Dual tank/fuel gauge problem

[USSkoval]

I have another question about my '79 crew cab, this one involves the dual tanks and fuel gauge. It seems that the passenger side tank is the only one that works, but I really can't tell for sure. I do know that it is the one being used right now. Also, the gas gauge does not work at all. It was showing about 1/2 tank at all times before I removed the cluster.

Can a non-op valve cause the gas gauge not to work, or would it be the switch? Or maybe just the gauge is bad? I don't know how to test anything because the only diagram I can find shows a 4-wire switch. My switch in the dash has 5 wires... pink, pink/black, pink/white, light green, and light blue. The wires all appear to be OK.