I had an auto electrician check for an electrical fault and he found the wiring on the fuel sender had a problem. Since he fixed this problem my fuel gauge has been out by a quarter of a tank. Showing empty with reserve light on when it should be reading a quarter of a tank and not showing empty. I have had it back to him three times and every time he gives it back to me it is exactly the same thing. I want to take it elsewhere but finding a Jag mechanic in my location is like finding a needle in a haystack. Could it be that the float on the sender has been placed in upside down? I had a problem with the fuel tank when the pump was replaced and they put the fuel tank seal rubber in upside down causing bad fuel vapor smells in the cabin.

 

Rather than reading low(er) when close to 1/4 tank, what does the gauge read when you fill up? Does it show 3/4 full then?

If the sender has been removed from the tank, could it be that the float lever wire was bent accidentally? It could also have been banged around and presumably bent when the fuel pump was replaced.

Just a guess BTW

Larry

 

Fairly easy to run the tank down below half and remove the boot linings to access the sender unit. Just make sure you use a brass drift to turn the locking ring, you don’t want any errant sparks in that area

 

Needs to be closer to 1/4 tank to remove the sender without fuel spilling out.

I would find a new or 'known good' sender and plug it in the harness. Work it up & down to see the gauge readings. If the readings are the same, then blame the wiring or car electronics. If the readings are NORMAL, then you have your answer.

bob

 

My main question is: Can the float mechanism be put in upside down. Yes or No ?

 

Hi ubetido,

I am not positive, but I think it would be difficult to install the sender upside down because the float and its bent rod (part 7 in the diagram below) would probably come into contact with the fuel pump module (part 8), which is mounted centrally in the tank. Your gauge might not be a quarter-tank off, but rather show a fixed reading if the float was immobilized against the pump. I suppose it's possible that the rod might not be fully immobilized but have a reduced range of motion.



I seem to recall that that the flange on the sending unit is indexed so that once you get it into position it notches into the proper orientation and is held there by the locking ring.

As Larry, Robman25 and Bob have mentioned, it's easy enough, with the fuel in the tank no more than 1/4 full or so, to remove the sender and check it for a bent rod as Larry suggested.

Another thought is that since the sender is just a variable resistor, if some resistance was added to the circuit, the gauge would be off by some consistent degree. Perhaps the connector wasn't properly seated, or the wiring harness was damaged, or the mechanic did something silly like apply dielectric grease to the resistive track on the sender or inside its electrical connector?

Cheers,

Don

 

Agree with Don, if you have dielectric grease anywhere except on the outside of connectors, you can really screw up the 40's sensor readings.

When I first got my XJ40 I was under the misconception that using this grease in a connector would keep corrosion away and ensure a "better" connection. Not the case!!

In these cars, the small voltage changes read by the computer can be affected by the insulating quality of the grease.

If you have any dielectric grease inside the TPS, o2, CTS, MAF, CPS or any other plugs I suggest removing it ASAP and giving the connector a good blast of contact cleaner before re-assembly.

Larry