The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - 1973-1991 Dual fuel tank systems theory of operation

**hatzie** · 01-04-2018, 09:55 AM

**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.

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.

Motorized tank switch details...

The above switch depiction is a Polarity Reversing DPDT switch used to turn a DC motors forward or backward depending on position. GM used a proprietary switch but this 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.
You can see below that... On the 1987 RV series TBI trucks the selected fuel pump receives Power on the wire from the switch so it will activate because it has power and the shared frame ground. The UN-Selected pump receives Ground on the wire from the switch thus shutting it down because it now has two ground wires and no power.

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

TBI TANK SWITCH SET TO LH

1981-1986 & 1981-1991 Diesel TANK SWITCH SET TO LH

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.
***Important NOTE*** I recently ran into an AC Delco D7809C switch with a terminal locator bump setup for the 89-91 NL2 harness plug #12004139. I'd never seen a locator on the switches til I got saddled with that one. Another board member has ended up with one as well so it's not a one off occurrence... It's easily remedied. Remove the terminal and cut a clearance notch in the plastic connector to slide past the locator lump then re-seat the connector. Or buy a different switch.

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.

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.

A Hot Start Fuel Pump Controller is mounted behind the dash on some 5.7L and all 7.4L TBI trucks.

More pictures of the harness plug behind the instrument panel are in this thread. -->http://67-72chevytrucks.com/vboard/s...d.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.

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.

Converting TBI to Carburetor senders without butchering the truck harness

Converting the senders from TBI to single wire on an NL2 dual tank truck is Extremely simple. As long as the orignal NL2 sub harness is in one piece... There's no need to pull new wiring into the truck and there's no need to butcher the existing sub harness so it's irreversible.
If the TBI senders are no longer serviceable you can liberate the wiring needed for the changeover from the senders.

Unplug the two position Weatherpak sender plug from the NL2 harness in the truck. Leave the truck harness alone til you plug in your new sender pigtails.
Cut the sender ground wire off at the top of the TBI sender and crimp a 1/4" female disconnect to the end of the wire for the carburetor sender tank ground.
Cut the 2 wires routed to the 2 position Weatherpaks off the at the top of the TBI senders with at least 6" of wire.
Purchase or otherwise source a molded GM fuel sender pin connector with at least 12" of wire.
Connect the molded pin plug pigtail to the sender wire on the Weatherpak plug you liberated in step 3.
Loop the end of the fuel pump wire over @ 1" and cover it with adhesive heatshrink to seal the end.
Tape the fuel pump wire to the sender wire and cover the resulting sender pigtail with split poly loom.
Plug the two position Weatherpak plugs into the truck harness and the tank.
Plug the tank ground you made in step 1 onto the tank sender ring ground tang.

If the senders and fuel pumps are serviceable... don't chop em up. You can sell them or give em to someone that needs em.
You still need to make up the pigtails but it's not difficult.

If your senders are working and you aren't comfortable crimping Weatherpak terminals and seals onto a wire you can buy Weatherpak repair pigtails and make them up just as you would've with cutoffs from the original senders.

I'd just roll the connectors from scratch but I have the tools and the parts onhand.

Parts:

two 2 position Weatherpak tower shells,
two female Weatherpak terminals,
two Weatherpak cable seals,
two Weatherpak cavity seals,
several feet of 12ga SXL black wire,
several feet of expanding mesh wire sleeve,
molded pin sender connector pigtails,
two ring terminals,
two 1/4" female disconnect terminals.

Slip some expanding mesh sleeve over the molded pin connector pigtail leaving @ 2" uncovered wire then strip and crimp the Weatherpak seals and terminals on the ends.
Make up two black sender to frame ground wires that will reach... Strip and crimp the ring terminals on one end, slide some expanding mesh over the wire leaving @ 2" uncovered then strip and crimp on the Female disconnect terminal.
Insert the cavity seal in the fuel pump power wire position of the Weatherpak shell. This safely terminates the connection without chopping it off the truck harness.
Insert the Weatherpak terminal in the sender position of the Weatherpak shell.
Plug the parts into the truck and test.

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.
This is a picture of the molded pin connector in the fuel gauge wire near the valve courtesy of Dukagora. I believe it's the same connector as the end that plugs into the pin on the senders.
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 soup to nuts... from the fuse panel/ground bar to the dash switch and down to the valve and gauge wire disconnect on the frame. 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.

