What’s this for

[Captain J]

Can someone tell me what this knob for?
1972 GMC Sierra
Truck have only one in cab fuel tank

[68bowtie]

I bet it’s anti-theft

[DeadheadNM]

It's a switch to toggle between the sending units for the in-cab fuel tank and the now missing but once present auxiliary fuel tank.

Post up a photo of the build decal (SPID) on the glove box door if present

[Killer Bee]

to add to that, if you follow the fuel supply line from the tank underneath the cab and you might find evidence of it's removal..

[special-K]

That's the '72 auxiliary fuel tank gauge switch to read levels in both tanks. There should be a hole in the floor between driver seat and door where the control valve was mounted. Might even still have the valve and placard in place. I guess someone removed the tank. Might have filled the filler hole in the upper forward part of the bedside or switched beds

[Captain J]

Originally Posted by 68bowtie

I bet it’s anti-theft

🤣🤣🤪🤪🤪
It might be.

[Captain J]

Originally Posted by DeadheadNM

It's a switch to toggle between the sending units for the in-cab fuel tank and the now missing but once present auxiliary fuel tank.
Not sure,
Post up a photo of the build decal (SPID) on the glove box door if present

I’ll thanks

[Captain J]

Originally Posted by Killer Bee

to add to that, if you follow the fuel supply line from the tank underneath the cab and you might find evidence of it's removal..

I’ll check,

[Captain J]

Originally Posted by special-K

That's the '72 auxiliary fuel tank gauge switch to read levels in both tanks. There should be a hole in the floor between driver seat and door where the control valve was mounted. Might even still have the valve and placard in place. I guess someone removed the tank. Might have filled the filler hole in the upper forward part of the bedside or switched beds

But where was the aux fuel tank located?
As I saw no evidence of aux fuel tank, but I’ll look for the valve and placard.
Thanks

[MJN]

Originally Posted by Captain J

But where was the aux fuel tank located?
As I saw no evidence of aux fuel tank, but I’ll look for the valve and placard.
Thanks

Aux fuel tanks were under the box, between the bedside and the frame rail. If there is no filler hole in either bed side I have seen aux fuel tanks that had fill necks inside the rear wheel housing in front of the rear wheel.

[jocko]

NL2 RPO was what got you a single, driver's side only aux fuel tank setup, which included the fuel gauge selector in your picture and a left (only) bedside fuel filler - i.e. a fuel cap just like the cab tank cap, but located on the bed, as shown below. The fuel flow selection valve was on the driver's side floor just inside the door sill and in front of the seat belt retractor. No passenger side fuel fills were available from the factory nor were wheelwell fill tanks - but there were several different vendors that offered dual saddle tank installs - but they would not have had the switch shown in post 1 though. Dealers could also install saddle tanks - but these setups only had the cuts in the bedside for the little metal, lockable fuel filler access doors or wheelwell filler necks. Do you have NL2 on your SPID? If no filler neck on bed, then it's been filled or bed is not original to the truck (assuming NL2 is on your SPID).

[special-K]

Originally Posted by Captain J

But where was the aux fuel tank located?
As I saw no evidence of aux fuel tank, but I’ll look for the valve and placard.
Thanks

Driver side front between bedside and chassis. The other thing, if it still has the same bed, would be the filler tube cover. About 3"-4" wide square "duct" from top of bedside to floor that covered the fuel fill tube.

[Captain J]

Originally Posted by DeadheadNM

It's a switch to toggle between the sending units for the in-cab fuel tank and the now missing but once present auxiliary fuel tank.

