How are the factory dual saddle fuel tanks supposed to be routed?

[WranglerTJForum]

This new to me 1973 K10 that I picked up has some janky stuff going on with the dual saddle fuel tanks. From what I can tell it appears that the previous owner installed an aftermarket pump that pumps fuel from one tank to another. There is an aftermarket switch on the dashboard that he labeled, "fuel tank transfer". It doesn't work, but I traced the wiring back and it goes to this pump.

One side of the pump has a hose going into the passenger fuel tank. The other side of the pump has a hose that ties into the drivers side fuel filler vent hose. This seems weird to me, but all I can figure is that when the switch is engaged it is supposed to transfer fuel from one tank to the other.

Anyways, I don't like this setup and I'd like to revert to the way it's supposed to work from the factory. The factory fuel transfer valve is there but as you can see from the photos it's inoperable at the moment.

I am wondering if there is a diagram somewhere that shows how it's supposed to be setup from the factory.

I'm going to assume that there is a fuel pump in each tank (I've got a Holley Sniper EFI setup on the truck) and that both of the pumps are routed to the transfer valve and the switch on the underside of the drivers side dash that says "aux / main" switches the valve from one tank to another. Is that correct?

Does that same switch also engage the fuel sending unit in the currently in-use tank?

Any help is appreciated here. I just want to make this function the way it was supposed to from the factory. Whatever this guy did seems pointless to say the least.

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[Ricko1966]

Aren't you the same guy that wanted to go rear tank? I'm not sure why you wanted to go rear tank,I don't see a big difference in capacity. The way the dual tanks are supposed to work is they are both plumbed to a pollack valve,the proper switch in the dash switches your fuel supply as to whether you are drawing off left tank or right tank. Let's say you are running on left tank and notice you are down to 1/4 tank you can flip the switch the gauge will switch and you will run off the right tank,you watch that tank until it gets low then fill them both up on longer runs. Other wise just use left tank one week,right tank the next so your gas doesn't go stale. Factory 1973s used a single mechanical pump on the engine that drew fuel from the tanks. So you will either need to use ,1987 sending units and appropriate fuel pumps. Or use a single or dual inline electric pumps. If you use in tank pumps you may want to go with 87 tanks too,they had baffles to keep fuel slosh from uncovering the pickup screen. If you stay above 1/4 tank it would probably never be an issue. But a low tank on a big hill or around a fast turn might cause the engine to die from fuel starvation. Sorry I don't have a dighram for you but it's not complicated. Look here scroll across the pics one of them has it all laid out.

https://classicparts.com/1974-95-gas-tank-selector-valve-kit?srsltid=AfmBOoovWOThptGajwvUEXtf_sfPhZVEA2S2MO_xKG0LV84Joargt_Br

 

[WranglerTJForum]

Aren't you the same guy that wanted to go rear tank? I'm not sure why you wanted to go rear tank,I don't see a big difference in capacity. The way the dual tanks are supposed to work is they are both plumbed to a pollack valve,the proper switch in the dash switches your fuel supply as to whether you are drawing off left tank or right tank. Let's say you are running on left tank and notice you are down to 1/4 tank you can flip the switch the gauge will switch and you will run off the right tank,you watch that tank until it gets low then fill them both up on longer runs. Other wise just use left tank one week,right tank the next so your gas doesn't go stale. Factory 1973s used a single mechanical pump on the engine that drew fuel from the tanks. So you will either need to use ,1987 sending units and appropriate fuel pumps. Or use a single or dual inline electric pumps. If you use in tank pumps you may want to go with 87 tanks too,they had baffles to keep fuel slosh from uncovering the pickup screen. If you stay above 1/4 tank it would probably never be an issue. But a low tank on a big hill or around a fast turn might cause the engine to die from fuel starvation. Sorry I don't have a dighram for you but it's not complicated. Look here scroll across the pics one of them has it all laid out.

https://classicparts.com/1974-95-gas-tank-selector-valve-kit?srsltid=AfmBOoovWOThptGajwvUEXtf_sfPhZVEA2S2MO_xKG0LV84Joargt_Br

Yep, that’s me. I decided I’m just going to keep the dual saddle tanks, I just have to figure out how to get the system working.

That setup you linked to mentions it’s for 74 and up. The one that is on my truck now (though it isn’t hooked up) looks exactly like this:

1973-80 Square Body Chevy & GMC Truck Dual Tank Selector Valve, Single Wire, 3 Port without Fuel Return Lines

1973-80 Square Body Chevy & GMC Truck Dual Tank Selector Valve

www.usa1industries.com

I’m assuming this is the right part?

I may go with those baffled tanks at some point but right now I just have to get it to work.

