Fuel sending Unit dilemma

[Oldsdank81]

Working on my 81 cutlass
Going through the motions of adding an aftermarket fuel gauge to my cluster and upon wiring the sender and turning the key it pegs to full. I’ve tested everything and believe it’s the sending unit itself. Redid all the grounds ran a fresh wire from the sender to the gauge and still the same thing.

I figured I order a new one to see if anything changes. This cutlass was originally a 3.8 v6 with the two outlet sending unit. I’ve since installed a small block v8 and After searching the net it’s recomending a 3 outlet unit for the carb
My question is should I stick with what came stock. Or can the other sending unit work.
What is the extra line for exactly?

Thanks and I appreciate any and all feedback

 

[454GrandPrix]

Going through the motions of adding an aftermarket fuel gauge to my cluster and upon wiring the sender and turning the key it pegs to full.

Which of the two sender wires is connected to your gauge? Did you perhaps accidentally connect the gauge to the sender's ground input rather than the output?

EDIT: In other words, if you simply unplug the gauge wire from the sending unit (and leave it hanging in mid air), does the needle now read dead empty?

 

[Oldsdank81]

Which of the two sender wires is connected to your gauge? Did you perhaps accidentally connect the gauge to the sender's ground input rather than the output?

EDIT: In other words, if you simply unplug the gauge wire from the sending unit (and leave it hanging in mid air), does the needle now read dead empty?

I don’t believe so. I tapped into the pigtail directly from the sender a pink wire.
If I unplug the sender wire at the gauge it goes back to E.
Soon as it makes contact with the post it pegs right up again.
I haven’t dropped the tank so I’m unsure the condition of the connections on the sender itself.

 

[Ribbedroof]

What sender ohm range did the gauge specify? They're not all the same, and there are many different ohm ranges used, even within GM.

You usually have to either buy a gauge that fits the sending unit range, or more commonly, buy a sender that is specified by the gauge manufacturer to work with their gauge.

Your original sender operates between 0 (empty) and 95 (full) ohms

 

[Oldsdank81]

It’s a GM specific gauge. 0 ohms empty/90 ohms full.

 

[454GrandPrix]

If I unplug the sender wire at the gauge it goes back to E.
Soon as it makes contact with the post it pegs right up again.

That does sound like the sending unit output wire is incorrectly outputting full ground (assuming it really is the 90 ohm unit).

I haven’t dropped the tank so I’m unsure the condition of the connections on the sender itself.

And it seems this will need to be the next step in your troubleshooting.