I'm at a complete loss on this one and I think I've stared at this so long that I cannot wrap my head around it anymore and could use some additional input.

With that, here goes...
I recently pulled my factory cluster out of the car, along with a previous stand alone wiring harness. Everything regarding the fuel gauge was working fine at this time. I have installed a Terminator X ECU and a Holley Pro Dash. Ultimately, I found that the fuel gauge would read full when the car was running, which I narrowed down to the actual pump itself running, as the culprit. I also excluded the dash and tested the leads directly with a multimeter.
The meter, when testing the sender circuit when the pump is powered, shows infinite (1) ohms and .42 volts. This explains the gauge reading, but not what's causing it.
The only changes in the wiring were to connect the fuel pump power to the new ECU and a tap into the pink wire in the cluster harness in the dash, for the sender.
The sender ohms correctly though the entire harness and fluctuates accordingly, based on jacking up the rear of the car, as an example. Ohms were tested at the cluster, C313 bulkhead connector and tank harness.
I am running a Racetronix hot wire kit and have been for many years. This kit grounds it's relay, the tank harness and the body harness through a ring terminal to a chassis ground. I have regrounded this twice to the chassis. The only outlier here is that I have not cut and crimped a new terminal; the current is undamaged, heat shrunk, clean and secure.
The fuel pump itself runs without any issue and will still do so through the factory ground, with the aforementioned chassis ground removed (as a test).
I've ran test grounds directly to the battery from several different test locations and have several chassis and engine grounds.
Pulling the pump relay yields no change in ohms when power is applied to the pump circuit up to it (the relay), as expected.
I even pulled the ECU fuel power lead apart and tested the pump directly, just because I was running out of ideas. Same results.

I'm at a loss at this point. I'm hoping that I've stared at this so long that I've missed something obvious. I've dug through every thread I could find on here and other sources, thrice. Maybe something failed just because it felt like it over the winter; I haven't a clue.

Hoping for a sanity check from you fine folks. In advance, thank you for your time!

 

Try taking the fuel sender ground wire and disconnect it from car harness and connect it to your gauges ground wire at dash. I had a similar issue on a different swap. Everytime the fuel pump ran the gauge would read differently. I added multiple grounds to the frame even running directly to battery and it made no difference. Apparently the gauge & sender ground need to be tied together so it always has the same reference.

 

I'll give this a shot and see if it makes any difference, it's one of the few things I haven't tried at this point.

I did some additional testing yesterday and with direct ground and power at the fuel tank pigtail, I received the same results when the pump ran.

I need to wire in the factory cluster just to see what results I get, for sanity sake, as well at this point.

 

No dice on all accounts. I had a spare cluster laying around with a loose, working fuel gauge and tapped it in to power/ground and the sender wire. Same ordeal, unfortunately, but as expected.

At this point, has to be something in the top of the tank pigtail or inside the tank. If I'm going to the lengths of dropping it, I'm putting in an Aeromotive Stealth tank and being done with it.

I appreciate your feedback, 2knight!

 

No problem but take the sender grounding wire and run it to the gauges ground wire not the other way around. It should work unless something else is going on but easy enough to try.