Actually there were three valves used between 1973 & 1991. None of the systems are rocket science to repair once you understand what makes em tick.
On both 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 and the other half of the valve to the return port
-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.
-To-The-Engine only has 2 lines. Feed and Return.
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1973 & 1974 & some 1975 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.
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1975/76-1980 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.
--Six port valve was used on trucks that came with return lines.
--Three port valves were for trucks without return lines.
--Both valves are obsolete. Standard Motor Products sells a three port version 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.
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1981-1991 squares used a six port motorized valve made by Pollack. These are still being produced by Pollack on Jan 1st 2018.
-The "universal" valves that claim they're three port usually have all three return line ports capped. You can use either just be sure to cap any unused ports to keep dirt out of the valve guts.
-The later motorized valve remains in the last switched position unlike the earlier solenoid valves.
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Electrical;
-The Pollack valve uses five of six cavities in a Six position Weatherpack plug. A-E are used. F is plugged with a seal.
-Internal to the valve are two diodes to prevent power being routed to the motor from D or E once the valve shuttle has reached the end of its' travel.
-The motorized valve has a SPDT switch to control which sender feeds the gauge shortening the wiring considerably.
----The gauge/Sender wires are on positions A, B, & C of the valve.
------Senders from the tanks are on switched terminals A & C.
------The gauge output is on Terminal B. This wire runs to the molded sender wire connector on the RH frame rail. It allowed GM to just drop in the NL2 as a sub harness without having two main harnesses.
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Motorized tank switch details...
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-The above switch is a generic Polarity Reversing DPDT switch used to turn DC motors backwards. GM used a proprietary switch but this picture exposes what the GM switch actually is doing.
--Two of the switch contacts do not have terminals. These contacts are bridged back to the other switched terminals diagonally across the switch.
--Output to the DC motor in the valve comes from the common terminals of the switch.
--Power and Ground run to one switched terminal of each half of the switch.
--When the switch is toggled to the opposite position the LH & RH MOTOR terminals of the switch reverse polarity. + becomes - and - becomes +. The valve motor then runs in the direction dictated by the polarity it's receiving on D & E til the stop on the valve shuttle cams the internal gauge and motor switches over to the switch leg with a diode that blocks the incoming current. The motor then stops til the dash switch is toggled back to the opposite position.
--1981-1996 used The AC Delco D7003 Momentary ON-OFF-ON switch that has "DEPRESS FULLY" emblazoned on the bezel. DO NOT USE THE D7003 SWITCH... I ALWAYS REPLACE WITH THE AC Delco D7809 or Standard DS-1807 1987-1991 R/V 10-30 "TBI" ON-ON switch.
Since the 1975/76-1991 NL2 dual tank systems used a sub-harness that plugs into the RH tank sender wire on the RH frame rail and the fuse panel under the dash. It's fairly easy to retrofit and or update to the later model system.
You can swap a 1981 -1991 motorized Pollack valve sub harness into a 73-80 truck. You need the entire later model NL2 sub harness from the fuse panel/ground bar to the dash switch. Two wires run through the firewall to the valve and one wire from terminal B of the valve to the RH frame rail disconnect of the "production tank" sender wire.
BTW I've used the Delphi Packard 56 switch plug for a 73-80 system on a generic polarity reversing switch and rolled my own harness to convert a single tank truck to a dual tank truck. Much easier to drill a 1/2" hole for a generic polarity reversing toggle switch than cut and file a square hole for a GM rocker switch. The toggle pointed to the enabled tank so no bezel was required.