Since the previous owner installed a Holley Sniper EFI he would have had to have changed out the fuel pump in the main tank at the very least. I’m guessing that since the main tank is new and shiny looking that he used an in-tank fuel pump when he purchased that new main tank and likely just left the aux tank alone and unfunctional which is why nothing works.

Sounds like I’m going to have to drop both tanks and see what I’ve got going on here.

 

[Ricko1966]

That is the early style valve if it works use it. If not I'd use the newer stuff that valve requires a constant current to hold the valve in one direction. I suppose that's not a big deal but I'd still go with the new style that just switches not requiring a constant power to hold on.

 

[Grit dog]

But someone else said drop a $500 fuel cell in it and it would basically install itself…….and solve his worldly problems….ffs
Reference the thread above this about the 87 no fuel etc. Then look up fuel routing diagram. Yeah you have a mess that would take 2 pages to explain but isn’t really that difficult.
Do some research and reading and then report back.
Yes what is there is janky af it appears.
PS look before assuming because we can’t look for you.

 

[Grit dog]

Ya, buys Holley Sniper, uses hardware store plumbing fittings (at least galvanized…roflmao) screams **** show.
Be were of what else is a fr o engineered.
If it runs right currently and you don’t age the time money or gumption to pop the bed off and just fix the fuel delivery system properly, I’d just check for leaks and enjoy it.

 

[Grit dog]

PS, your truck looks freakin awesome!

 

[Grit dog]

Also a 1974 would never have had a pump in a tank or an electric fuel pump at all and even an efi suqrebody pump wouldn’t run a sniper and if you can’t determine which pump(s) do what then you need to figure that out first.
And likely chit can the whole mess.
AND no baffles and high pressure efi is also a no no so there’s that.
Start readin. You’re not the first or 1000th person with a high pressure efi aftermarket system. There’s a pile of info on this forum alone.

 

[Broken85]

It sounds to me that the PO didn’t know how to do any wiring and he couldn’t figure out how to switch between two different in tank fuel pumps so he just wired one and kept transferring fuel like an idiot.
There are full dual tank conversion kits that you can buy to switch it back to standard on Amazon, and they are not expensive. Look for a complete Pollack kit. Below is the 80ish-87 system that used a pollack valve Instead of that original for 1973 you have. I attached my 3rd grade wiring diagram too for an 80’s truck to wire two intank pumps to the Holley sniper. You can do it with one relay, but I used two for redundancy. I have seen other guys skip the relays and wire power through the switch, I’m sure it works fine but I am not a fan of pulling power through a switch if I can avoid it. Your wires may be different colors I think, I don’t know, but hopefully this gets you started. Use a set of Holley 12-308 in tank pumps $$$, you won’t need to run the return line.

If you want it to look stock, Use a tank switch from an 87 that is constant. All other years are only momentary. They are like $10 on LMC.

BTW, ripping all this out and making it right and staying with the saddle tanks is better than deleting a spare tire and messing with a burb tank. You will be happier in the end to know you did it the right way. If you need more help, report back and we will get you straightened out.

 

[Broken85]

That is the early style valve if it works use it. If not I'd use the newer stuff that valve requires a constant current to hold the valve in one direction. I suppose that's not a big deal but I'd still go with the new style that just switches not requiring a constant power to hold on.

GM never used two different Pollack valves. The pollack valves on the 87’ and the pre-87’ are the same and can handle constant power when switch or momentary power. I used this as a trigger wire for the relay to activate the pump in the correct side. I am not sure if the pre 80’ can handle constant voltage, but maybe.

 

[AuroraGirl]

GM never used two different Pollack valves. The pollack valves on the 87’ and the pre-87’ are the same and can handle constant power when switch or momentary power. I used this as a trigger wire for the relay to activate the pump in the correct side. I am not sure if the pre 80’ can handle constant voltage, but maybe.

the pre 1980 switches?

The 1980 only differs because the 1 wire valve had a module which didnt actuate it everytime you turned the key if the thing just operated recently, so think key on, crank, then no start, you try to crank again and the module prevents the solenoid from actuating again if you went all the way to key off

It was an attempt to extend the life of the solenoid which was notably short lived in the 70s and early 80s

 

[WranglerTJForum]

It sounds to me that the PO didn’t know how to do any wiring and he couldn’t figure out how to switch between two different in tank fuel pumps so he just wired one and kept transferring fuel like an idiot.
There are full dual tank conversion kits that you can buy to switch it back to standard on Amazon, and they are not expensive. Look for a complete Pollack kit. Below is the 80ish-87 system that used a pollack valve Instead of that original for 1973 you have. I attached my 3rd grade wiring diagram too for an 80’s truck to wire two intank pumps to the Holley sniper. You can do it with one relay, but I used two for redundancy. I have seen other guys skip the relays and wire power through the switch, I’m sure it works fine but I am not a fan of pulling power through a switch if I can avoid it. Your wires may be different colors I think, I don’t know, but hopefully this gets you started. Use a set of Holley 12-308 in tank pumps $$$, you won’t need to run the return line.

