Fuel gauge and wiring diagram for 1978 Suburban C20

[dogfart]

I have just bought a 1978 Suburban C20 from northern ohio. It has its share of rust and bumps and bruises one of those being the fuel gauge doesnt work. Using a bunch of youtube videos, I was able to do some testing and verify that the problem probably isnt the gauge. When starting the gauge slowly rises from E, going over full on some cold starts and after the car starts then is slowly goes back down to slightly above E. I was thinking that it was likely the sending unit but when I was under the car the other day I noticed that coming up along the passenger frame rail just entering the bottom of the engine bay, there is a wire that seems to be cut or burned off as it comes up where the exhaust is. The problem is I don't see where the other end is. I can tell from looking at the glass fusebox and other wiring under the hood that someone has messed around with some wiring so that's not great. The problem is I dont know where the other end of this wire goes. From what I can see the only wire that goes along the passenger side frame is the fuel send unit so that makes me think that may be to blame for my fuel gauge not working but everything is covered in 40 year old decaying undercoating so I cant see the color of the wire . Ive looked around this forum and others and despite lots of similar issues about troubleshooting fuel gauge things, no one has the same issue that I do that Ive found. So I have 2 questions 1) does someone have a 1978 C20 wiring diagram where I can full trace the fuel wire from the sender to the gauge? I'm trying to find where I need to hook up this mystery cut wire in the engine bay and hopefully magically get my fuel gauge working. 2) Please verify what the behavior of my fuel gauge indicates, if anything at all. It seems that the needle rise is normal behavior on start, not sure what the always settling about an 1/8th of a tank means.

Any help would be greatly appreciated. Thanks!

 

[Jgonick]

LINK- to wiring diagrams

 

[75gmck25]

On a pickup there is a tan or pink sending unit wire that runs up the passenger side frame rail, then across the back between the engine and the firewall, and connects to the wiring for the instrument panel.

 

[dogfart]

Thanks for the help guys. Using your tips and some trial and error I was able to backtrack the fuel sender wire from the bulkhead to the spliced wire with the green end that the previous owner had inexplicably wired to the solenoid on the firewall (see attached pic). When I removed that from the solenoid and started the burb the gauge went to 3 o'clock. I then took the wire from the bulkhead and spliced it into what I suspected was the orphaned wire below the vehicle (see pic) and started the burb. Same issue, gauge went to 3 o'clock which made me think it's broke in another spot between the engine and the tank. So on Amazon I ordered I new fuel sender pigtail wire (https://a.co/d/ap8bthF) that I spliced into a new wire and ran it from the sender wire in the engine bay to the back of the burb. Luckily i have the spare tire well out cause im replacing it due to rust so I was able to reach in between the bulb floor and the top of the gas tank without dropping it, unplugged the old sending wire and plugging in the new one I just made and started the burb. The gauge went to about 3/4 full, which is plausible given where it should be. Gonna keep my eye on it as I drive it but I have hope that this could be resolved. Thanks again for the suggestions guys.