1988 F-150 4.9 efi
front tank reads over full all the time, r4ar reads empty all the time. question is - is reading empty an indication of open circuit or is it a short?

 

Can't say for sure it could be that the senders have simply failed in opposite directions. Usually they fail low.

 

here in the rust belt I have found that when a fuel gauge reads way past full..its usually a lost ground because of corrosion...either on the sending unit terminal ot the frame where its grounded.........its almost a given here that old truck dont have reliable fuel gauges....fix the ground & its fixes the reading.

 

Sounds like my 88. Been that way since I bought it in 1995. I had it at the dealer for some work and asked them to check the gauge problem. Ended up being the sending units. I keep thinking I'll have it fixed one day, but until then I'll keep using my odometer.

 

On my 88 Mustang years ago the gauge read past full when the sender failed (open circuit). I found the sender wire in the trunk and cut it. Then spliced in a 50ohm resistor from Radioshack that cost about $2 to give the illusion that the tank had gas. The gauge then read somewhere between 1/2 and F. I believe the Ford resistance range is usually 0ohms (empty) - 75ohms (full) or so. Anyway with this test I knew the sender was bad. Bought a new one and replaced it and its worked longer than the original at this point.

HTH

88 Mustang
1/2 of a 90 F250HD