I have a 2006 F250 extended cab with 5.4l engine and 38 gallon fuel tank and 160,000 miles. When cold, the gauge reads fine, but after warm up it goes to E and shows low fuel. I just installed a new Ford OEM pump/sending unit (yes $500) combination and problem is still there. when tank was down I checked wiring harness and plug next to tank and found no problems. Unit is still the same. New and old sending units seem fine ( I was not going to go to trouble of dropping tank and not changing the unit out). Both new and old sending units read 15 ohms on empty and 160 ohms on full and seem to change readings evenly through out range. I then found a couple of wires "naked" and rubbing the top front shock bolt in the engine compartment.. I traced these wires into the main harness that was coming from the rear of the truck that also feeds the pump/sending unit in the tank. Insuloated these wires thinking that was the problem.. Sorry, no such luck. My question is this? Does the sending unit hook to the gauge directly, or does it report to the PCM and then PCM reports the reading to the gauge. IF this is the case, then the gauge or the PCM could be at fault. I thought the naked wires I found could be bringing the resistance of the sending unit down towards zero and making the tank look empty.
ps. I also had a brake controller fault ( trailer wires are in the same harness) at times. However, I found water in the rear connection plug and unplugged it. The fault disappeared and did not reappear even after I dried it out and put dielectric grease in the plug.

Any help would be appreciated.