Switching from dual to single tank... How do I fix the wiring?

Ok so my flatbed is the combination of two trucks:

Chassis and Drivetrain: 1979 GMC 1 Ton which originally had dual tanks. It was a plain jane work truck
Non Power windows, manual locks, rubber floor mats, etc.

Cab: 1984 GMC 1/2 Ton with dual tanks, power windows, locks, etc.

So its the old school selector valve from the original one ton. But fuel tank and sender are from the later model 1/2 ton....

Can someone please give me a step by step on how to make the fuel gauge read from the single tank whick is mounted behind the cab on the right hand side.

Hope Ive provided all the needed info, if not let me know...

This forum is awesome,

Thanks for the help.

Steve

Find your sender wire its probly pink running down the driver side. Cut as far back as you can connect to new sender. Remove all other wires. Its what i did still works

Still working on my flatbed... Fuel selector issue not solved yet... NOPHO84K30 recommended that I trace the pink wire on the drivers side and then run it to the passenger side and connect it to the sender...

What about the selecter valve on the dash? Is it still used? And if it is still used which one should I use? I have the "toggle RH-toggle LH" switch from the original 1979 1-ton and also the "momentary LH/RH" switch from the 1984 1/2 ton which is the cab Im using....

Thanks for all the help...

Steve

Anybody???

If you have the 84 donor why not swap the entire NL2 dual tank harness over.
You'll be able to use the 6-port 81-91 motorized Pollack valve, that you can still buy, instead of the obsolete and unavailable 73-80 6-port solenoid valve.

The 84 will have several easy disconnect points. Don't cut the wires.
The cab half should have a disconnect in the engine bay where it exits through the firewall. In 84 it's probably a molded plug on the firewall. The molded plug or wiring grommet can be pushed through into the cab by working around the perimeter with something that's not sharp or pokey.

Inside the cab you'll have the power and ground at the fuse panel and above the E-Brake pedal as well as the switch plug... It's probably tied in various places as well. I recommend you change the momentary switch over to the Delco D7809C ON-ON switch that GM used on the 87-91 RV series.
You should probably grab the six position grounding buss bar from the 84 kick panel because I don't believe the 79 will have one. It makes a great place to ground a CB and Trailer controller. It'll bolt to the kick panel under the ground bolt and ring terminal arrangement GM used in 1979.

Disconnect from the 84 tank senders and push/pull the wires to the valve.
Unplug the Weatherpak at the valve.
Remove the valve and bring it along. It probably still works.
Disconnect the PINK sender wire on the RH frame rail from the '84 NL2 sub-harness you're liberating. This connection is a molded plug. It looks like the sender connector on the cab side because it is. Single tank systems just plug this connection right into the sender or a short sender pigtail.
There will likely be several places where the NL2 harness is tied up so you'll need to get it loose at those places. Probably the pine-tree push in retainers wrapped to the sub harness with harness tape.

I would also liberate the 1984 PINK wire from the bulkhead disconnect to the frame rail if it's not damaged. Don't cut it. You can remove the Packard 56 terminals from the bulkhead plug fairly easily.

Pull out the 79 tank harness and switch. Don't chop it up. If it's not damaged you might be able to sell it.

On the 79 you'll find a molded disconnect on the frame. Probably in a similar location to the placement on the 84. This is where the PINK wire from the 84 valve harness will plug in.
Mount the Pollack valve and plug it in.
Route the sender wires and plug em in.
Tie the sub harness up and route the plug to the firewall hole.

You will have to make the dash hole slightly larger to fit the newer switch.
You may be able to plug the Pak Con power connector directly to the 79 fuse panel.
The ground connection can be made to the buss bar you liberated from the 84 and mounted above the e-brake on the kick panel.
The grommet or molded plug may be a mild adventure to get situated in the firewall hole but it shouldn't be too bad.
Tie up the wiring so it doesn't get tangled in the pedals or passengers' feet.


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You may want to consider using the 84 cab harness to get creature comforts like power windows, intermittent wipers, and the column mounted dimmer but that's a bit more involved and includes swapping steering columns... In the grand scheme of things it's not too bad.
The power door stuff will bolt right up inside the 79 door shells and the additional interior lighting is kinda nice.

Here's a How-To on removing Packard 56 terminals.
This one includes Pak Con terminals like GM used in the fuse panels and wiper motors as well...
This guy is using screwdrivers like you can easily find at Sears, ACE, or the Borg box stores.


