Temperture sensor /gauge..

[JORGIE95961]

So i m starting to slap my truckck back together and i realized my temp guge was on hot. now i remember it working just a a year ago when i tested . now it pegs to hot. if i remove it and gound the wire it bounces an pegs to hot. .. i replaced sensor and same deal.. Now i got to thinking and none of my grounds straps are on , but i just dont think thats the issue.. and i know some say you have top get the right sensor.. i got mine from oreilys, but the old and the new did the same thing.. i do have a multi meter can anyone dumify a way to test this. as it what settings to place the ,ulti meter at and where to touch to test..... anything helps