That may change my testing setup. I had already figured it was the cluster, and the owner was pushing so I bought a used one and it was very clean, likely out of a wreck, and I had to transfer a couple of the gauges from the original to get the full set back that the truck was equipped with, but there was no change in the behavior. It was cheap, and a calculated gamble. Cost less than a single trip to the dealer to scan it, so that's where he wanted to spend the money. Knowing as much as I do know, with the schematics and information you've provided, I probably would not have done that. But it's done. That's one more reason I am beginning to suspect the controller. I have another 8600 in the yard, but it's a 2012, so entirely different setup. If I had an identical model I might be tempted to swap in it's controller just to turn the key on and let the gauges self test and see what happens. Not actually sure I don't, so I might look around. I've eliminated all the wiring between the senders and the 36 pin connector and subbed in known good wire, so this is getting puzzling and looking less like a simple wiring issue to me.
We have a Diesel Laptop system, but I never use it and don't know if it would show anything. I figure if it would, the truck guys would have found and fixed it by now. It's been sitting on my list for at least a year.
I'll take that schematic and see what changes there are and adjust to suit and see what happens. I'll be back!
Quick glance says ground the grey (pin 26) and one lead of a 75 ohm resistor (half a tank for two tanks) and connect the other lead to pin 5.
Another thought that may be a clue. When you key on, it POSTS, then everything reads fine. Then a second set of tests apparently happens and the air pressure is analyzed and if recognized as low, the light comes on and the piezo beeper starts....it is at that point that the fuel gauge does it's thing....red light, gauge around to 6:00.