I'm pretty decent at electricical. I've had this same issue on my wifes 67 jeep Gladiator. New sender ( 2 different styles) read correctly out of the tank when hooked up to the truck harness. It appears to read correctly when installed disconnected from the truck harness and checked with an ohmeter. I have an adjustable resitance box. When I connect that to the harness at the tank, the factory gauge the gauge reads correctly. I ran new wiring from the sender by passing the cluster connection directly to the gauge. The result? it reads 3/4 at full, still has plenty of fuel on E. Doesn't make any sence to me.
Doug
same thing happened to me when i installed a new stainless sender/pickup in the replacement tank i installed in my charger.
i checked the resistance of the sender before installation, and it was spot on. just to insure it had a good ground for the sending unit, i soldered another wire from the sender case to the pickup tube inside the tank, then soldered another wire to the outside pickup tube, to ground to the body sheet metal in a newly drilled hole. however, once installed it reads all over the place. i have never seen F, and a couple of times when it said E, it only took 5 gallons to fill the tank !
the original, super rusty sending unit read almost 3/4 of a tank when it was replaced, and i drained out almost 15 gallons of "weed killer" from the original tank.
