I have a series 1 580SL when the fuel tank is full with fuel the needle on guage reads pegs way over full and stays that way untill you get to about 3/4 full and then it starts jumping every 2 seconds or so from 3/4 full to peg.Then when you get to around 1/2 full it still jumps but not as frequent probally ever 5-6 seconds.Then when it drops to 1/4 tank it quits jumping all together.I replaced the sending unit a couple of years ago and it worked fine untill the last 2 months or so and it started doing this.
I bet its sending unit again could be the wire going from the sending unit to the guage is chaffed and shorting out unhook the wire from the sending unit, tape the end and try to leave it in the original position, run it for a day (of course dont run out of fuel) and see if the guage jumps then. If its got a short, it should jump of course you could always replace the wire it could be the guage, but I doubt it
Mine is doing something similar, wanting to show all the way full even when about empty. I took the cover off today and this was what was underneath. I am going to clean all this tomorrow and check the resistance in the sender. I washed all this recently and I suspect the moisture mixed with the oil, dirt etc to partially short the sensor out. Does anyone know the ohm range? And does the fuel sender vary the resistance to ground similar to the shuttle oil temp sender? And yes, that is where a rat made a bed in the corner. :bash Mine is a 580SE so yours may look different. BTW.. in the lower right corner on the 1st photo is the hyd tank vent and pressure relief valves. I been looking to find & clean them rascals also.
Well I got this mess all cleaned up and I still have the same problem. Fuel gusge shows totally full all the time. even with the switch off. I need a new sensor for the tank. The thing measures like 1.2 meg ohms, & when I took it all out the tiny wire for the sensor is broken. My fuel guage works different to the temp switches. With an open circuit the fuel guage goes all the way to the top. When I ground the wire at the sender, the guage goes all the way to the bottom. I suppose I will order the 9801324 sender from Case for $65. I wonder if they redesigned this think because the parts drawing does not seem to show the LONG float. I am going to remove my whole tank so I can clean and paint the cavity; what a mess.
Good luck bowen and maybe your new one will last longer than mine did.The best i can tell mine on my SL is bad again i just replaced mine a couple of years ago.
My fuel gauge on my 416B Cat does the same thing. When full it behaves normally and the needle stablaizes over Full but when it I get down to less than full it jumps all over the place.
IF your is like mine read the resistance like this. If you have an open circuit or real high resistance the little variable resistor wire inside the tank is probably broken. ANY variable resistor works fine until the wiper reaches a broken wire then you only got resistance maybe from moisture /fuel etc. I would expect the guage to go to full. On mine the sensor is an open winding that actually sits naked, covered with fuel.. kinda strange to me. (See close up of the float pivot point) You can see that moisture in the tank rusted part of my sender assy. I suppose that's one reason to keep the fuel tank FULL.
Fuel sensors with a mechanical wiper design, typically last a long time unless there is something abrasive present to prematurely cause them to wear out. If the fuel sensor is having to be replaced frequently, then I would suggest the tank be checked for debris/contamination/rust in the fuel. As noted in this thread, the wiper assembly sits directly in the fuel. Once the wiper, or its contact area, gets damaged then erratic fuel gauge can result. The engine is somewhat protected by the fuel filter from fuel contamination, but the fuel sender is not. I would also point out the source of contamination may not originate in the backhoe’s fuel tank, but may come from tank the backhoe was filled from.