'72 GMC Jimmy fuel guage problem
 

The sending unit on my Jimmy was changed by my father shortly before I bought the truck from him and there is still a problem. The guage pegws itself at the 3 o'clock position and doesn't accurately show the fuel level. Do I need to change the sending unit again or is this more likely an electrical problem??

If you unplug the sender, the gauge should read well past full. If you ground the brown wire from the sender, then the gauge should read empty. If that is the case then the problem is in the sender unit, sender grounding or the wire from the back to the front. The sender actuates the dash gauge by "grounding" it in effect, so if the sender is not grounded completely then the gauge can end up anywhere.

An easy location to do your gauge tests is at the fuse panel as the wire from the sender plugs into the panel.

Jim

sending unit
 

Thanks Jim,
You probably just saved me an entire weekend of troubleshooting.

I just had the same problem with my snow plow truck. I turned out to be the wire between the fuse box and the sending unit. I replaced it and now I have the option to run out of gas on purpose.

Common place for that wire to short out is under the chrome trim at the bottom of the door.