96 Temp gauge troubleshooting tips

My 1996's temp gauge has always read at the mark excatly in the middle of 100 and 210 so I guess that is 155. It does start at 100 cold but when warmed up never goes higher than that 155 mark. I flushed the radiator and installed a new 195 T stat, no change. I installed a new temp sending unit in the block, no change. I ran the truck with a thermometer in the radiator and when the T stat opened and water circulated it was 190. I used a infared no contact temp gage on the t stat housing said 185, and on block by sending unit 180. Does any one know what resistance the sending unit should read at 100,210,and 260. I have a box I can hook up to the sending unit wire and simulator various resistance vaule and see if the dash gauge moves to the proper reading. All grounds to engine and frame were cleaned and the other dash gauges read proper. All replies are appreciated !!!

Also if I check the reading through the OBDII port with my scan tool I get 198 F. So what tells the dash gauge its reading ? computer ? or is a temp sensor hooked to the dash gauge ? Anyone with a schematic or wiring diagram ?