Trying to fix up dual tank fuel system

[Dleslie212]

I bought a 77 K10 a few weeks ago, and decided I want to start fixing it up slowly. The main issue I've been having is a fuel delivery problem, which I think I've solved with a new fuel pump. The fuel gauge does not work, and I think that's the next issue I want to tackle.

The truck has dual tanks with a selector switch in the dash to switch between tanks, and I don't get a fuel level reading for either tank. The PO said that the passenger side tank hasn't worked in forever, it's been run only on the driver side tank for years. This morning, I went ahead and dropped the passenger side tank and removed the sending unit. I'm planning on going ahead and picking up two new sending units, and a new fuel selector valve and fixing the junk wiring for them.

I know there are two different fuel systems - one with a return line, and one without. When I replaced the fuel pump, the one I removed had only an inlet and an outlet, no third port for a return line. BUT, the truck has two fuel lines running from the engine back to the fuel selector valve, but only the inlet was hooked up. The second line was just disconnected and hanging there Also, I found an old fuel pump in the back of the truck that DID have the return line port. So when I replaced the pump a few days ago, I replaced it with a pump that had the return, and hooked the return line up. Everything is running for the most part.

The fuel selector vavle has six ports - I'm assuming that is pressure and return from drivers tank, pressure and return from passenger tank, and then pressure and return from the engine. Can anybody tell me if that is correct? The sending unit i removed from the passenger tank has three ports, but had one of them plugged off with a plastic bolt. Can anybody tell me if I'm right in thinking that one port is vent, one is pressure and one is return?

Next is the fuel selector valve. I've looked on LMC Truck, and they make either a 3 port or 6 port valve. 3 port would be for no returns, and 6 would be for returns? Also, from what I've read, the selector valve defaults to passenger side tank in the absence of 12v power, and switches to drivers tank when it DOES get 12v. The valve on my selector valve has only 1 wire going to it, and it grounds to the frame through the valve body. Does this sound correct? If there's only 1 wire, how would the fuel gauge be able to tell the fuel level in two different tanks?

 

[Charlie]

 

[Rickf]

Before you buy new sending units, put an ohm meter and test them. As the float moves up & down the reading should change.

 

[Dleslie212]

Before you buy new sending units, put an ohm meter and test them. As the float moves up & down the reading should change.

I appreciate the advice, but I'm just going to buy new sending units anyway. The one I pulled out already must have been original - the "filter sock" part is just shredded and the whole thing looks like it's in rough shape. I'm just going to buy two new units and the new valve, that way I know they're good. I'm just worried about getting the units and valves with the correct number of ports

 

[dvdswan]

You are correct on the 6 port being one side is supply and one side is return. If you have the means, if you are doing one tank at a time, blow out the lines to make sure there is no crude in them. I ran into this issue giving me intermittent fuel supply problems (I didn't have the return on mine).

The one wire is correct as well. Fuel injected models will have 5 wires IIRC because of the fuel pumps in the tanks. You can test the selector valve by grounding it and running a 12 v source to the post and you should hear it click.

The switch on the dash has 5 wires IIRC, one from each tank for the gauge, one to the gauge, one to the switching valve, and power.

I don't recall which is the primary tank but it sounds correct about the pass. side.

Welcome to the site BTW.

 

[Dleslie212]

The switch on the dash has 5 wires IIRC, one from each tank for the gauge, one to the gauge, one to the switching valve, and power.

So then I should be seeing a single wire from each sending unit hding towards the cab to tie into the selector switch, and from there they head into the gauge?

Thanks for the welcome!

 

[Remo76C15]

I appreciate the advice, but I'm just going to buy new sending units anyway. The one I pulled out already must have been original - the "filter sock" part is just shredded and the whole thing looks like it's in rough shape. I'm just going to buy two new units and the new valve, that way I know they're good. I'm just worried about getting the units and valves with the correct number of ports

Good call to replace.

 

[dvdswan]

So then I should be seeing a single wire from each sending unit hding towards the cab to tie into the selector switch, and from there they head into the gauge?

Thanks for the welcome!

One wire from the fuel gauge to tank selector switch, one wire from each fuel tank to tank selector switch, one wire from switching valve to tank selector switch, and power.

Look at the wiring diagram for your year here... https://www.gmsquarebody.com/threads/1973-1991-wiring-manuals.14498/

It will be under aux tank near the bottom.

 

[Dleslie212]

One wire from the fuel gauge to tank selector switch, one wire from each fuel tank to tank selector switch, one wire from switching valve to tank selector switch, and power.

Look at the wiring diagram for your year here... https://www.gmsquarebody.com/threads/1973-1991-wiring-manuals.14498/

It will be under aux tank near the bottom.

Thank you. Unfortunately, I can't read a wiring diagram to save my life. It may as well be Chinese.

I think I have everything figured out except the switching valve. I wanted to replace it, but it seem that the six port one wire valves are impossible to find anymore

 

[dvdswan]

Walmart shows it (LOL) - https://www.walmart.com/ip/ACDelco-...MI3Nju19-15AIVCKrsCh18gA-9EAQYAyABEgJQmvD_BwE

You can use the 5 wire one...

You must be registered for see images attach

Or this...

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[Jason Reed]

Very cool to post this.

