guage problem

on my 87, changed coolant temp sensor (3 times!), temp still shows on the guage really low like its reading lower than it should, it doea deflect full scale however when grounded, fan kicks in on the second (of the 4) marks half way between 100 and 200. left the engine for 15 minutes then tested the coolant in the bottle with a candy temp sensor it read 165, so i reckon that the temp is fine and everything is kicking in, IT DID TAKE approx 20 mins for the engine toreach temp at which the fan kicked in (running at idle). So how can I further check the guage? without pulling of the instrument panel. I am assuming from the above I am not running too cold otherwise the electric fan would not have cut in.

advice please