That's an excellent overview of the fluid routing.
This is how the electrical portion of the 1981-1991 Auxillary Fuel Tank Regular Production Option NL2 works.
I posted this on another board.
**1981-1991
Motorized Valve system... 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.
- Pollack 42-159 Motorized valves are available with 65PSI limits for some, but not all, later EFI systems. The originals were rated for lower pressures.
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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. This 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...
The following three panes should be pretty decent visuals of what I was saying with the last two above bullets.
NOTE: Stock 1981-1986 & Diesels don't have a 2 position Weatherpack disconnect at the sender with Fuel Pump Power (just a single sender wire from A or D on the valve), and obviously no in-tank fuel pumps, they are otherwise identical in function to the TBI setup.
TBI TANK SWITCH SET TO RH
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TBI TANK SWITCH SET TO LH
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1981-1986 & 1981-1991 Diesel TANK SWITCH SET TO LH
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NOTE ON SWITCHES:
- Usually, but not always, the switch is at fault for a non operational valve. The switch is cheap as in $10-$15. Change it if the wiring looks OK and the valve doesn't work.
- 1981-1991 I ALWAYS REPLACE the motorized valve switch with the 1987-1991 RV series AC Delco D7809 or Standard DS-1807 1987-1991 R/V 10-30 "TBI" ON-ON switch. I never have been able to figure out why GM used a centre off switch on the early 80's trucks... The Pollack motorized fuel valve always had the over-travel protection diodes and switch built in.
- For those of you that want a "Correct restoration" 1981-1986 GM used The AC Delco D7003 Momentary ON-OFF-ON switch that has "DEPRESS FULLY" emblazoned on the bezel. I NEVER USE THE D7003 SWITCH... IMHO it's a problem child because of the momentary action.
1987-1991 RV series TBI Fuel Pump Relay for TBI tank switch and fuel pumps.
Year specific information can be found in the 1987-1991 Emissions and Driveability service publications. You can find them in my manuals post in the Library.
The TBI fuel pump relay (AC Delco 158240 Multi-Purpose SPDT Relay) can be found on the firewall between the transmission tunnel and the Heater/AC box.
Fuel Pump relay without the transmission kickdown relay. You can see the pump test terminal dangling beside the split loom.
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A Hot Start Fuel Pump Controller is mounted behind the dash on some 5.7L and all 7.4L TBI trucks.
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More pictures of the harness plug behind the instrument panel are in this thread. -->
http://67-72chevytrucks.com/vboard/showthread.php?t=757299
RV series TBI Fuel Pump Relay test terminal.
The Fuel Pump Test Terminal that's present on all RV series TBI trucks is shown pulled away from the harness.
This picture also includes a transmission kickdown relay mounted next to the fuel pump relay. The kickdown relay is supposed to be mounted nearer the RH side of the engine bay (US passengers' side) on the tabs. The two relays are the same part and it's been 30 years, give or take, since it rolled off the assembly line...
You can also find this AC Delco 158240 multi-purpose SPDT sealed Metripak 280/630 relay and mounting tab used elsewhere in the engine bay for DRL on some Canadian models, Aux cooling fan, Glow Plug Inhibit, and probably other things. The replacement plug is a Delphi Metripak hybrid 280/630 # 12052287 or AC Delco PT1115 is the plug with a repair pigtail.
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Fuel Pump Relay Control
- When the ignition switch is turned ON the Electronic Control Module (ECM) will energize the coil in the fuel pump relay. The ECM will energize the fuel pump relay coil as long as the engine is cranking or running and the ECM is receiving distributor reference pulses. If there are no reference pulses the ECM will cut power to the fuel pump relay coil within 2 seconds after ignition ON.
- As a backup system to the fuel pump relay, the fuel pump can also be turned on by the oil pressure switch. The oil pressure switch is, normally, open and closes when oil pressure reaches about 28 kPa (4 psi). Fuel Pump Power through this switch is also cut if the engine stops running as the oil pressure will drop.
- If the fuel pump relay fails, the oil pressure switch will run the fuel pump to allow you to drive to a safe location and repair the problem. However. You will experience extended cranking times because the fuel pump will not run at all til the starter builds oil pressure. This will shorten oil pressure switch and starter lifespans if the failed fuel pump relay is not diagnosed and corrected.
- The pump will deliver fuel to the Throttle Body where the system pressure is controlled to about 9 to 13 psi. Excess fuel is returned to the fuel tank by the pressure regulator.
- A Hot fuel module is used on all 7.4L and some 5.7L engines to correct a hot restart (vapor lock) during a high ambient temperature condition. It is designed to over-ride the ECM 2 second pump operation and will run the fuel pump for 20 seconds at initial ignition switch "ON”.
- The dual tank switch harness connects switch power at the "Bulkhead Connector", replacing the fuel pump power wire with the NL2 harness, in the below Fuel Pump Relay schematic.
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All years NL2 system NOTES:
- 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 power comes from the fuse panel under the dash or the TBI fuel pump power connector. It's fairly easy to retrofit and or update to a 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.