If you want it to look stock, Use a tank switch from an 87 that is constant. All other years are only momentary. They are like $10 on LMC.

BTW, ripping all this out and making it right and staying with the saddle tanks is better than deleting a spare tire and messing with a burb tank. You will be happier in the end to know you did it the right way. If you need more help, report back and we will get you straightened out.

Thank you, this is tremendously helpful.

Looking at it more last night, I can see that it's a fairly simple system. However, this is my first square body and I've also never owned a vehicle with a dual fuel tank setup. Hell, I didn't even know such a think existed until now. I've been tracing wires and lines all over so I know what is what. Those diagrams you made clear up the confusion.

I agree with you though. After rethinking the whole single fuel tank conversion I think it will just be easier to get these dual fuel tanks working correctly.

I ordered a set of Holley 12-308 in-tank pumps last night.

Before I purchase it, I assume this is the Pollack kit you're referring to?

Amazon.com: Pollak 42-308P 3 Port Fuel Selector Valve Kit : Automotive

Buy Pollak 42-308P 3 Port Fuel Selector Valve Kit: Selector Valves - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

That one appears to be a 3-port unit. This one here has 6-ports I believe.

https://classicparts.com/1974-95-gas-tank-selector-valve-kit?srsltid=AfmBOorZ2fWRRzVlnI0MHFGv15e7XA8tk2YWEbihF5Mgul_NjGa2jpwx

I'm going to assume the 3-port one is the one I need since I wouldn't need return lines if I'm running the Holley 12-308 pumps with my Sniper.

I'm also going to assume that the switch in that comes with that Pollack valve I linked to is not the one I want and instead I want this one?

1987 Square Body Chevy & GMC Truck Dual Gas Tank Selector Switch

1987 Square Body Chevy & GMC Truck Dual Gas Tank Selector Switch

www.usa1industries.com

My dash doesn't have a cutout for that switch on it. Instead, my dash has this switch:

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That switch is mounted right under the steering column on the left side.

Can that switch be used instead of the one lined to above? You know, just to keep a factory appearance and avoid cutting a hole?

Sorry for all the questions. To start with I just want to make sure I save myself a hassle by ordering the correct parts the first time.

 

[AuroraGirl]

Thank you, this is tremendously helpful.

Looking at it more last night, I can see that it's a fairly simple system. However, this is my first square body and I've also never owned a vehicle with a dual fuel tank setup. Hell, I didn't even know such a think existed until now. I've been tracing wires and lines all over so I know what is what. Those diagrams you made clear up the confusion.

I agree with you though. After rethinking the whole single fuel tank conversion I think it will just be easier to get these dual fuel tanks working correctly.

I ordered a set of Holley 12-308 in-tank pumps last night.

Before I purchase it, I assume this is the Pollack kit you're referring to?

Amazon.com: Pollak 42-308P 3 Port Fuel Selector Valve Kit : Automotive

Buy Pollak 42-308P 3 Port Fuel Selector Valve Kit: Selector Valves - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

I'm going to assume that the switch in that is not the one I want and instead I want this one?

1987 Square Body Chevy & GMC Truck Dual Gas Tank Selector Switch

1987 Square Body Chevy & GMC Truck Dual Gas Tank Selector Switch

www.usa1industries.com

My dash doesn't have a cutout for that switch on it. Instead, my dash has this switch:

You must be registered for see images attach

That switch is mounted right under the steering column on the left side.

Can that switch be used instead of the one lined to above? You know, just to keep a factory appearance and avoid cutting a hole?

Can you verify if your tanks are both saddle tanks or if there is any aftermarket tanks?

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Heres an example of that brand in a 1971

my dad has a 76 or 78 ( i forget off the top of my mind) with an auxillary tank , not GM original but heres what it looks like , has no sending unit

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@Grit dog also has a 70s truck with an aftermarket. I only bring this up because if youre not dealing with 2 GM saddle tanks, you should definitely go to one so you can simplify the work and not have a rusty tank

 

[AuroraGirl]

Can that switch be used instead of the one lined to above? You know, just to keep a factory appearance and avoid cutting a hole?

for your purpose, i would only use the 87 switch if youre using the pollak valve and using baffled tanks for the in tank pumps

The switch above is for switching the sending unit most likely

 

[idahovette]

That switch shown in post #13 is exactly like the one was in my 75 K20 with the added on left tank. Controlled everything and worked beautifully for 30 years..... til I removed it.......bad idea.....you can ALWAYS use fuel in a Square