Dual Gas tank Wireing?

[Esteban86K5]

On the wiring harness there are 4 wires, blue, green, brown, and a brown w/ a white strip where do they go?

Asking for a friend.

THANKS!!!

 

[Emmettology 101]

Read the instructions!

 

[JeremyK5]

Here's some info i copied out of a couple posts i saw last year.

"The questions about dual tank wiring are so frequent, but I never seem to keep be able to find previous replies.

The wiring varies a little from year to year, but is still easy to figure out.

Switch in the dash - dual pole, dual throw. One half switches the gas gauge from one tank to the other and the other half activates the selenoid for the switching of the fuel lines.

Five wires on the switch - gas gauge, left tank sending unit, right tank senging unit, power (from fuse box) and selenoid (to activate it).
All the tank & gauge wiring and the wire to the selenoid (four wires total) come through the firewall and down under the truck.

Selenoid (under the truck) - some have three ports and only switch the supply line. Some have six ports, and switch both supply and return lines. The selenoid is mounted under the truck, on the bottom of the cab between the fuel tanks.

Gas gauge - uses a tan colored wire that comes through the firewall, across behind the distributor, and down the frame rail to the right side (at least it does on my '75). This wire then comes back up the frame and through the firewall to connect to the dash switch. I know the routing seems very convoluted, but this simplified the factory options for single and dual tanks.

The wires from each sending unit (one tan, one tan/white) come back along the frame rail (along with the selenoid wire) and through the firewall to the dash switch."

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"recently helped my dad install new gas tanks & fuel lines on his '77 GMC. It sounds like you have the same setup on your truck.

The electrical wiring should be as follows:

*The tan wire running down the passenger side of the frame is the sending unit wire. On single tank trucks it plugs directly onto the sending unit. On dual tank trucks it plugs into a corresponding tan wire on the dual tank "extension harness".

*The dual tank "extension harness" has a tan/white stripe wire that plugs onto the passenger side tank sending unit. It has a light blue wire that plugs onto the driver side tank sending unit. And it has a light green wire that plugs onto the solenoid valve.

Those 4 wires (tan, tan/white stripe, light blue, and light green) all run up along the driver side frame rail and go through a plug/socket in the firewall. They connect to the in-dash switch as follows:

*Top left terminal on switch = light blue
*Top right terminal on switch = light green
*Middle left terminal = tan
*Middle right terminal = pink/black stripe (connects to fuse panel "IGN")
*Bottom left terminal = tan/white stripe
*Bottom right terminal = unused


The 3 lines from each tank sending unit are as follows:

Large 3/8" = main feed to fuel pump
Medium 5/16" = tank vent
Small 1/4" = return line from fuel pump

On trucks with heavy duty emissions equipment, the 5/16" lines just have little vent caps (like those found on a rearend) right near the tanks. But on trucks with the charcoal vapor canister(s), those lines run up to the canister(s).

The solenoid valve has a total of 6 fittings. There are two 3/8" inlet fittings that face back towards the tanks and one 3/8" outlet fitting that faces forward. The 3/8" outlet fitting goes to the 3/8" fuel line on the frame rail that leads up to the fuel pump. Each of the 3/8" inlet fittings goes to the 3/8" fitting on each of the fuel tank sending units.

Similarly, there are two 1/4" return line fittings on the back of the solenoid that connect to the corresponding tank 1/4" sending unit fittings. And the 1/4" fitting on the front of the solenoid valve goes to the 1/4" fuel return line on the frame rail that comes back from the fuel pump."

 

[diesel4me]

Someone did a lot of research!

Wow!--what a lot of info that post has!--I pulled my hair out on my 82 K20 diesel with dual tanks,and I had the wiring diagram!(A diesel is slightly different,they have another wire to both sending units for the "water in fuel" light--but similar otherwise,different color wires too)..I found out someone chopped out the fuel selector valve,its switch,and a bunch of hoses!--also noticed the drivers side tank leaks,so I just use the other tank--lucky I was able to get the gauge working!(bad tank has bad sending unit too!)--I dont go too far,one tank is enough!(It would take almost 100 bucks to fill both! )...thats a keeper though,that post with all the dual tank info!

 

[Esteban86K5]

That will hopefully help him out. I appreciate it.

THANKS!!!