Okay so one of dads old rigs has an issue with his fuel gauge. It goes from full to 3/4 of a tank only. It will run totally empty and the gauge will not move from 3/4's full. The odd thing is not only does the gauge seem to not be working but the low fuel light doesn't work either when the tank ran empty.

This wasn't noticed until relatively recently. This particular truck dad bowed the frame above the rear end. We cut the frame in half behind the cab and grafted a new half to it. Also made it a dually at the same time. Anyways since the mint bed we had on the truck folded up like an accordion when the frame bent we opted to stick a flatbed on it. This worked well but we noticed immediately it wasn't a dually bed (it was on a D350 I bought). The truck had a C&C dually rear in it. Anyways in the time we fixed the frame and placed the flatbed on it we noticed in driving it that the fuel gauge wasn't working properly. He hated having a single wheel bed on it. So we found a dually flatbed and when I pulled the single bed off I put a brand new float in the tank. Thinking that would take care of the issue. Well guess what. It didn't. I haven't had a chance to break out a meter but I did use another RWAL harness by plugging it into the factory blue plug under the dash and running it out the door and plugging it in to the fuel tank. Well the same result. So I then used another whole sending unit (whole piece in the tank) and this time the gauge went all the way to empty and the low fuel light came on. Now I tried to slowly raise the float to see if the gauge would move off of empty but it wouldn't move off of empty. The problem with the spare sending unit is I received it with a truck and don't know its condition. So my question is has anyone ever had this exact issue? A fuel gauge only reading from full to 3/4 tank? When I removed the complete sending unit before the dually bed it seemed to float up and down and I was sure I put it in the tank so that the fuel lines and faced up towards the drivers front wheel and the plug faced back towards the drivers side rear wheel. So I am pretty sure the float is obstacle free and should have free range of motion in the tank. Any suggestions without dropping the tank? Thanks everyone

 

Dang no one? Well today I was able to get under the rig and using another whole tank sending unit (not just the float) plugged into the trucks harness. Walla! It went from 3/4 tank to a little less than a half tank. Where it should be. I then pulled the one out of the tank and swapped the new float over to the good unit and re-installed it into the tank. I had just enough room to get the whole unit out/in with the tank in the truck. I didn't get a chance to start testing the wiring and the pins inside the sending unit to see where in lies the problem. Just glad to finally get it done. Especially before any snow.

 

Well my luck wasn't that good. Today I went and filled the truck up and the darn gauge didn't move off of a half tank. So now I am back to square one. I have tried another gauge cluster, another harness from the HVAL blue plug back through the door to the fuel tank, and now another sending unit with a brand new float in the tank. It went from moving from full to 3/4 to now with the factory harness and gauge with a different sending unit and new float to staying at a half tank. Dang for such a simple circuit I am getting my butt kicked. I have used a test light with a long lead on the positive side to find I do have a ground on the two black leads. I also have used a DVOM to see the float show me its resistance. At full it was showing around 5ohms and went up over 14ohms at empty. What the heck am I missing?

 

I replaced the sender unit on mine. I also modified it with a cover plate and light spring over the float arm pivot. The plate that I used came out of a 2nd gen gasser. Over time the plastic pivot pin wears out and the arm with the rheostat contactor can pull away from the rest of the rheostat and cause the gauge to lose signal. My old one was fried but I preemptively modified the new one. My fuel gauge would only work sporadically. Now it works as it should.
I thought I did a write up about it but I cant find it. If you do a Google search you'll find the articles I took my inspiration from.

 

Mr Fusion but I installed a new sender/float with a brand new Mopar unit. It actually was doing the same thing with the old one as it did with a new one. I thought I had it as it was pretty much on 3/4 tank all the time unless it was filled up. Well Friday I pulled the whole unit out of the tank and installed a new float/sender on a totally different unit (piece the float screws to and the fuel lines go to). When the gauge went from 3/4 to half I thought I had it. Well it didn't fix it, just went from moving from full to 3/4 to now staying on .5.

 

Bummer. Maybe check the orientation of the float or start chasing wires looking for corrosion.

 

So still no luck. I recently got back under to look at it. What I noticed is when I unplug it with the key on the gauge goes to E like you would expect it to. What has me baffled is the fact that it goes to F when I fill the tank up and seems to work as it should when the fuel level goes to 3/4 of tank and then just sits there. It literally makes no sense. I cant believe no one else has had anything similar.

 

Silly question.

Are you aligning the arrow on the tank, with the arrow on the fuel reservoir?

You most likely are, but I had to ask. The reservoir position can be tweaked accidentally when tightening the seal ring

 

I wish it was that simple. I have had the complete sending unit out on three separate occasions. Once before I put the flatbed on it. I figured it was a bad gauge sender so I installed a brand new Mopar unit. Since the tank was full at the time I thought it was fixed because it went to full. I then pulled it a second time with the flatbed on it to check the float. Then I even unhooked the plug and plugged it into a spare tank unit and slowly raised the float with no change to the gauge. I am thinking there is a grounding issue but I literally have changed everything out temporarily except the main dash harness. The complete in tank unit, the sending unit, the RWAL harness, and the gauge. One thing that has been done since the frame repair was the installation of a flatbed. I had one bed (a single wheel) and then swapped a dually width bed. Before the frame bending the gauge worked and it had a regular bed on it. Could the LED lights be the problem?

 

Having a similar problem with mine. My fuel gauge when i gill it up only goes to 3/4 but will run out and showes it so is it a problem with my gauge or tank?

 

you can test the gauge resistance to see if gauge is at fault, someone might correct me, but i think values are 0-65 ohms empty to full, you can visibly see if resistor is physically worn, some times contact can be adjusted, worst case, rebuilder repair or renew

 

Looking over this whole thread, I would come to the conclusion that something is mechanically interfering with the sending unit mechanism, Perhaps it's oriented wrong in the tank and hitting the side.

 

Sir you are 100% correct. For me at least. Some more information. The truck in question for me was bought back around 2000 from a Dodge dealer in Erie PA. Its body was ate with rust. A few years after buying it my dad had a problem with the truck not wanting to start if it was parked on an incline. We traced the problem then back to the fuel return line having a small pint hole that allowed the fuel to drain into the tank. So we ran new fuel lines then, pulled the tank to install a new sending unit, and put back together. Now I am not certain why but fast forward about 10yrs and my dad started having issues with the gauge. We pulled and installed a new sending unit thinking that was the problem but it wasn't. I literally changed everything in this circuit. I pulled the gauge cluster out and tried another dash, ran another chassis harness back. I mean everything. Then I decided to swap the whole in tank unit and bam. I noticed that the arrow on the sending unit didn't exactly line up with the mark on the tank. I loosened the ring and spun it (less than an inch) and I heard the float fall down inside the tank. Mind you this truck has a flatbed that allows access to the top of the tank without dropping it. So I would recommend that if in the future anyone has an issue of their gauge reading from full to 3/4 and then not going lower or the dummy light coming on to immediately check the clock rotation of the entire sending unit.