well i finally found what we have all been looking for - a tank full of diesel that stays full and never drops at all. The gauge is pinned at the fullest mark and has been for some time.
The last time this happened was when my rusted exhaust pipe broke off at the end and broke the wire that goes to the sender, but then i fixed that and the gauge seemed accurate for well over a year after that. Not too long ago i topped off the diesel tank, probably filled it to the max, and since then the gauge has not dropped except every once in a while it drops a bit while driving, to about the 3/4 mark which is where the fuel level probably is.
It really seems like the wire from the sender is intact and with good connections all the way to the front of the vehicle - then it goes inside a piece of flexible conduit and i have no idea what it looks like from there on.
could a bad ground wire also cause this problem? the ground seems good too but i can never know for sure.
Any ideas how to trouble shoot this?
thanks,
paul

When I first got my truck, I thought the seller was nice enough to fill up the tank. After 50 miles of driving it home, I was really happy with my MPG. The gauge didn't move a bit:eek:. I fueled it up and I was surprised again when I topped if off and it took almost 11 gallons:D(16gal tank).
Anyways, I believe your right about the ground wire. If you ground the pink wire from the fuel gauge it should point to Empty.

The most common cause is a bad sending unit ground.

When the fuel sending unit wire is open circuit, the guage goes to full, when grounded, it goes to empty. A bad ground is the same as an open circuit. Check the ground, then check the sending unit.

Sometimes, the keeper that holds the strap that runs from the float rheostat to the guage connection pin comes loose and loses it's contact. I've found tightening this connection has fixed some sending units. If it's not loose, the rheostat is likely defective. If so, I'd replace the sender.

I've got the same fuel tank...always full!I have chased mine down to it being the fuel sending unit.I'll have to drop the tank this weekend and fix it.

The dual tank setup will sometimes transfer fuel from one side to the other if the return valve in the switchover valve gets stuck. If it does, it pumps fuel from one tank, dumps return line fuel into the other until the tank gets full, then it goes on the ground. Interesting but expensive! If it's just the guage thats not moving, most likely the sender.

When the fuel sending unit wire is open circuit, the guage goes to full, when grounded, it goes to empty. A bad ground is the same as an open circuit. Check the ground, then check the sending unit.

Sometimes, the keeper that holds the strap that runs from the float rheostat to the guage connection pin comes loose and loses it's contact. I've found tightening this connection has fixed some sending units. If it's not loose, the rheostat is likely defective. If so, I'd replace the sender.


does anyone know how to tighten the keeper that Fred refers to here? Does he mean to tighten it at the connection pin or at the rheostat?
I still can't get the sending unit to work. It was fine until one day i topped off the tank, perhaps a bit too full, then the gauge always stayed on full and never drops. It does not seem like the sender is actually shot but i have checked the ground and also the pink wire that goes to the gauge seems to be in tact, although i only get a reading of 6 volts when i test it - not sure if that is correct or not.
Is there anything else to try?
thanks,
paul

Did you try running a clip lead from the sending unit to a good ground? That would assure you that the ground is good...

yes i tried that but no luck. I also took the pink wire that supplies power to the sender and connected it to the sender ground and watched the dash gauge drop to empty so at least i know the power and ground wires are in tact.
the problems seems like the rheostat thing does not have a good connection at one of the two ends but i don't understand how to tighten it up - it doesn't look like there is any kind of bolt to tighten.

Paul

I take it you have the sending unit out of the tank then, right?