end of my rope with fuel gauge

I've tried everything. Fuel gauge on my 93 xj hasn't worked for several years.
I thought I was going to get to the bottom of it this summer. Please read carefully so we avoid unnecessary answers.

Fuel gauge jumps on start up, but as soon as running drops to empty and empty light comes on and stays on, regardless of the fuel level.
I recently got a spare gauge cluster thinking I would be set, exact same things happens to new cluster.
First thing I did was replace the sending unit in tank, verified that the old unit was inoperative, and verified that the new unit was sending proper resistance.
Next, since that didn't solve the problem, ran a complete new circuit from the sending unit to ground battery direct, and to the cluster to eliminate any bad wiring or breaks in the lines on their way. No change.
I am getting proper resistance readings all the way to the cluster.
I even tried wiring the two fuel gauges separately of the clusters, both with no results.
I have verified the two fuel gauges actually work with a separate sending unit I have, both work the full sweep of the gauges.
Not sure what to do next, I guess a separate aftermarket fuel gauge, but I'm not convinced that will work either after all I've been through.
Everything else works on the cluster with no problems.
One odd thing I have come across while diagnosing, when I ground out the sending wire on the fuel gauge, (it should go to full when grounded) the jeep runs VERY rough, to the point of actually stalling, strange.
Any help would be appreciated.

What color wires did you run up to the gauge from the sending unit? Also, did you ohm test the ground circuit at the tank sender?

What does the colour wire matter? I ran a completely new ground wire from the sender to battery direct. Sending unit wire is B1 at the cluster, dark blue. Thats the only wire to the gauge, other went to second battery in cargo area.

tkjeeper said:

What does the colour wire matter? I ran a completely new ground wire from the sender to battery direct. Sending unit wire is B1 at the cluster, dark blue. Thats the only wire to the gauge, other went to second battery in cargo area.

Click to expand...

I guess I phased that wrong. I was asking what wire color you replaced(which you indicated was blue) to make sure that you hooked up the correct wire in the correct spot.

"First thing I did was replace the sending unit in tank, verified that the old unit was inoperative, and verified that the new unit was sending proper resistance."

From what year vehicle was the donor sending unit from?

same year, so was the donor cluster

so.....thats it?