[Shiny]

If there was a resistive short causing you to measure 4 ohms low at the gauge when the tank is half full, the short is a resistance in parallel that brings 35 ohms down to 31 ohms.

1/Rt = 1/R1 + 1/R2, or 1/31 = 1/35 + 1/Rshort

Rshort = 271 ohms

Unfortunately, the resistance of a short like this is still higher than the "full" value of ~90 ohms. That dashes hopes of using the gauge to detect the short by unplugging the sending unit.

What other circuits are shared by the fuse for the gauge? If the gauge was the only circuit on the fuse, you could measure current at the fuse box with and without the sending unit.... but I suspect there will be other loads on that same fuse to make this fruitless.

This is a tough one..

I guess if the short was stable, you could add 5 ohms (or whatever) in series with the sending unit and get your gauge to read correctly. The chances of a "stable" short like this sounds pretty slim though...

You need some smarter help diagnosing this one!

Mike