Dual tank wiring

[Lonestardale]

Why is the tan fuel gauge wire that runs up the right frame rail routed so long? Its gotta be near 20 feet of wire! 1976 C20 dual tanks. It runs from the firewall down the left frame rail then across to the right frame rail, then forward to the starter heat guard tube, then back behind the engine at the firewall and into the fuseblock. All to go from the switch to the gauge.

 

[75gmck25]

I have similar wiring on my '75, and I think it is set up that way to facilitate dealer-installed dual tanks, or to make it easier to add a factory aux tank.

In this timeframe, if you have a single tank it would be on the passenger side, and there would be a tan wire running from the gauge, across behind the engine and down through the metal tube and the passenger side frame rail to the "main" tank. No other wiring is needed.

When they add a 2nd tank they add a wiring loom that runs from the DPDT dash switch, through a plug in the firewall and down the driver's side frame rail This loom has wires for solenoid power, left side sending wire (tan/white), right side sending wire (tan), and connection to the gauge (despite the fact that the dash gauge is only about a foot away from the switch location).

- After running it down the frame rail, at the driver's side tank the tan/white wire comes out of the loom and connects to the sending unit. The other wires continue across under the truck, supported by the bracket for the fuel lines.
- At the passenger side tank they pop apart the original wire connection to the sending unit and connect two wires in the switch so that the switch is now "in between" the tank sending unit wire and the sending unit itself (switch now connected to the passenger side tank and to the gauge wire).
- They also connect the solenoid power wire wire to the solenoid (12 volts can be applied to the solenoid).

The switch on the dash is a dual pole, dual throw switch, so it can switch two things between two sets of wires. It switches the sending unit wire, and also controls power to the solenoid.
- The first half of the switch has the center pole connected to the wire coming from the dash gauge. Remember that from the dash this wire runs all the way down the passenger side rail (stock on all trucks) , but is then extended with the wiring loom so it run across under the truck, then back up the driver's side rail to the switch. This is a really long wire.
The two side poles on this half of the switch have one side connected to the sending unit wire from the left tank and one side connected to the right tank. The switch just flips the sending unit/gauge connection from one tank to the other.
- The other half of the switch has the center pole connected to power from the fuse box, and one side connected to the wire that powers the solenoid. When switched to the aux side it provides power to the solenoid, and on the main position it is just open (no power).

This is not an easy wiring scheme to explain with words, and a simple diagram would probably make more sense. However, the GM wiring diagram does not have any overlay of the truck body itself, so even with the diagram the length of these wires and actual location is not obvious.

Bruce

 

[OldBlueDually]

This is my next project to tackle on my 76 GMC crew cab with dual tanks.

75gmc, I see you said the wiring diagram kind of sucks from GM, but do you have one to post, or does anyone for that matter? I found some diagrams for the fuel/return/vapor line which makes sense to me, but I don't "think" with my Olds 455 I will be running a return, nor the vapor line.

However, this trucks wiring has been sooooo cobbled up and modified it makes me sick, so I just spent this last week fixing a huge mess under the hood. The dual tanks are next and I want to be sure I know what I am doing before digging into it!

That tan wire you mention on mine was cut under the hood just hanging there. The remainder was cut on the passenger side frame just under the passenger door is where I found the end of it taped up. The person mounted a square electric fuel pump to the outside of the passenger frame and from there forward removed the fuel line that was originally on the truck. I plan (or should I say hope) to have this fuel system wired to original and hope it all works.

Any diagrams would be great!

 

[dvdswan]

76 aux tank wiring diagram.

Just remember there are 2 Tan wires at the DPDT switch. One for the gauge and one from the main tank (pass. side). Fuel gage wire is in the center position (pin 4) opposite of the pink/black (power (pin 1)). Green wire for the solenoid (pin 2) is above pink/black and the 2 tanks are above/below the fuel gauge wire.

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[Dave M]

This is my next project to tackle on my 76 GMC crew cab with dual tanks.

75gmc, I see you said the wiring diagram kind of sucks from GM, but do you have one to post, or does anyone for that matter? I found some diagrams for the fuel/return/vapor line which makes sense to me, but I don't "think" with my Olds 455 I will be running a return, nor the vapor line.

However, this trucks wiring has been sooooo cobbled up and modified it makes me sick, so I just spent this last week fixing a huge mess under the hood. The dual tanks are next and I want to be sure I know what I am doing before digging into it!

That tan wire you mention on mine was cut under the hood just hanging there. The remainder was cut on the passenger side frame just under the passenger door is where I found the end of it taped up. The person mounted a square electric fuel pump to the outside of the passenger frame and from there forward removed the fuel line that was originally on the truck. I plan (or should I say hope) to have this fuel system wired to original and hope it all works.

Any diagrams would be great!

I had to start from scratch with mine, you can check it out here.
https://www.gmsquarebody.com/threads/fitting-of-twin-gas-tanks.28968/#post-561334

 

[OldBlueDually]

Thanks guys!!!!! I will have to do some studying on what you both posted.

Thanks again!!

 

[OldBlueDually]

Thanks again for the info, I was able to verify the harness was correct and not chopped up. Tan wire on pass side I need to redo. I hooked up my meter to the tan wire end and it had a signal from the wire cut under the hood!

