Dual tank wiring

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Lonestardale

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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

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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

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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

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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

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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

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

Thanks again!!
 

OldBlueDually

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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.
 

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