[TDS-MN]

On my '85, there is a single orange wire on the top of the sending unit, and when that wire comes off it's post, I get the same reading on the gas gauge in the dash. The issue started after I had my tank replaced, and the wire kept coming off frequently for a while after.

I was able to re-connect it working between the cab and bed, using a long pry bar and one of those bendy-spring grabbers you can get at the auto parts store for a couple dollars. Was able to line the plug up with the grabber, and then push it down onto the post with pressure from the pry bar. I never have tried to see what access might be like from underneath the truck, mostly only because this first happened during the winter and I wasn't motovated to crawl under, when I had a system that worked for me.

I know i've seen it expalind, but can never keep it straight. An open ground forces the needle to go all the way to one direction,a nd a sharted ground forces it to go the other way. Wish I could tell you for sure which was which. But your symptoms match what mine was doing, hope you can find the solution.