Not sure if my tank solenoid is any good ..that's next.

 

[OldBlueDually]

Gents---need some help here. My fuel gauge has NEVER worked, and I am working on it now. Turns out I had that long@s$ tan wire on the passengers side frame rail terminated wrong. I want to fix this first before proceeding if it doesn't fix the fuel guage.

Simple question: Where in the hell is that long passengers side frame rail tan wire supposed to go on the firewall!! To another lonely non-existent wire, or to the fuse block, or??

My round plug coming out of the firewall has tan, tan/white, green, and blue--This goes down the drivers frame rail, blue stops at drivers fuel tank, remainder goes across..blah blah blah...it is that freaking tan wire with the little boot on my passengers side frame rail. I ran a new tan wire up the passengers side, across my firewall and now it is connected to nothing and has nowhere to connect.

My fuel selector seems to work. Before I drove the truck it had old fuel in each tank. I hooked an electric pump to the line up front, turned it on and pumped fuel out from one tank and when it ran dry I flipped the selector switch and it pulled fuel out of the other tank--so the switch works.

This is frustrating, not matter which schematics I look at, nothing makes sense to where that tan wire is supposed to go. Even on my parts truck they re-wired that so it does't make sense either (1/2 ton 76 GMC with dual tanks).

I've looked here;

Complete 73-87 Wiring Diagrams

Complete 73-87 Wiring Diagrams

forum.73-87chevytrucks.com

I have also looked at these here;

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[dragstews]

Simpler days back in the early 50's ...
Only for the fuel gauge ...

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[OldBlueDually]

Well, nevermind my above post about having it terminated incorrect. I remembered that if I take that connection and ground it out, my gauge should go to E, sure enough it did. So that tells me either I have a bad sending unit, or a bad ground. I will assume a ground.

Of course the ground is in the most accessible spot.....in between the fricken frame and cab/box....so real easy to get to Will have to try to get the grounds cleaned up now. Then if that doesn't fix it, on to the next.

 

[OldBlueDually]

Well, I'm 50% there

Did more work on this. Pass side ground was bad, terminal fell off, so I terminated a wire to existing with new terminal, cleaned frame, used star washer and reattached. Drivers side ground was loose, so took it all apart, cleaned all surfaces and terminal, used a star washer with new fastener and reattached it.

Turned on the key and crossed my fingers--pass side still goes past full. Drivers side works. I also hear the fuel selector solenoid engage (light thunk kinda sound).

I reached up to pass side tank and felt around to find the main wire to sending unit (tan/white), and using a long pry bar with my other hand I was able to carefully jiggle the connector off the top of the sending unit, and plugged it back in. That tank still goes past full. So I then cleaned a spot in the seam/web on the pass tank to shiny bare metal, same with a spot on my frame and grounded it that way, and it still goes past full.

Wondering if maybe I've got a faulty sending unit??

 

[75gmck25]

How long since that passenger tank has been used routinely?

The sending unit resistance sweep is a very simple mechanism, with a piece of copper bar/wire that sweeps up and down another piece of metal with varying resistance. When there is no gas in the tank most of the metal sweep mechanism is exposed to air, and it can get quite corroded. Its also possible that the sweep is broken or the wire (inside the tank) running to it is disconnected.

If the tank has only have a few gallons in it, I have been able to drop each tank without too much trouble. The bolts that hold the brackets are relatively easy to reach, and the screws for the tank filler are also easy to remove from the bed side. The challenge is the rubber lines running to the sending unit may not be long enough to get much drop of the tank, and the old rubber lines are usually in bad shape. The last time I worked on mine I just replaced all the rubber lines at the same, since they all had serious cracking on the surface, and one of the vent lines was broken open.

I used a piece of 2x8 and a floor jack to hold up the tank while I unbolted it, and then gradually lowered the jack.

 

[OldBlueDually]

How long since that passenger tank has been used routinely?

The sending unit resistance sweep is a very simple mechanism, with a piece of copper bar/wire that sweeps up and down another piece of metal with varying resistance. When there is no gas in the tank most of the metal sweep mechanism is exposed to air, and it can get quite corroded. Its also possible that the sweep is broken or the wire (inside the tank) running to it is disconnected.

If the tank has only have a few gallons in it, I have been able to drop each tank without too much trouble. The bolts that hold the brackets are relatively easy to reach, and the screws for the tank filler are also easy to remove from the bed side. The challenge is the rubber lines running to the sending unit may not be long enough to get much drop of the tank, and the old rubber lines are usually in bad shape. The last time I worked on mine I just replaced all the rubber lines at the same, since they all had serious cracking on the surface, and one of the vent lines was broken open.

I used a piece of 2x8 and a floor jack to hold up the tank while I unbolted it, and then gradually lowered the jack.

I've actually been using the passengers side tank constantly since the truck has been on the road, I've never put fuel into the drivers side.

You're right, these lines are a bit firm but look to have been replaced ages ago. Hose clamps are accessible to disconnect from solenoid/selector, but I'd probably just cut them and put on new. I did see the 4 bolts per tank saddle on the inside of the frame.

So it is probably corroded or a broken internal wire then?? Sounds like it works on the same principal as my old remotes on my slot car race track sets

Thanks for the info and advice! Sucks I recently filled up that tank too...