You can pull the switch from the dash, mark the wires and remove them. then ohm out the switch to see if there is a short.
Stolen from another forum.
http://67-72chevytrucks.com/vboard/showthread.php?t=754061
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.
Here's a thread on rebuilding the manual valve pak.
https://67-72chevytrucks.com/vboard/...d.php?t=829266
**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.
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--Three port valves were for trucks without return lines.
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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.
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Contrasting color diagram to show function...
Not GM wire harness colors... The wire colors vary by year and can be found in the wiring diagrams for your year truck.
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**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.
***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.
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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/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.
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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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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:
