Dual fuel tanks and valves

[fnny6770]

My 1980 GMC 1 ton dually has dual fuel tanks on looking at the switch does anyone know which color wires or number goes to the left tank or the right tank? I'm trying to find information on how to troubleshoot the passenger side fuel tank has been replaced the driver's side has not been replaced yet but I have the new one, so for now I'm just running fuel out of the passenger side or I'm trying to. My son had this truck for a few years and he started fixing things and now he doesn't remember which wires go to what tank? Where the valve is it switches from one tank to the other, ? I would like help in trying to figure out if it's switched over to the right tank or not because right now it will not start the fuel pump doesn't seem to be pumping fuel into the filter so I bought a new fuel pump to change it because it's an old fuel pump and it's probably in need of being replaced anyway.

 

[fnny6770]

I forgot to mention the gas gauge stays in the full no matter where I position this switch

 

[Doppleganger]

Either of these any use?

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[fnny6770]

Thank you I will use these diagrams and see what I can find, my question too is where is this selector valve that switches the fuel from one tank to the other?

 

[Copakekid]

I have an 84 GMC C1500 with dual tanks. My switching valve is on the passenger side near the fuel tank. Follow the fuel lines coming out of each tank and it should lead you to the valve.
Mine has a heat/ dust shield covering it.