1973-1991 Dual fuel tank systems theory of operation

[gmachinz]

A lot of times its just the simple things. The fuel sender has an arm attached to the float which makes contact with the resistance board as full levels move up and down. Throughout the full motion you get anywhere from 0-96 ohms on average. Most of the time if you have more than 50K miles on a stock sending unit, that resistance board will be worn enough that the gauge is off noticeably. Most worn units I’ve seen is from half tank and less-most people dont keep their tanks topped off while never dipping below a half tank for example. The high mileage resistance boards are always worn out at half tank and less which is why the gauge reads fine on full but near a half tank the gauge tends to wander all over the place. Quality of the sending units are worse with offshore replacements than GM or NOS ones as well.

[jtownes84]

will you please respond to my pm thanks!

[gmachinz]

PM sent again.

[hatzie]

Originally Posted by 808_67C10

HA! So, I started the truck this morning and the gas gauge read right on full.... then, after driving about 2-3 miles, she topped out about 1/4" past Full - I swear, these square bodies and their electrical issues. I miss my 67 c10 so bad right now! LOL

3:00 on the gauge is usually your ground from the sender ring to the frame... It can be a damaged wire or dodgy connection between the sender and the gauge but usually it's the short little ground wire.

Try grounding the fuel tank to the frame with some jumper cables. If that fixes it the ground wire is your 3:00 needle problem...

[808_67C10]

Originally Posted by hatzie

3:00 on the gauge is usually your ground from the sender ring to the frame... It can be a damaged wire or dodgy connection between the sender and the gauge but usually it's the short little ground wire.

Try grounding the fuel tank to the frame with some jumper cables. If that fixes it the ground wire is your 3:00 needle problem...

That is what it was... HATZIE - you are amazing! Thank you!

[44th Miss Inf]

Lots of great info here. Thanks

[v30crewcab]

91 tbi truck, took out fuel pumps, and hard tubed it for a diesel swap. I'm getting power to the switching valve, but I think I've lost the ground due to unplugging the fuel pumps, am I correct? whats the simplest way to convert the tbi setup for a diesel setup? do I just need to ground the two wires that were originally power for the fuel pump? or do I need to hook the two together? factory wiring diagrams leave out alot!

[hatzie]

Originally Posted by v30crewcab

91 tbi truck, took out fuel pumps, and hard tubed it for a diesel swap. I'm getting power to the switching valve, but I think I've lost the ground due to unplugging the fuel pumps, am I correct? whats the simplest way to convert the tbi setup for a diesel setup? do I just need to ground the two wires that were originally power for the fuel pump? or do I need to hook the two together? factory wiring diagrams leave out alot!

NO!! You haven't lost the ground... The valve motor wires from the dash switch change polarity depending on the switch position. The dash switch is a polarity reversing switch commonly used to control the direction of DC motors. Go back and read the 81-91 section in post #2 of this thread. Pay close attention to the power routing diagrams I drew up.

The only wiring modifications you need to make are at the two position Weatherpak plugs attached to the pigtails dangling from each sender. DO NOT DO ANYTHING WITH THE TWO POSITION PLUGS ON THE ENDS OF THE NL2 VEHICLE HARNESS.

Remove the fuel pump power wire from the Weatherpak plugs in each sender pigtail and replace it with a green Weatherpak cavity plug. Fold over the fuel pump wire and tuck it into the split poly loom around the sender pigtail.
You only need to mod the sender pigtail. AGAIN!!! DO NOT DO ANYTHING WITH THE TWO POSITION PLUGS ON THE ENDS OF THE NL2 VEHICLE HARNESS. If you ground these wires you'll have a fire because either could be hot depending on the dash switch position.

[v30crewcab]

ok, so it should still be switching without the fuel pumps in the loop? I've got power and ground at the switch, but the valve is not switching, even with a new switching valve. I'll have to investigate more. Thanks.

[bigoilman]

Great collection of info here! Thank you. I'm replacing everything in my '77 dually and only found the dash switch as D7089C, I'm assuming the part number in Post #2 was just a typo.

It doesn't say "DEPRESS FULLY" nor did my original switch. It doesn't behave like a momentary switch, should it? It clicks firmly into either position and there is no center position.

[gmachinz]

If you are going with EFI, you have the right switch. You need a constant switch (not momentary) because the switch also runs the fuel pump relay. On my conversion kits I ise a LH and RH relay for the fuel pumps and they also control the LH/RH shuttle valving inside the frame mounted selector valve.

