Wiring on 2005 AJ 270

I am having some wiring issues on my Amberjack. The first one is the fuel gauge reads E all the time. (it has done this since I bought the boat) I took the sending unit out and checked it with an omh meter, I tested good when the float was moved up and down. I then checked the voltage on the hot wire 0. It appears that the wiring goes to the engine someplace because it shows up on the smart gauges but I can't figure out where. The second problem I am having is I am attempting to install a ACR. I separated the batteries with the new switch and placed the #2 battery as the starting battery. I am attempting to isolate all other equipment to run on the #1 battery bank which is 3 deep cycle batteries. (I know, overkill for this size boat but I am paranoid about dead batteries offshore) My problem is that after separating everything I seem to have missed something because the engine will crank but not start unless I turn the switch to join all batteries.

This might help for the fuel sender. Selected electrical schematics for a 2006 AJ. Page 2 shows the diagram for the fuel sender. It seems the sender does connect to the engine ECU.

I have seen the wiring diagram, it really leaves a lot of questions unanswered. I just need to find where the it joins the ECU.