There's no need to cut up donor vehicle harnesses. You can liberate any obsolete connector shell you need from a donor vehicle fairly easily.

I keep putting off doing a wiring how-to writeup with explanations of GM connector tooling, wire strippers, how to get good crimps etc.

This video is a pretty decent explanation of extraction and insertion.

I don’t know how far you have gone in the adding or deleting tanks or whatever. It’s simple issue. Assuming your sending unit is good. Ensure ground wire is good from sending unit ring to frame. Then ensure you have a continuous wire (no breaks) from sending unit post to gauge. That wire is pink normally.

By no breaks I mean disconnected. Splices are ok.

As the sending unit creates resistance in pink wire it moves needle on gauge. Sending unit MUST have good ground wire though.

I was just trying to figure out how to do this to my truck

Thanks guys!! I got the fuel gauge fixed!! Now on to the next problem.... I do a new post for it...

echo7bravo said:

I don’t know how far you have gone in the adding or deleting tanks or whatever. It’s simple issue. Assuming your sending unit is good. Ensure ground wire is good from sending unit ring to frame. Then ensure you have a continuous wire (no breaks) from sending unit post to gauge. That wire is pink normally.

By no breaks I mean disconnected. Splices are ok.

As the sending unit creates resistance in pink wire it moves needle on gauge. Sending unit MUST have good ground wire though.

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@echo7bravo: This is how my setup is current connected, as you said: pink wire from engine harness -> down driver side frame -> connects to pink wire going across frame -> aux/pass tank sending unit. However, my fuel gauge is not working still. I still have the dual tank selector switch on the dash, and the selector valve by the tanks; is there a way to bypass those? Any thoughts on getting the gauge to work again?

TravisUT said:

@echo7bravo: This is how my setup is current connected, as you said: pink wire from engine harness -> down driver side frame -> connects to pink wire going across frame -> aux/pass tank sending unit. However, my fuel gauge is not working still. I still have the dual tank selector switch on the dash, and the selector valve by the tanks; is there a way to bypass those? Any thoughts on getting the gauge to work again?

Click to expand...

The pink wire on the drivers side frame is part of an expansion harness for the selector switch. You can unplug the entire harness from the 4-pin connector on the firewall near where the throttle cable goes through the firewall and take everything out all the way around. There is a connector near the selector valve where that pink wire connects to another wire that comes from the heat shield near the starter wires. That wire that runs near the starter wires is the actual original single tank sending unit wire. When you disconnect the harness that crosses over behind the cab from the sending units and the wire going over the sending unit, that wire that runs near the starter will then connect directly to the passenger side tank and then everything will function just like a single tank truck would have from the factory. Let me know if that was too confusing, I can take pictures of my entire dual tank harness and the connector in question. I just removed the whole thing and spliced into it for my blazer tank conversion on my 1980 K25

I took out the fuel gauge, removed the needle and removed the gauge motor from the dash plate.
I then put two gauge motors side by side in the dash plate.
I repainted the gauge plate and used model train decals to show two fuel gauges, side by side.
I them used the short, like an oil pressure gauge, needle on both motors.
I rewired each so the left tank went to the left gauge and the same for the right.
This way, I see what is in both tanks at all times.
I then just look at the selector switch to see what tank I am running on.
No more switching to an empty tank and it looks factory.

Bextreme04 said:

The pink wire on the drivers side frame is part of an expansion harness for the selector switch. You can unplug the entire harness from the 4-pin connector on the firewall near where the throttle cable goes through the firewall and take everything out all the way around. There is a connector near the selector valve where that pink wire connects to another wire that comes from the heat shield near the starter wires. That wire that runs near the starter wires is the actual original single tank sending unit wire. When you disconnect the harness that crosses over behind the cab from the sending units and the wire going over the sending unit, that wire that runs near the starter will then connect directly to the passenger side tank and then everything will function just like a single tank truck would have from the factory. Let me know if that was too confusing, I can take pictures of my entire dual tank harness and the connector in question. I just removed the whole thing and spliced into it for my blazer tank conversion on my 1980 K25

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Any idea what the harness that crosses over behind the cab from the sending units does? I think its a 2 wire harness. Mine has a broken wire and my passenger tank doesn't work.

shinkle said:

Any idea what the harness that crosses over behind the cab from the sending units does? I think its a 2 wire harness. Mine has a broken wire and my passenger tank doesn't work.

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It connects all the things to the switch.