fuel gauge malfunction

[anthony.cain.18]

Hey guys.. I have a 1985 GMC high sierra swb 3/4 ton. recently had fuel gauge failure and I did some trouble shooting and the fuel gauge was not the issue. I replaced the sending unit and now the gauge works but only goes to 1/4 when full.

[anthony.cain.18]

I've tried other gauges and they all do the same thing.

[rich weyand]

Reading 1/4 at full means the resistor on the back of the gauge is open-circuit.

Here's one source:
https://www.classicmuscle.com/p-6237-fuel-gauge-resistor.aspx

[anthony.cain.18]

What does that mean ? Open circuit?

[rich weyand]

The resistor is broken.

[anthony.cain.18]

On all the gauges I've tried even though they were working in the trucks I pulled them from?

[zieg85]

I don't know if this applies but I once had a 72 that did not have factory gauges, just had the indicator lights and the gauge fuse was blown, mine did the same thing about the 1/4 tank when full.

[bd]

What does the gauge do if you unplug the sending unit with the key on?

Then what does the gauge do if you ground the gauge wire that connects to the sending unit?

[rich weyand]

On all the gauges I've tried even though they were working in the trucks I pulled them from?

If you used the same resistor on the back of the gauges, yes.

Do what bd said, and let us know.