I have a 1993 F150 XLT supercab with 5.8L, automatic, and 2WD. It has dual fuel tanks (18 rear & 17 gallon midship). Other than leveling kit and aftermarket radio the truck is stock, in great shape, and only has 128k miles on it. Problem: Neither tank read the level and the rear pump wouldn’t work (midship pumps okay). I measured 10~12 volts at the rear pump connector but there is no fuel pressure on the fuel rail so I decided to replace both FDM (pump and sending unit). I bought it recently and the previous owner told me he replaced both the fuel pumps but I didn’t believe him so I replaced them myself. First I dropped the rear tank, replaced the FDM, reinstalled the tank, but still no dash level reading and the truck would not run (just like before I replaced the rear FDM). So then I dropped the midship tank, replaced the FDM and tested it before I reinstalled the tank. When I moved the float (outside the tank) I watched the gauge in the dash go up and down. Then I reinstalled the midship tank and when I put fuel back in the tank it would not read the level on the dash but it cranked up and ran okay. I confirmed voltage to the pump and continuity in the wire to the sending unit between the FDM connector and the tank selector switch but I couldn’t figure out how to check continuity between the FDM connector and the dash. To recap...I replaced both FDMs and I’m in the exact same place. And if it trust the last owner then both FDMs (and fuel filter) have been replaced twice in the last 1,000 miles. I have voltage on both pump connectors but the truck only runs on the midship tank. And neither tank will read the level. Do you think I have a clogged fuel line between the rear tank and midship tank? What could be causing both tank levels not to read? Any hep is much appreciated!

 

It sounds like more of an electrical problem to me. Can you hear the pumps priming when you turn the key? It also couldn't hurt to check out the selector switch as well as the relay. The pumps and level sender should run independently of each other. And these trucks seem to be really finicky with fuel pumps. I replaced both of my low pressure pumps with ones from napa, and within a year they both went out. But then again, a year is about the most Ive gotten from any napa part.
But long story short, these trucks seem to like motorcraft pumps and pretty much nothing else.