Gas gauge misreading

My 1986 C20 crew cab has dual fuel tanks. They have both been replaced within the last 2 years and the sending units are not much older than that.

The other day I noticed that the level in my one tank was quite a bit lower than I thought it should be. I had parked in a different part of town over night and suspected that maybe some kids had siphoned some gas from my tank.

I refueled that tank, but the level didn't show a full tank. In fact, it would not read above 3/8 of a tank (before I refueled, it read 1/4).

I tried switching the tanks over to the other one, but it too would only read between 1/4 and 3/8.

The valve and solenoid are working as I can hear it change position when I move the switch from the right tank to the left and back again.

Some people have suggested that it may be a bad ground behind the dash causing the gauge to read incorrectly, but I have not been able to locate a bad ground.

Does this mean I should head to the wrecking yard or the dealer and try another gauge? Or is there another area of the electrical system where I should do some more investigating before replacing the gauge?

Thanks for any and all input.

75 Addict
Does the gauge read correctly when the other tank is selected?

Couple of ideas to check: The connections on the back of the dash switch for tank selection, and both the ground connection and sending unit connection at the tank itself.

I think the dash cluster wiring consists of 1 big printed circuit and the gauge just "plugs" into the cluster, so I'm not sure about a ground problem there.