I have added a sending unit and gauge to the old 96 12V and the gauge just barely moves when the thing is good and hot. Even with pulling the trailer it barely makes 120 F. Does that seem right or have I used the wrong line?

 

Im not too knowledgeable on the 12v but it sounds like its fine. Most gauges only go to 250 F and if your not really pushing the truck there should not be a reason for it to get extremely hot. Is the line for the fluid itself or just for temp reading gauge?