[bigoilman]

Thanks for the clarification. Looking forward to installing your harness when it arrives!

[hatzie]

Originally Posted by bigoilman

Great collection of info here! Thank you. I'm replacing everything in my '77 dually and only found the dash switch as D7089C, I'm assuming the part number in Post #2 was just a typo.

It doesn't say "DEPRESS FULLY" nor did my original switch. It doesn't behave like a momentary switch, should it? It clicks firmly into either position and there is no center position.

The OEM 1977 system is the solenoid valve setup outlined in post #1.

Post #2 is for the 81-91 systems. That part number is not a typo for the 81-91 systems. I use that switch for all of the motorized valve systems regardless.

[AustinBelair]

You guys are super smart when it comes to this. I've got my 87 dual tank TBI that has the 4.3L V6 and I put all new parts in it and I cannot get the drivers side tank to fire. The pump primes with the key on and it will crank but I'm not getting fuel in the throttle body. I've checked grounds etc. All new stuff.

New tanks
New pumps/sending unit assemblies etc
New selector valve
New dash tank selector switch
New fuel filter


The passenger side works perfectly. I can key it on the DR side and I hear it prime but will not fire when cranking. Every now and then, it seems like it wants to start but it just cranks. I did find that the previous owner installed a "Kill switch" toggle switch mounted to the dr side kick panel. They basically cut one wire behind the dash selector switch and routed to the toggle then out of the toggle and back to the cut wire on the body side.

I removed the toggle and it still won't fire but works perfect on the pass side.

I'm lost.

[hatzie]

There were a handful of Right Hand Drive 73-91 squarebody trucks made for UK standard roads in South Africa, Australia, New Zealand, etc. There are even a few that were imported back into North America in the last 30 to 40 years.

This is why I always use LH & RH in my descriptions and not driver/passenger sides.

Industry standard nomenclature for Left and Right hand sides of a vehicle... you'll find these used in the service manuals.

• LH is the seated driver's Left hand.
• RH is the seated drivers' Right hand.

Are you positive that all six of the rubber feed and return lines on the valve are routed to the proper steel feed and return lines?

Are you positive the LH fuel pump is the one that's running and not the RH pump?
You can check this at the two position fuel sender/pump disconnect to the LH sender using a test lamp. It should light for 2 seconds or so when you turn the ignition to ON.

[GonicGM]

Thanks for posting. Responding for watching.

[AustinBelair]

Originally Posted by hatzie

There were a handful of Right Hand Drive 73-91 squarebody trucks made for UK standard roads in South Africa, Australia, New Zealand, etc. There are even a few that were imported back into North America in the last 30 to 40 years.

This is why I always use LH & RH in my descriptions and not driver/passenger sides.

Industry standard nomenclature for Left and Right hand sides of a vehicle... you'll find these used in the service manuals.

• LH is the seated driver's Left hand.
• RH is the seated drivers' Right hand.

Are you positive that all six of the rubber feed and return lines on the valve are routed to the proper steel feed and return lines?

Are you positive the LH fuel pump is the one that's running and not the RH pump?
You can check this at the two position fuel sender/pump disconnect to the LH sender using a test lamp. It should light for 2 seconds or so when you turn the ignition to ON.

Yep, I get power there on the LH for 2 seconds then light goes off.

[hatzie]

Then it's down to the hose routing at the valve or the fuel pump itself.

If you take the bed off or at least slide it back a foot or two it's easier to work on this area than working in the dark over your head under the truck.

Don't use an impact on the bed bolts. You'll waller out the holes in the bed. If they are stuck just cut em off under the bed and replace them. Nut splitters work on the bed bolts too.

[AustinBelair]

Originally Posted by hatzie

Then it's down to the hose routing at the valve or the fuel pump itself.

If you take the bed off or at least slide it back a foot or two it's easier to work on this area than working in the dark over your head under the truck.

Don't use an impact on the bed bolts. You'll waller out the holes in the bed. If they are stuck just cut em off under the bed and replace them. Nut splitters work on the bed bolts too.

Do I have to worry about tail light wiring and other stuff or just the bed bolts?

[hatzie]

The taillight pigtail unplugs from the front-back lighting harness on the tail end of the LH frame rail.
If the factory lighting harness disconnect plugs aren't bodged somehow you unplug it. Not a big deal.