 

[75gmck25]

I believe the dual tank plumbing for my '75 is the same as your '77. Passenger tank is the primary in early years (GM later switched to using driver's side as primary)

Solenoid under the truck only needs one wire because it is already grounded to the frame, and the single post is just 12 volts on/off. When "off" it defaults to passenger side, and when 12 volts is applied it switches to driver's side.

Switch under the dash controls switching of sending unit wires. The wiring path is a little screwy because it was designed to allow for a factory or dealer-installed 2nd tank. With a single tank the sending unit wire from gauge goes through the dash into engine compartment and across the truck firewall to the passenger side frame rail and down to the tank. Near the tank there is a pop-apart wiring connector in the tan colored wire, inside the frame rail so you can disconnect it and drop the tank.

When dual tanks are added they first add the dash switch.
- Dash switch is a dual pole dual throw (six terminals) and five terminals are connected.
- One half of the dash switch has the center terminal connected to 12 volts from the fuse box. One side of the switch is open (no connection) and the other side is the twelve volt line to the solenoid. All it does is apply 12 volts to the solenoid when in the aux position.
- Other half of switch is connected to the dash gauge wire in the center, and the two other poles are aux vs. main tanks sending unit. It switches the dash gauge between sending units.

Then you get to the complicated wiring path for the sending unit wires.
- The wiring runs from the dash switch over to the firewall and into the engine compartment, and then down the driver's side frame rail to the driver's side tank.
- At the driver's side tank one wire "drops off" and connects to the aux sending unit (tan/white).
- The other wires cross under the truck with the metal fuel lines running to the passenger side tank.
- Near the passenger side tank these are the connections.
-- Power wire from the switch connects to the solenoid
-- Passenger side tank sending unit wire (tan) connects to sending unit (remember the pop-apart connector)
-- Wire from switch to dash gauge connects to the gauge wire (remember, the wire from the gauge already runs from the dash down that frame rail?). I think that wire is also tan.


Bruce

 

[Dleslie212]

I did find a few places online that show a picture of the correct single wire, six port fuel selector valve, but in every case, they listed the incorrect part number (the same is true for the WalMart link above, just take a look at the reviews)

Unfortunately, before I even got a chance to test my single wire, six port selector valve, I broke off one of the plastic nipples like a dummy when I was trying to remove one of the fuel hoses.

So a week or two later, here is where I am at. I dropped both tanks and replaced both sending units. The driver side sending unit ground was not even connected to the sending unit anymore. The tubing is all new, and tubing, grounds, sending unit wires and power wire are hanging there, waiting to be connected to a new fuel selector valve. I also removed the instrument cluster so I could check all the gauges for proper operation - they all seem to be working correctly when hooked to 12v and ground using some alligator clip extension deals i made that hook to the back of the gauge, and then to the battery. The gauges actually all look in great condition - almost like brand new. I'm wondering if somebody replaced them all at some point? Unfortunate, same cannot be said for the plastic housing and the printed circuit board. The PO cut a big hung out of the plastic housing and the lens in order to run two new wires to a ghetto ass horn button he installed. Anyway, just seeing the fuel gauge move by itself after hooking up those alligator clips was awesome - at least I know that works now. The circuit board has a small little chunk that looks like its missing.

I ordered a new fuel selector valve, pigtail and switch. Here are the part numbers

Fuel selector valve - AC Delco U7000
https://www.amazon.com/gp/product/B0013FY5KW/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

Selector valve pigtail - WVE 1P1132
https://www.amazon.com/gp/product/B00E3IE5U6/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1


Selector Switch - AC Delco D7003
https://www.amazon.com/gp/product/B000C9MN2C/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

After looking at the diagram dvdswan posted above, I am pretty sure I bought the incorrect switch. My switch has only four ports on the back, and it looks like it should have six. I am pretty positive I read a post here a few weeks ago from zambonie (?) about converting from a six port one wire to a new style, so I'm going to keep searching for that.

In the meantime, if anybody can advise on a part number for the correct switch? Id prefer to keep a stock looking switch instead of using one like shown in the pictures above. Thanks all for the help!

 

[Dleslie212]

Bruce -

Thank you for the response. Just reading about the path your wiring takes makes me think mine has been messed with. Both of my fuel sending wires run up the driver side frame rail. Also, each wire has probably three or four splices in it, that are just twisted together and exposed. No wire nut, no electrical tape, nothing. Same for the single 12v power wire that fed the old fuel selector valve - it has three different splices, but those are at least taped over. I plan on redoing all those wires up to the switch, once I know exactly what I'm doing.

 

[bikerdave]

I have A 74 TO 79 k/C 10, the cab is a 74 {according to the registration} c10 the frame is a 78 i think. dual tanks. whoever owned it before me bought it one piece at a time, and did a complete frame off restoration. so i have a 74 cab 78 gauge cluster and dual tanks with 2 tan wires and a green wire hanging under the frame a tan wire that hooks to the fuel gauge hanging off the fire wall. no place in the dash for a switch,{ I have 2, one with 4 prongs and one with 5} which i can cut or put in a couple toggles. cant figure out how to hook up the fuel gauge. the power to the valve is easy on /off. any thoughts on the wiring from 2 tan wires to 1 tan wire for the fuel gauge. this should be easy but having a brain fart here. thanks