There were three fuel valves used between 1973 & 1991. None of the systems are rocket science to repair once you understand what makes em tick. On all three systems... -The fuel is routed from one of the tanks to a common OUT port. Only one tank at a time is valved open to the output port. -If the truck is equipped with a fuel return line the common return port from the engine is switched from tank-to-tank as well. -On systems with a return line... The return line should be routed to the tank that fuel is being drawn from. If you mix up the return lines you can overfill the tank not being used. Hosing down the guy in the next lane with raw fuel may not garner a friendly reaction. -Both LH tank lines into the same side and both RH fuel lines into the opposite side. -the COMMON side of the valve To/From-The-Engine only has 1 or 2 lines. Feed and, if equipped, Return. NOTE: It's worth noting the 1981-1991 systems likely all have return lines. I haven't run into one without. **1973 & 1974 & some 1975 Cable System... had a cable operated valve with a SPDT switch behind the dash knee panel that activated via the cable to run the fuel gauge. If you google it you'll find a thread on Chris's board about the valves with pictures of this setup. This is LONG obsolete. Folks replaced them with the 75/76-80 system when the parts ran out but there are a few survivors. NOTE: I'm not sure whether this system used the NL2 RPO or not... The 1975 wiring diagrams list the cable system and then NL2 in separate panes. **1975/76-1980 Solenoid system... used a single wire solenoid valve with three or six ports. --The valves returned to a rest position as soon as power was removed. In 1980 GM added a retained power relay to keep the valve energized in the AUX position while cranking. <-- Click to see the post about this timer relay. --The AC Delco 467513 Six port valve or a similar function unit was used on trucks that came with return lines. These are not being produced by Delco or any other manufacturer. If you feel like spending big bucks search that part number they occasionally show up on evilbay. --Three port valves were for trucks without return lines. NOTE: Standard Motor Products sells a three port version FV1 but there are no six port solenoid valves available. Folks have used the 1981-1991 Pollack valves but generally they butcher the wiring. -Four wires run through the firewall. --Two tank sender wires. --One Dash Gauge feeder wire. --One Solenoid Valve power wire. -The dash switch is a five terminal DPDT (Double Pole Double Throw) ON-ON rocker switch. ----Each switch is called a pole and each switch position is called a throw... ------Double Pole means there are two switches actuated by one rocker. ------Double Throw means there are two different switch positions with a common input or output terminal. ------ON-ON means there are two ON positions with no centre OFF position. --The reason there are five and not six terminals is to key the switch to the connector such that it can only be plugged in one way. You can use a standard 6 terminal DPDT ON-ON rocker or toggle switch by clipping off one of the switch terminals to match the original switch. --On the three terminal side of the switch there's one wire direct to the dash gauge in the common position and the fuel level sender wires to the switched terminals. This selects which sender is routed to the dash gauge. --The two terminal side of the switch is still a double throw switch but it's missing one of the switch terminals to key the connector. This is so it can't be plugged in backwards. It also assures that the unwashed masses will buy switches from GM and not McMaster Carr or Grainger or... It has one power wire from the fuse panel, or a relay in 1980, on the centre common terminal and a green wire that routes power to the solenoid valve when the switch is set in the AUX position. This is a depiction of the internal switch action.