Post up a photo of the build decal (SPID) on the glove box door if present

Decal says it has an aux fuel tank, surprise surprise,
🧐

[Captain J]

Originally Posted by jocko

NL2 RPO was what got you a single, driver's side only aux fuel tank setup, which included the fuel gauge selector in your picture and a left (only) bedside fuel filler - i.e. a fuel cap just like the cab tank cap, but located on the bed, as shown below. The fuel flow selection valve was on the driver's side floor just inside the door sill and in front of the seat belt retractor. No passenger side fuel fills were available from the factory nor were wheelwell fill tanks - but there were several different vendors that offered dual saddle tank installs - but they would not have had the switch shown in post 1 though. Dealers could also install saddle tanks - but these setups only had the cuts in the bedside for the little metal, lockable fuel filler access doors or wheelwell filler necks. Do you have NL2 on your SPID? If no filler neck on bed, then it's been filled or bed is not original to the truck (assuming NL2 is on your SPID).

Couldn’t find either of them, must remove all.

But where is the location of the aux tank and how it’s connected, size of the tank. I like to re install it if easy.

[Captain J]

Originally Posted by jocko

NL2 RPO was what got you a single, driver's side only aux fuel tank setup, which included the fuel gauge selector in your picture and a left (only) bedside fuel filler - i.e. a fuel cap just like the cab tank cap, but located on the bed, as shown below. The fuel flow selection valve was on the driver's side floor just inside the door sill and in front of the seat belt retractor. No passenger side fuel fills were available from the factory nor were wheelwell fill tanks - but there were several different vendors that offered dual saddle tank installs - but they would not have had the switch shown in post 1 though. Dealers could also install saddle tanks - but these setups only had the cuts in the bedside for the little metal, lockable fuel filler access doors or wheelwell filler necks. Do you have NL2 on your SPID? If no filler neck on bed, then it's been filled or bed is not original to the truck (assuming NL2 is on your SPID).

I’m not familiar with NL2 & SPID!

[sick472]

SPID = service parts identification... It's the sticker that was put on the inside of the glovebox door that has the options that where ordered with the truck. NL2 was the aux tank option code.

[Captain J]

Originally Posted by sick472

SPID = service parts identification... It's the sticker that was put on the inside of the glovebox door that has the options that where ordered with the truck. NL2 was the aux tank option code.

It has that code NL2, but none exist

[special-K]

Originally Posted by special-K

Driver side front between bedside and chassis. The other thing, if it still has the same bed, would be the filler tube cover. About 3"-4" wide square "duct" from top of bedside to floor that covered the fuel fill tube.

Originally Posted by Captain J

Couldn’t find either of them, must remove all.

But where is the location of the aux tank and how it’s connected, size of the tank. I like to re install it if easy.

I answered a couple of those questions. The fuel line runs from top of tank to the valve in the floor and from valve to fuel pump. The valve is body is a "block", I would call it, like the t-block in your rear brake lines. You'd have to ask for pictures for proper routing.

The tank is something like 10-12 gallons if I remember correctly.

There is also a piece I'd call a guard that mounts across the front of the valve to protect it.

[Captain J]

Originally Posted by special-K

I answered a couple of those questions. The fuel line runs from top of tank to the valve in the floor and from valve to fuel pump. The valve is body is a "block", I would call it, like the t-block in your rear brake lines. You'd have to ask for pictures for proper routing.

The tank is something like 10-12 gallons if I remember correctly.

There is also a piece I'd call a guard that mounts across the front of the valve to protect it.

Thanks, I found the fuel filler capped (on the left side) painted over, the valve is removed, no fuel lines.
Again thanks for your help 👋🏻

[57taskforce]

Here’s the NL2 tank and filler from my K20.

[Cesar Diaz]

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[Captain J]

I wish I can, I’m 3500miles away, cost of shipping exceeds the value of the switch.

[doowopper]

Are replacement SPIDs available?

Thanks,
Roger

[LockDoc]

Originally Posted by doowopper

Are replacement SPIDs available?

Thanks,
Roger

Yes they are.

http://outintheshop.com/

A lot of other good info on there also.

Jeff is a member on here and a really nice guy.

LockDoc

[doowopper]

Originally Posted by LockDoc

Yes they are.

http://outintheshop.com/

A lot of other good info on there also.

Jeff is a member on here and a really nice guy.

LockDoc

Doc, thanks a million. That site is a keeper.

Roger