01-04-2018, 09:55 AM	#2
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,913	1981-1991 NL2 Dual fuel tank systems theory of operation 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. 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. Motorized tank switch details...* The above switch depiction is a Polarity Reversing DPDT switch used to turn a DC motors forward or backward depending on position. GM used a proprietary switch but this 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. You can see below that... On the 1987 RV series TBI trucks the selected fuel pump receives Power on the wire from the switch so it will activate because it has power and the shared frame ground. The UN-Selected pump receives Ground on the wire from the switch thus shutting it down because it now has two ground wires and no power. 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 TBI TANK SWITCH SET TO LH 1981-1986 & 1981-1991 Diesel TANK SWITCH SET TO LH 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. *Important NOTE* I recently ran into an AC Delco D7809C switch with a terminal locator bump setup for the 89-91 NL2 harness plug #12004139. I'd never seen a locator on the switches til I got saddled with that one. Another board member has ended up with one as well so it's not a one off occurrence... It's easily remedied. Remove the terminal and cut a clearance notch in the plastic connector to slide past the locator lump then re-seat the connector. Or buy a different switch. 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. 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. A Hot Start Fuel Pump Controller is mounted behind the dash on some 5.7L and all 7.4L TBI trucks. More pictures of the harness plug behind the instrument panel are in this thread. -->http://67-72chevytrucks.com/vboard/s...d.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. 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. Converting TBI to Carburetor senders without butchering the truck harness Converting the senders from TBI to single wire on an NL2 dual tank truck is Extremely simple. As long as the orignal NL2 sub harness is in one piece... There's no need to pull new wiring into the truck and there's no need to butcher the existing sub harness so it's irreversible. If the TBI senders are no longer serviceable you can liberate the wiring needed for the changeover from the senders. Unplug the two position Weatherpak sender plug from the NL2 harness in the truck. Leave the truck harness alone til you plug in your new sender pigtails. Cut the sender ground wire off at the top of the TBI sender and crimp a 1/4" female disconnect to the end of the wire for the carburetor sender tank ground. Cut the 2 wires routed to the 2 position Weatherpaks off the at the top of the TBI senders with at least 6" of wire. Purchase or otherwise source a molded GM fuel sender pin connector with at least 12" of wire. Connect the molded pin plug pigtail to the sender wire on the Weatherpak plug you liberated in step 3. Loop the end of the fuel pump wire over @ 1" and cover it with adhesive heatshrink to seal the end. Tape the fuel pump wire to the sender wire and cover the resulting sender pigtail with split poly loom. Plug the two position Weatherpak plugs into the truck harness and the tank. Plug the tank ground you made in step 1 onto the tank sender ring ground tang. If the senders and fuel pumps are serviceable... don't chop em up. You can sell them or give em to someone that needs em. You still need to make up the pigtails but it's not difficult. If your senders are working and you aren't comfortable crimping Weatherpak terminals and seals onto a wire you can buy Weatherpak repair pigtails and make them up just as you would've with cutoffs from the original senders. I'd just roll the connectors from scratch but I have the tools and the parts onhand. Parts: two 2 position Weatherpak tower shells, two female Weatherpak terminals, two Weatherpak cable seals, two Weatherpak cavity seals, several feet of 12ga SXL black wire, several feet of expanding mesh wire sleeve, molded pin sender connector pigtails, two ring terminals, two 1/4" female disconnect terminals. Slip some expanding mesh sleeve over the molded pin connector pigtail leaving @ 2" uncovered wire then strip and crimp the Weatherpak seals and terminals on the ends. Make up two black sender to frame ground wires that will reach... Strip and crimp the ring terminals on one end, slide some expanding mesh over the wire leaving @ 2" uncovered then strip and crimp on the Female disconnect terminal. Insert the cavity seal in the fuel pump power wire position of the Weatherpak shell. This safely terminates the connection without chopping it off the truck harness. Insert the Weatherpak terminal in the sender position of the Weatherpak shell. Plug the parts into the truck and test. 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. This is a picture of the molded pin connector in the fuel gauge wire near the valve courtesy of Dukagora. I believe it's the same connector as the end that plugs into the pin on the senders. 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 soup to nuts... from the fuse panel/ground bar to the dash switch and down to the valve and gauge wire disconnect on the frame. 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. __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful. Last edited by hatzie; 04-04-2019 at 12:08 PM. Reason: Add TBI to Carburetor sender conversion.