Dual tank switch problems

[Navy_chief]

Guys,
I just replaced my Fuel tank switch and it is doing the same thing as the old switch so they are both good I assume.

Anyways here is what is happening the right tank is working correctly on the fuel gauge as soon as you switch it to the left side it pegs way past the full mark what could be causing this? I know there is fuel in both tanks? And this is for the 1977 GMC Desert Fox.
Thanks Matt

[79k30dually]

Have you checked resistance through the tank units at the switch? Possibly a ground issue at the tank unit. I've changed more switching valves than switches. Double check the grounds first. Good luck, Ron the snipe.

[hatzie]

You've probably discovered that 1/2 of the DPDT dash switch handles switching the gauge wiring on the early trucks like your 1977. The centre wire is the gauge and the angled outside wires are the senders. The other switch half only has 2 wires populated. This half feeds IGN power to the solenoid valve when it's switched to AUX.

When a tank pegs the gauge needle way past full it's usually a broken or missing tank ground. Sometimes it's a damaged sender wire on that side. Very rarely it's a bad sender.

Sender resistance to ground rises as the tank is filled.
0 ohms or a dead short to ground = empty.
90 ohms to ground = full.
Open or very high resistance = way past full.