78 K10 single to dual tank

iamtherealJayy

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So as a few of you know I’ve had a 1978 Chevrolet K10 bonanza for a little over a year now, I’ve just recently started driving it after restoring most of it. The truck is a work truck so it has a utility bed on it that has a filler on each side, the truck is originally a passenger side fill. What is the easiest way of adding a driver side tank? Buy another tank, pickup unit, hoses, and what else? It would be awesome to somehow make it full from drivers side only but I could live with having to fill independent each side. I’ve read a few forums about people having fittings welded to their gas tank etc, but I live in a smaller town where people don’t really do that stuff. If anyone could point me in the correct direction that would be much appreciated. I have seen that the passenger tank you can just swap to the drivers side but the pickup would be backwards, I’m currently building a filler neck for the passenger side out of 1-1/2” flexible fuel hose. My only question with if they’re independent of each other how could I know the fuel level in each tank since the truck is original single tank so there’s no switches to swap from tank to tank. Could I just wire in a three way toggle switch to swap the input wire from each tank between the gauge? And do they make some sort of valve to swap which tank is flowing fuel to the front?
 

dvdswan

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You will need:

switching valve preferably the 3-port with single wire connection.
wire for the fuel level
dash switch
not really needed but you could buy/find the crossover pipe/tube

as you mentioned:
tank
tank mounts
tank straps
fuel sender
hose - IIRC it's 3/8"

If you look in the reference library under 78 wiring diagrams you will find the aux tank wiring diagram and see that it is super simple.

If I missed anything I'm sure someone will note it.
 

iamtherealJayy

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Is there a way of doing it other than factory? I don’t know that I’d be able to find affordable factory style parts
 

Grit dog

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To make it switch like original, you’d need all the factory stuff. Cab switch, switching valve, OE type fuel pickup and sender.
Honestly, it’s not “that much” more $.
Switch is like $15-20
Valve is $30 for universal or $60-80 for OE.
New tank approx $100.
Fuel pickup/sender $40

Hoses and wire etc is the same either way.
You need the tank and sender for sure
$50-100 gets you the switching valve and switch.
Vs $30-40 for a universal switching valve and $10/free for a toggle switch.
 

iamtherealJayy

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If I did it the “correct/factory” way would I be able to mount the switch wherever? Or would I have to mount it where it’s generally mounted? I don’t want to cut anymore holes than this dash already has, I’d love to put it in one of the existing holes somewhere, it is just a work truck, but it would be a hell of a lot nicer to have 40ga capacity instead of 20. Refilling would suck but we’ll cross that bridge when we get there. Fuel here is at 4.39 a gallon currently and rising 10 cents every couple days. Where can I buy the proper valves etc? Lmc or brothers have either of those? Tank and straps should be easy to find and 3/8” fuel hose is easy. I’ve never looked into the dual tank wiring valves switches etc though.
Also can’t forget new straps for the tank! Lol
 

fast 99

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It is fairly easy to do. I spliced into the harness just behind the cluster for the gauge. Attached those wires to the center and one side of a 6 pin change over switch. GM did it the same way just much cleaner. Used one side of the switch for the single wire to the changeover valve, other side for the gauge. Mounted the switch in the lower left corner of the bezel. If it's a 78 shouldn't have a return line from the pump. If it does a 6 port change over valve [not 3 port] will be required. I mounted the valve on the crossmember below the front of the bed on the right. Then run lines from the tanks to the valve and valve to pump. Will need to run a wire from the new tank sender to the dash switch.
 

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iamtherealJayy

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Pump does not have a return, I’m not sure if I’m imagining the wiring correctly or not but could I put the switch like lower corner of bezel like yours but have the two sender wires come to somewhere other than just before the gauge? I don’t know that I understood it correctly but I’d like single wire going to gauge and the switch change just what the input wire is kinda like toggle left is drivers side tank flip switch and the gauge swaps to passenger side? And is the valve for fuel flow electric somehow?
 

Juggernaut

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I have a factory harness, factory switch, and a brand new 3 port valve if you're interested. The valve has 5/16 hose fittings, but that shouldn't be a problem on a near stock work truck. Came out of a 78 K20
 

fast 99

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On the later trucks like the one I pictured [81] the original GM parts also changed over the gauge within the valve. 1 circuit ran to the cluster from the valve. My truck was modified like you want to do. There is an offer for all the factory parts from the same year to adapt it. I would get those parts then decide what is the best/easiest way to do it. Usually, factory parts will be better quality.
 

iamtherealJayy

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I’d like to do it as simple as possible, just a second tank I can toggle fuel from, but I’d also like the fuel level to be shown on the gauge too. Would having a 3 toggle switch allow you to have the wire from one sender going in at position 1, then having the other tank be position 3? Would that turn off input from one tank and allow input from second tank? Or is it more complex than I’m hoping? I’d like primary to be drivers side and passenger to become the “extra” even though truck is factory passenger side only. Or supposedly, had a dual tank bed on it when I got it.
 

SquareRoot

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Is there a way of doing it other than factory? I don’t know that I’d be able to find affordable factory style parts
Why? I've done it. Rock auto has everything you need.
 

iamtherealJayy

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I’ll write up a build list and pricing and weigh my options on it, this isn’t a needs to be done right now kinda thing. I haven’t even burned a full tank of fuel yet. I’ll post on here everythint needed and general pricing after I get everything gathered up.
 

iamtherealJayy

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Finally got time to make up a list
Tank~80-100
Straps~150
Sender~40
Switch, valve, pigtail harness~200
Misc(hoses clamps) ~50
Possibly forgetting small things but the majority of what you’d need is here. So roughly $400-$500 depending where you get your parts. 20 gallon tanks range from 80 to 100 so you could in theory do it cheaper I just went with averages.
 

fast 99

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Is there a pick and pull near you? Way cheaper there. 6 bolts a couple screws, a wire cutter and it's yours.
 

75gmck25

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I added a 2nd tank to my K25 and using pick and pull parts it was about $75-$100. I then bought a new sending unit from Autozone, since the old one was rusty inside and gave flaky readings.

Another option is to use a 40 gallon Suburban tank mounted in the spare tire area, and put the filler up inside the bed corner. There are at least a couple internet articles on how to do it. RockAuto currently has the Spectra brand 40 gallon tank on close out for $90.
 

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