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Old 07-30-2018, 02:20 PM   #26
gmachinz
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Re: 1973-1991 Dual fuel tank systems theory of operation

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.
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Old 08-01-2018, 11:27 AM   #27
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Re: 1973-1991 Dual fuel tank systems theory of operation

will you please respond to my pm thanks!
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Old 08-01-2018, 01:06 PM   #28
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Re: 1973-1991 Dual fuel tank systems theory of operation

PM sent again.
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Old 08-03-2018, 10:46 AM   #29
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Re: 1973-1991 Dual fuel tank systems theory of operation

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Originally Posted by 808_67C10 View Post
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...
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Old 08-03-2018, 07:12 PM   #30
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Re: 1973-1991 Dual fuel tank systems theory of operation

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Originally Posted by hatzie View Post
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!
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Old 08-07-2018, 06:37 AM   #31
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Re: 1973-1991 Dual fuel tank systems theory of operation

Lots of great info here. Thanks
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Old 08-09-2018, 09:44 PM   #32
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Re: 1973-1991 Dual fuel tank systems theory of operation

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!
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Old 08-10-2018, 12:02 AM   #33
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Re: 1973-1991 Dual fuel tank systems theory of operation

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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.
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Old 08-10-2018, 09:00 AM   #34
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Re: 1973-1991 Dual fuel tank systems theory of operation

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.
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Old 09-23-2018, 11:55 AM   #35
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Re: 1973-1991 Dual fuel tank systems theory of operation

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.
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Old 09-23-2018, 12:03 PM   #36
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Re: 1973-1991 Dual fuel tank systems theory of operation

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.
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Old 09-23-2018, 01:17 PM   #37
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Re: 1973-1991 Dual fuel tank systems theory of operation

Thanks for the clarification. Looking forward to installing your harness when it arrives!
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Old 09-23-2018, 02:09 PM   #38
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Re: 1973-1991 Dual fuel tank systems theory of operation

Quote:
Originally Posted by bigoilman View Post
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.
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2009 Impala SS LS4 V8


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Old 12-26-2018, 01:15 PM   #39
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Re: 1973-1991 Dual fuel tank systems theory of operation

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.
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Old 12-26-2018, 01:53 PM   #40
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Re: 1973-1991 Dual fuel tank systems theory of operation

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.
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1986 M1008 CUCV SOLD
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8


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Old 12-28-2018, 10:57 AM   #41
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Re: 1973-1991 Dual fuel tank systems theory of operation

Thanks for posting. Responding for watching.
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Old 01-03-2019, 06:11 PM   #42
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Re: 1973-1991 Dual fuel tank systems theory of operation

Quote:
Originally Posted by hatzie View Post
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.
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Old 01-03-2019, 06:36 PM   #43
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Re: 1973-1991 Dual fuel tank systems theory of operation

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.
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1969 Dodge Polara 500 B383, A833 SOLD
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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...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; 01-03-2019 at 06:44 PM.
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Old 01-03-2019, 08:48 PM   #44
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Re: 1973-1991 Dual fuel tank systems theory of operation

Quote:
Originally Posted by hatzie View Post
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?
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Old 01-03-2019, 11:34 PM   #45
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Re: 1973-1991 Dual fuel tank systems theory of operation

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.
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1986 M1008 CUCV SOLD
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8


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Old 01-11-2019, 08:12 AM   #46
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Re: 1973-1991 Dual fuel tank systems theory of operation

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 ?
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