21 hardbody(even tho they say its a 90 ) base model , pickup , original owner , 400,800+ miles (still runs like a champ) any help highly appreciated.

What makes this problem better or worse?

Not sure hasnt done betrer or worse yet.

How long has your 1990 Nissan D21 had this problem?

About 2 weeks