Hello everyone, I'm new to this site, I have a 1988 Hess Eisenhart XJS convertible.
I'm having trouble with the fuel gauge, I recently removed both fuel level sensors from the Upper and lower tanks and after I reinstall them the fuel gauge reads full all the time when the ignition is on. I've ran through several tanks of gas but it still reads full. I also disconnected the battery for more than 24 hours to see if that would reset something still the gauge reads full. When I turn the ignition on the fuel gauge goes from empty to full and stays there until I turn the ignition off. I'm hoping that someone can give me some direction on how I could get the fuel gauge to work properly again?

 

That's going to be tough one. The tank setup is completely unique to the Hess & Eisenhardt and the fuel gauge is notoriously inaccurate, and the two senders would never work properly in unison.

IIRC, the one recommendation was to disconnect the upper tank sender and just rely on the lower tank one. You might search the archives of the jag-lovers.org website as well as here, but it's going to really hard to find good info.

Good luck.

 

You may have already found the XJS Help book, but if not check it our here:

Kirby Palm's Jaguar XJ-S Help Book

See page 594 in particular.

Cheers!

 

Thank you, I will check it out.

 

Ok, what how I have figured out so far is that if I hold the lower tank rheostat in my hand with the electrical wires connected to it, the fuel gauge works properly but if I install the rheostat back on top of the fuel level float that is installed in the tank then the fuel gauge will go all way up to full. I then removed the electrical wires from the fuel pump thinking that there might be a short, still no change in my results.

 

What happens if you remove the wires from the upper tank rheostat?

 

Nothing works then. Have you or anyone heard that if you disconnect the fuel level sensors like I did that you would then have to run through a full tank of gas before the fuel gauge would reset and start working again?

 

No, I haven't heard of that, but you could try disconnecting the battery for a while. I think there was circuit board in the H&E that sent the signals from the sensors to the gauge, so a 'hard reset' might work.

I thought of something else, but memory could be faulty on this (it's nearly 20 years since I sold my H&E conv)...

I seem to recall that the two senders were similar, but that installation of each was different and if they weren't installed correctly the signals would get messed up. One is horizontal while the other is vertical I think, but can't remember which is which. You might double check that that each one is oriented correctly.

 

Yeah, I already tried the battery disconnect, that didn't do it. I always take pictures before I start disassembling things so that I can put them back correctly. The front tank is horizontaly mounted the rear tank is vertically mounted both rheostats are the same except the front has an extra collar around it. I remember coming across a thread several months ago where somebody was having the same trouble and they said in their thread that they understood that you have to fill the tank and then use up at least 14.7 gallons of gas before the fuel gauge would start to work properly. I can't seem to find that quote again but I am in the process of using up my fuel to see if that will actually work. It might take me a while, I don't drive the car very much.

 

Disconnecting battery have no efect. The dials are only resistor, which are not connected with ECU, only with clock pointer.
Which is the coil and the deflection depends on the input resistance
Both dials are connected in series:

Interior tank
empty full
90 Ohm 0-2 Ohm
Boot tank
empty full
180 Ohm 0-2 Ohm
Input to clock
empty full
270 Ohm 0-5 Ohm

Disconnection wires do nothing, only say to clock pointer, that dial have infinity resistor, which is tank "empty".
So you can:
1. disconnect wires on both dials, clock shoukld show "EMPTY".
2. disconnect wires on both dials, connect boot dial wires together, interior dial wires together, clock should show "FULL".
If not, the error is apparently in the clock.
For next work you need variable resistence (2 pcs, range cca 0-200 Ohms) and Ohmmeter (VOM).
Disconnect interior dial, connect wires to resistor, with VOM set to 0 Ohms.
Disconnect boot dial, connect wires to resistor, with VOM set to 0 Ohms.
On the clock should be "FULL".
Now set the boot resistor to 180 Ohms, on the clock should be cca "HALF".
So you can with resistor simulate "emptying" and "filling" both tanks.
If not, clock ponter is probably bad.

Rochester dials:
Interior fuel tank, top mounting
Gear Number : N6540 59C89 4 holes
Dial : 6540 Senior TwinSiteŽ Sender (apparently), 0 - 90 Ohm range
Hall´s effect dialing, 2 wires

Boot fuel tank, side mounting
Gear Number : N6744-8 D89 5 holes
Dial : P5ABXS0284 0-180 Ohm range
Hall´s effect dialing, 2 wires

Maybe it will help

 

Roman try this site. 6500 Series Senior Adjustable 4-Bolt Gauges