[travlinman]

Quick question for u. 82 Chevy c-30, I'm eliminating the driver side tank and selector valve and will run new lines from the tank to the pump. How do I get my gauge to read correctly off the single tank? I'm assuming I'll need to bypass the dash switch in some way? What about the wiring and connector from the old valve ?

[dmack91]

Hatzie,

Thanks for all of the great info.

[biggsteve97]

The factory dual tank harness failed completely in my 87 Silverado R10 a about 2 years ago and was wondering if anyone had or knows where to get a new dual tank harness for my truck. Mine was so bad when I removed its not worth using it to fab up another one. First lost the LH tank 10 years ago and repaired the harness multiple times through the years. About 5 years ago it started to go through pumps and senders in the RH tank about every 4-6 months and even damaged 2 fuel gauges somehow. Then one day last year it lost power in the harness to both tanks completely all the way up to the firewall. I have replaced the dash switch and the Pollack valve multiple times through the years and have managed to find Ac Delco parts every time but the truck has had a very hard life and driven almost every day since new.

[hatzie]

I assume the wiring from the fuel pump relay to the Metripak disconnect on the NL2 wiring harness is OK.

If you have the whole NL2 harness it's not difficult to build a brand new one using the old harness as a layout guide. If you cut it in a couple places or it's broken in a few places just duct tape it together. You aren't trying to make it work again you're in need of a layout and wire length guide. Even if it's chewed up pretty badly it'll show where the wires need to be terminated and how long you need to make em.

I usually lay one of these small harnesses on a piece of plywood and drive in finish nails after the terminations and next to any breakout points.

The NL2 sub harness is actually in two sections.

1. The Weatherpak and Metripak 480 firewall disconnects to the switch and ground inside the cab.
2. Two position Weatherpak disconnect at the firewall to the Weatherpak valve plug, and the Weatherpaks at the senders.
   [**]There's a single wire from the B terminal on the six position Weatherpak at the valve to a disconnect in the fuel gauge wire on the RH frame rail. Some of the later TBI trucks used a Weatherpak at this position. If yours is a molded rubber connector and this wire is chewed up I'd replace it with a single position Weatherpak and new wire.

You can buy the 2 position Weatherpak shells, Metripak 480 shells, all three terminal families (Weatherpak, Metripak 480, & Packard 56), and the Metripak/Weatherpak wire seals from Mouser Electronics.
The connector shells inside the cab are obsolete but the terminals are easy to source.

Download the 1989 wiring diagrams and print off page 34 (PDF page 42) on 11x17 paper. The connector shell part numbers are on the diagram along with the wire gauges and colors. I'd use 14ga SXL jacketed wire. You can get project assortments in 25' lengths from the Wirebarn.
The original harness was wrapped in harness wrap. This is not electrical tape it's a special friction activated tape. I'd use Split poly loom with Tesa harness tape at the joints and maybe wrap the split poly every 6" or so with two wraps of Electrical tape to keep the split closed.
https://www.amazon.com/Tesa-Wire-Loo.../dp/B00EH6IZ6Y

There are two splices in this harness, near the six position Weatherpak on the valve, for the fuel pump power feeds. I'd solder these joints and slip adhesive lined "marine" heatshrink over the connections.

Delphi 12085270 & 12085271 ratcheting terminal crimp tools are very versatile tools. They will do every original terminal in the square-body trucks from Packard and Pak Con to Metripak and Weatherpak. They're around $100 each but these are the most used crimp tools in my wiring box outside of my Daniels AF8 & AFM8.
There are some non ratcheting crimpers that are almost as versatile but they're unpleasant to use.
I'd buy the Delphi tools and sell em when you're finished... if you decide you don't want to keep em.

Gmachinz Harnessworx makes custom and factory type harnesses. I've never done business with him so I don't know what your experience will be.

[Ford Assassin]

So I am in the final stages of parts collecting of doing the LS swap on my 1988 R30. I am planning on swapping to Walbro 255 (GCA758-2) pumps in each tank. The first post mentioned the switch can only handle 65 psi of fuel pressure.

I was only planning on running a single fuel pressure regulator (01-04 Corvette, Wix 33737) in the engine bay and reusing as much of the stock TBI fuel lines as possible. This will allow the fuel switch to dictate where/which tank unused/bypassed fuel to either tank that is selected.

Anyone see a problem with doing this way?

Thanks