Fuel gauge reads past full

[74chevyc30]

I'm new to the square body world so any knowledge would help tremendously. I have a 1974 chevy c30, it has one fuel tank on the right side of the cab, and the fuel gauge itself reads around 1-2 o'clock with the key OFF and ON. But whenever I unhook the wire from the sending unit, the gauge reads 3 o'clock. I'm not sure what is going on here and I have no clue where to check first.

I'm a tractor technician as a career, and on tractors whenever the fuel gauge is not reading anything with the key on or off, you can turn the key on and touch the wire on the sending unit to a good ground and if the gauge moves then you know that the sending unit is Bad, but in this case the opposite is happening.

 

[bucket]

You may be on the right track. If you take the wire that goes TO (not on) the sending unit and ground it to the frame, the gauge should go to empty.

If it doesn't, then most likely it is a bad gauge or something wrong with the printed circuit on the back of the cluster. Or possibly a bad connection in the sender wire somewhere. You can remove the gauge and easily bench test it to determine if it's at fault or if you have to go looking elsewhere.

 

[ali_c20]

Good videos on gm gas gauges:

https://www.youtube.com/watch?v=ydmwbdXGC1Y

https://www.youtube.com/watch?v=RCP-xplvyh4

My vote: Ground

 

[Dejure]

Indication is, our gauges run 0 ohms at empty and 90 full.


That yours likely has an issue aside, one could adjust the reading by adding a resistor from the gauge to ground (in parallel to the existing circuit). That should take the 90 ohms down to, for example, 80 ohms.

Then, if one needed it up, it would just be a matter of putting the resistor on the end (in series)

It could be helpful if one wanted to tweek the reading of a gauge.

I'd have to work the formula, but if a sender read too high, simple formulas could tell us what value resistor needed to be added. One could experiment with a a low ohm pot (e.g., 0 - 40 ohms), or work the formulas.