Wiring Fuel Gage Dual Tank EFI

[Bcrookston]

Hello All,

We are not new to the forums, but this is our first post. My 15 year old daughter, my wife, and I have very much enjoyed working on our GMC, but we are stumped after long internet searches trying to get our fuel gage to work with dual tanks and EFI.

We have a 1978 k25 that we have attempted to marry with a Holley Sniper unit and new dual tanks, each tank with in-tank fuel pumps and rheostats, new fuel selector valve, and a new in-cab tank selector switch per the 1987 or later models with EFI. We have everything temporarily wired up and can select the main or auxiliary tanks successfully and drive the truck around the block no problem. But, where we are stumped is what wiring we need to do between the firewall, the selector switch, and the rheostats to get the fuel gage to work.

We installed a brand new fuel gage from LMC and it was working fine prior to EFI. We checked the rheostats before we hung each tank and they seem to be working fine. Any advice on the four wires coming through the firewall? They have been pulled out from behind the sbc and are:

Light blue (?aux tank?)
Light green (?abandon?)
Tan (?fuel selector valve?)
Tan and white (?main tank?)

Each rheostat has a ground running to the battery with the other wire going directly to the fuel selector valve. The schematic we have shows a single wire coming from the fuel selector valve forward along the frame to the fuel gage...but we have no luck when we connect this wire to the tan wire or any of the other three.

Many thanks again for any advice or help.

 

[SirRobyn0]

Hello All,

We are not new to the forums, but this is our first post. My 15 year old daughter, my wife, and I have very much enjoyed working on our GMC, but we are stumped after long internet searches trying to get our fuel gage to work with dual tanks and EFI.

We have a 1978 k25 that we have attempted to marry with a Holley Sniper unit and new dual tanks, each tank with in-tank fuel pumps and rheostats, new fuel selector valve, and a new in-cab tank selector switch per the 1987 or later models with EFI. We have everything temporarily wired up and can select the main or auxiliary tanks successfully and drive the truck around the block no problem. But, where we are stumped is what wiring we need to do between the firewall, the selector switch, and the rheostats to get the fuel gage to work.

We installed a brand new fuel gage from LMC and it was working fine prior to EFI. We checked the rheostats before we hung each tank and they seem to be working fine. Any advice on the four wires coming through the firewall? They have been pulled out from behind the sbc and are:

Light blue (?aux tank?)
Light green (?abandon?)
Tan (?fuel selector valve?)
Tan and white (?main tank?)

Each rheostat has a ground running to the battery with the other wire going directly to the fuel selector valve. The schematic we have shows a single wire coming from the fuel selector valve forward along the frame to the fuel gage...but we have no luck when we connect this wire to the tan wire or any of the other three.

Many thanks again for any advice or help.

I'd really like to try to help you but just don't know where to begin especially with the aftermarket fuel induction system. Would it help if you could post any schematics you have.

 

[Frankenchevy]

It should all be factory wiring from the sending units forward.

I’m assuming you used oem style tanks and senders?

There are wiring schematics here in the reference library.

The sniper will not change anything from the sender to the gauge. Mine all stayed the same with the exception of running the blue power wire to the sender to power the pump.

 

[Vbb199]

One day, I'm going to add a saddle tank to my 79' and when I do, I think I'm going to create a step by step tutorial on "how to" with EFI and force an admin to pin it because I think dual tanks, selector switches, and efi seems to pop up across the latest posts every month or two month

 

[SirRobyn0]

It should all be factory wiring from the sending units forward.

I’m assuming you used oem style tanks and senders?

There are wiring schematics here in the reference library.

The sniper will not change anything from the sender to the gauge. Mine all stayed the same with the exception of running the blue power wire to the sender to power the pump.

This is what I thought too, but I don't know after market EFI systems very well and wasn't sure how that would play in, if at all.

That being the case this is a diagram for the pollak 6 port valve. Brian, the OE GM systems used different color wires, but it will give you a very good idea of how the system functions which hopefully will help you figure out what to do.

You must be registered for see images attach

 

[mshawn]

Your selector switch controls the signal to the fuel gauge. In this schematict “b” on the comet or go to the gauge. A&C are coming in from each sender “A” is main tank sender and “C” is aux tank sender. So if you look at a schematic from and 1987 the color code for “B” will be for the gauge. When you look at a schematic for the year truck you have 1978 then the wire color for the gauge should be spliced to go to pin”B” on the selected valve connector. The EFI has nothing to do with it.

 

[mshawn]

After rereading your post . Did you add the selector switch and selector valve or was that a factory items?

 

[SquareRoot]

1973-1991 Dual fuel tank systems theory of operation - The 1947 - Present Chevrolet & GMC Truck Message Board Network

1973-1991 Dual fuel tank systems theory of operation The 1973 - 1987 Chevrolet & GMC Squarebody Pickups Message Board

67-72chevytrucks.com

Everything you want to know. @Vbb199 , someone beat you to it.

 

[Bcrookston]

Thank you everyone. With the info you shared we were able to sort it out and it is now working great. Much appreciated! Have a great day and week!

 

[Albrigap]

I made this for my 82 6.2 diesel.
This way I knew what was in each tank at all times.
I had to look at the tank selector switch to see what tank I was burning off from.
This worked great.
I done the wiring schematic myself and was sure before I did this mod.
I had to run a single wire for the one gauge from the tank selector valve.
PS: The diesel filler necks do not use the anti-siphone flipper and are easier to fill.

You must be registered for see images attach

You must be registered for see images attach

 

[Snoots]

Thank you everyone. With the info you shared we were able to sort it out and it is now working great. Much appreciated! Have a great day and week!

What was it? Inquiring minds want to know. Glad you got it worked out.

 

[87 Scottie]

1973-1991 Dual fuel tank systems theory of operation - The 1947 - Present Chevrolet & GMC Truck Message Board Network

1973-1991 Dual fuel tank systems theory of operation The 1973 - 1987 Chevrolet & GMC Squarebody Pickups Message Board

67-72chevytrucks.com

Everything you want to know. @Vbb199 , someone beat you to it.

This thread is a wealth of information regarding the dual tanks, gauge, switch, valve, wiring and more! wiring diagrams etc etc thank you for posting up the link!