1973-1991 Dual fuel tank systems theory of operation

[hatzie]

If it was just one tank I'd look at the sender or wiring but this is the same on both tanks. More to the point it's exactly the same on both tanks...

What's the same no matter which tank is selected??? The fuel gauge and the sender wire from terminal B of the valve to the gauge.

Typical Fuel Sender Resistances were within ±3Ω or so on my test gauges from several different years:

• E = 0-3Ω
• ¼= 26-32Ω
• ½= 42-48Ω
• ¾= 61-67Ω
• F = 87-93Ω

¾ is less resistance than F. The only way to get less resistance in this circuit is for the wire insulation to be rubbed through and make an intermittent ground connection. That's unlikely to be repeatable over time.

I'd say it's probably the gauge itself.

[808_67C10]

Originally Posted by hatzie

If it was just one tank I'd look at the sender or wiring but this is the same on both tanks and it's exactly the same on both tanks...

What's the same no matter which tank is selected??? The fuel gauge and the sender wire from terminal B of the valve to the gauge.

Typical Fuel Sender Resistances were within ±3Ω or so on my test gauges from several different years:

• E = 0-3Ω
• ¼= 26-32Ω
• ½= 42-48Ω
• ¾= 61-67Ω
• F = 87-93Ω

¾ is less resistance than F. The only way to get less resistance in this circuit is for the wire insulation to be rubbed through and make an intermittent ground connection. That's unlikely to be repeatable over time.

I'd say it's probably the gauge itself.

Right on Hatzie, I need to pull the dash apart to change my dash so, while I am in there, I will do some testing. thanks for the tolerances - as always, I appreciate your knowledge and guidance.

[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

[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!

[aotte1]

Thanks for sharing your knowledge on this subject. 1986 K10 short bed, needs right side tank replaced, suggestion on good replacement tank, and/or key factors to look for.

Thanks,
Les

[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!

[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.

[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.

[gmachinz]

PM sent again.

[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.

[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!

[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.

[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.

[GonicGM]

Thanks for posting. Responding for watching.

[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 ?