needing help with dual tanks please


Junior Member
alright here it goes. after more than a few hour of hair pulling i need some help desperately. i have a 78 chevy c20 4wd. when it recieved a heart transplant, i was unhooking all the wires and moving the harness out of the way. now as everyone knows there is a metal tube near the starter that all the wires that hook up to the solonoid, go through. i remove the wires from the solonoid and had to cut a couple to get them trough the tube. now i know that the positive cable from the battery and the power cable going to the cab hook up on the top post( looking up at starter from underneath ) and the wire that comes from the ignition switch that engages the starter goes on the little post on the right. i have a wire coming from the tanks or the switching valve. and i also have a wire coming from theharness going to the cab, these go where is my big question. the switching valve and the fuel gauge worked fine before i messed with it. lol!! and it tottally driving me nuts trying to figure this out. maybe i am missing the simple picture here i dunno. i have a factory gm service manual and a hynes manual and both only have wiring diagrams for single tank trucks. any help is greatly appriciated. thanks


Senior Member
Fairbanks AK
So now the tanks are batty? Hmmm, I know on my 83, the wiring for the selector valve / fuel level runs down the driver's side of the truck and crosses over between the bed and the cab on the cross member there. does your starter work? I'll sleep on it and hopefully come up with something tomorrow. Just used up all of my wiring patience getting all my autometers to dim with the stock gauges, and putting a new spedo cable in.....



there should be 2 wires going to the tank selenoid I believe that they run side by side on some and one down one frame rail one down the other there is a year split some where
anyway start at the selenoid and go forward If memory is correct I think the guage is a pink wire


Junior Member
thanks for the help so far

this place is great as well as all who participate too.

Boss, thanks for the welcome:D

Mtck, hey i iknow all about gauges fun!! i changed the originals out with sunpros in their place, in correct locations, volt, oil press, and temp, what a job . doing the headstand on the floorboards under the dash is quite fun.:p

Staley52, your user name makes me wonder.... we have a fatory here called AE staley and it sits on U.S. 52... two of them actually. maybe just a coincedence. but anyway, i broke out the flashlight tonite and looked with out having the frayed mind set. from the right tank is a wire that travels to the starter on the right frame rail. this wire may have once been pink( hard to tell now) after the spit shine i gave it it looks kinda orange-pink, so it must be the one, also the other unknown wire actually looks to be the same so i might hook them together and see what happens. like i said before i might be over looking the simple picture. i only have one wire coming from the switching valve itself and it does cross over the cross- member at front of the bed. and travel foward. that is as far as i went with it , since all i did was mess it up at the starter i hope:rolleyes: thanks again to everyone and keep them coming . i will get back with more info and pics of the rig


Junior Member
New to the site .Question for a Chevy Gruroo'S

I have a 75 Chevy short bed p/u bed with a 79 cab and since i made the conversion ,I have had to relocate all the wires,gas switching and sending units (2) are there but the new cab the 79 was only set up for single tank ,no biggie ,but I know to the gauges the wire is brown ,but I have (4) wires coming up from the dual tank system.FUN SO FAR?
I think when 12 volts is applied to a green wire? I hear the fuel switch on the frame rail wok but what is the wiring for the switch and fuel gauges ?
I knew I should have kept the dual tank switch from the 75 cab but I rolled the truck and the dash was a ***** to get into ,
Anyone got info on the wiring layout for the other (3) wire's?????
email me please before the snow flies and i need to keep on doing the mileage check thing all freaking winter !!!!!!!!!!!!!!!!


All you really need, is a double pole, double throw(6 posts) switch, and some wire.

One side of the switch: Run one wire from each tank sender wire to each side of the switch, then the center post of the switch to the dash gauge wire.

Other side of the switch: Find a place to draw power, that is hot only when the key is ON. Run the hot wire wire, to the center post, then from one side to valve itself.
Fill one tank, leave the other lower. Check to see if the gauge reads from the correct tank. If not, move the valve wire, to the other side post.

Mount the switch so that it throws side-side, instead of up-down. Then you won't need a marker to tell which tank you are using.

*Note* Be sure your chassis to body ground is attached, or it could give strange readings.
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Here's a schematic that may help. Sorry, I don't remember what years it applies to ...




Keep in mind that there are two different configurations out there. Unfortunately I don't know when GM made the switch. My 74 truck was wired such that the gas tank sender unit was selected by the dashboard-mounted tank selector switch independently of the solenoid valve. The result is that if the solenoid valve failed, the gas gauge could read the full tank while the valve was still stuck on the empty tank. Yes, this happened to me.

Because of this, GM changed the design of the solenoid valve so that the sending unit wiring also runs through the valve. This way, the gas gauge always reads the tank that's selected in the valve. If the valve fails, you may be stuck with a full tank that you can't use but at least your gas gauge will let you know that the tank you're connected to is empty.

The wiring for these two configurations is obviously different, but easy to determine. If the wires from the two sending units go to the solenoid valve on the frame you have the later configuration. If they go to the switch you have the early configuration.

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