Fuel Gauge passed full

[TruckIt75]

Hey everyone I have a ‘75 GMC Sierra Grande with dual tanks. We have it set up that we only run the drivers side tank. My issue is that the fuel gauge is way passed full and has been since we bought the truck. I recently changed the printed dash circuit, the contact pieces for the studs, the cluster housing, and swapped in LED bulbs.
I’m not sure where to head next with this. Any help is greatly appreciated

 

[Soundmound]

Sending unit.

 

[TruckIt75]

That was changed as well, I should I’ve mentioned that

 

[Bextreme04]

Make sure the ground wire from the sending unit to the frame is clean and attached securely. Also, does the gauge change when you change the selection on the tank switch?

 

[TruckIt75]

I’ll check that thank you. I’m not sure we never really messed with the switch. The truck was set up to run a single tank by the previous owner and there was no power going to the switch I believe

 

[75gmck25]

With dual tanks, the wiring path to the sending units is more complicated than you would expect, so maybe the PO screwed up the connections.

The wire from the dash cluster is tan and it runs down the passenger side frame rail to near that tank. If there was a single tank (passenger side in ‘75) it would just plug in there to the sending unit There should be a round connector inside the frame rail where it can be disconnected.

Wiring from the dash switch runs through the firewall and down the drivers side frame rail to the area of the drivers side tank.
- one tan/white wire comes out of rhe loom and connects to the driver's side sending unit. The rest of the wires cross to the passenger side.
- from there one wire comes comes out of the bundle and connects to the solenoid power terminal
- the connector in the gauge wire on that side is popped apart and other two loom wires are inserted so they connect to (1) gauge wire running up the frame rail and (2) sending unit wire from passenger side tank

If you only want to keep the drivers side tank you could simplify the gauge wiring. Just find the gauge wire on the passenger side, disconnect it inside the frame rail, and verify that it works properly with the gauge (full when grounded, etc.). Then extend it across and connect it to the sending unit on the drivers side tank.

Another possibility is that someone pulled the engine or wiring inside the engine compartment and cut the sending unit wire. From the gauge cluster it runs through the firewall and across the back of the engine compartment, and then down the passenger side frame rail. It is vulnerable where it runs across the firewall.

 

[Elliot W]

Since it hasn't been spelled out here and I just went through this, when the gauge reads past full, it means open ground (as in there is literally some discontinuity in the circuit somewhere between the gage and the sending unit ground). This could be anything in that circuit from bad terminal connections at the plug to the gauge cluster, cut wire as Bruce mentioned, a bad sending unit, or a bad ground at the end of the sending unit.

Best way to check is by doing continuity checks with an ohm meter. Alternatively you can take the tan wire Bruce mentioned and ground it to see if the gauge moves. Keep in mind that the ignition has to be on for the gauge to readjust, and sometimes the gauge moves slowly when adjusting to the ground.

 

[Jimbarry]

Try replacing the selector switch in the dash, that's what fixed mine.

 

[tebgm81]

I've been chasing this for awhile but think I am closing in on it. The wiring path mentioned above is good info for tracking. The grounds, continuity, and resistor on the back of the gauge need checking.
I think my problem originated last year when trying to get at the dash lights. I recently found the main dash circuit board plug wasn't fully seated.

 

[Elliot W]

That could do it as I believe the fuel gauge wire is on the end of the two column harness. Also check the metal transfer surface where the cluster meets the harness plug. The metal on the cluster basically looks like a gum wrapper metal. Mine was ripped off but fortunately I saw it so I had to repair it with HVAC tape.