Since I bought it, the fuel gauge in my '86 xj-s has been inaccurate. When filled to the brim the gauge would read 3/4 and bounce up/down erratically when driving over bumps until it reached about 1/2 tank; then it would act normal/stable and seemed to read accurately.

1. I replaced the fuel level sending unit. The old sender had rough arm travel and a float filled with gas so I was sure that this was the issue.

Alas, no change in the behavior of the gauge.

2. So out came the cluster and a meticulous cleaning of all contacts/grounds on the printed circuit and the addition of an auxiliary ground cable commenced.

Put it all back together and for about 20 seconds the gauge read totally full, only to drop back down to 3/4 when I wasn't looking.

3. Out came the cluster again for an even more thorough cleaning along with continuity tests with a multimeter. I also cleaned and gently pried the pins on the male cluster connectors out a bit so they would make a firmer connection.

Reinstalled, still at 3/4.

4. This took up the whole afternoon until I was so fed up that I replaced the gauge itself with one from a spare cluster I had in a display case.

Still no change. UGH.

So as it stands I think I can safely say I've ruled out the sending unit, printed circuit, cluster grounding issues, and the gauge itself; all that's left is......... the wiring itself? The flickering of the gauge suggests a poor connection.. but why only when the tank is near full? The gauge reading low means that there is additional resistance being added to the circuit somewhere, but where haven't I already checked?

Funny thing is the gauge didn't even bother that much until I tried to fix it and failed and now it's driving me crazy!

 

I will be interested how you fix this, same problem 89 V12.

 

What the OP has done will fix the gauge in 90% of cases. The fact that it did read correctly for a while proves he did the correct things. What I would try is:

• clean up the cluster earth strap (the steel strip that runs along the cluster inside top) and all its connections and the earth connections on the blue flexible circuit board that connect to it,
• add an extra earth to the cluster direct to the car frame
• test for a solid 12 volts at the sender feed.
• clean up the earths in the boot (RHS chassis rail just below the ECU) and the connections to the gauge sender
• If this does no good, there is a connector between the dashboard loom and the loom going back to the gauge sender. You could test for this being the problem by running a wire direct from the gauge sender variable output terminal to the cluster and seeing if that changed anything.

 

My Gauge does that too!

I fill her up to almost overflowing and She only registers 3/4 Full (sometimes)

But since I'm always Topping Her up, I've got so used to it now, that it really doesn't bother me anymore

 

Same story here.

I replaced the sender unit, redid some of the boot wiring connections...it'll occasionally get above 3/4 when I've topped it off, but it still flops around.

I had the instrument panel out about five years ago, but now I can't remember what it was I was trying to fix at the time :-). With winter practically here, it seems like a good time to pull the panel and go over it again. Since the fuel gauge and the charging indicator are the two biggest problems, I may just look at hard wiring those to see if that brings any improvement, along with Greg's suggestions.

Thanks,

John
1987 XJ-S V12

 

So I cleaned up the earth strap and the connections to it and still no luck. And as I said in the original post, adding an auxiliary ground wire was among the first steps I took. My next step will be to clean up those trunk grounds and conduct more electrical testing. But that will have to wait for a couple weeks as I will be traveling.

Since this is apparently such a common issue I'll be sure to come back with my findings and any insight on the issue that I might pick up along the way.

 

 

One more for the 3/4 full mark, never let it get low enough to see what happens when I get near empty.

 

 

Finally got time to work on the Jag today and the latest developments have me more stumped than ever.

First, I went to the gas station and filled the tank to the brim. The gauge as usual read between 3/4 and 4/4 right about at the bottom of the "F". And as usual the gauge was rapidly bouncing all over the place on the drive home. What I did notice was that when I braked, it would peg the gauge at Full and when I accelerated the gauge would drop down to 1/2. Cruising it would move about randomly over bumps and turns.

This led me to believe that something was wrong on the tank side rather than the gauge cluster side like I had originally suspected. To troubleshoot, I took the old sending unit, plugged it back in, and taped the arm in place at 100% full. The gauge still read 3/4 unsurprisingly but when I drove it around the block the gauge was still going schitzo even with a definitively stable reading coming from the sending unit.

Since I had already added an extra ground to the cluster, I added an extra ground to the sending unit which resulted in no change. Next steps will probably be to run wires from the trunk along the outside of the car to the cluster itself, bypassing everything to see what happens.

 

Yip, same here, 3/4 when full.

 

If the gauge goes up to full sometimes, (eg going over a bump, under braking etc), and assuming you do not have one of those anti-slosh modules as fitted to later cars, about which I know nothing, then I would suggest removing the sender and testing it manually. It might be, for instance, that the arm is so angled that the variable resistor track is not getting to the end of its travel, even when the tank it full. Or that the upper stop is somehow preventing it etc etc, and a bit of judicious bending will do it.

 

Can you take a few photos of the sender. I suspect there may be a bad connection in the unit. I never pulled mine apart when I had it out so I do not know what is inside it. But if there was a bad solder connection on the rheostat, or if it is a carbon pot, the carbon could be worn this could cause the problem.

 

So as I said in my original post, the first thing I did was replace the sending unit with an OEM Jag one. The only result that this had was making the gauge read *slightly* higher.

I plugged the old sender (which evidently was fine to begin with) back in outside of the fuel tank and tested the travel manually. Even with the sender arm at the end of its travel the gauge still only reads 3/4. So I then taped the sender arm around the upper travel stop and took it for a drive. The gauge still jumps all over the place even with a totally stable signal coming from the sender. So I've ruled out that it's fuel sloshing around because even with a steady signal the gauge still is behaving erratically.

I was under the dash yesterday with the scuttle removed trying to bring my climate control back to life when I noticed that I could see the back of the cluster from under the dash. The plugs are fully seated and definitely making good connections to the cluster that has already been meticulously gone through.

Greg you mentioned a connecter between the dashboard loom and the loom that goes back to the sending unit; do you know where that plug is located? It seems like it's the only variable in this circuit that I haven't already addressed.

 

Great test! I think it is under the console, but I am not 100% sure, herewith a few other ideas, FWIW:
I have forgotten exactly how the rheostat in the sender is wired, but I think there is a 12 volt input and a variable output that goes to the gauge, plus an extra wire that sends the low fuel light 12 volts when the arm is sufficiently low. If this is so, why not try a hot wired 12volts direct to the test sender (ie bypassing the loom 12v) if you have not already done so?. Also, have you checked the connectors onto the sender, they might have fractured wires? Finally, you could get someone to look at the gauge while you BRIEFLY put 12 volts into the output, you should see the gauge go straight to the top, if I remember correctly.
Rather than pull the interior apart looking for a loom plug, you could run a test wire from the sender output to the cluster and test what the gauge does or does not do, first.
Greg

 

CP7 is the connector under the console, this connector has almost all the wiring from front to rear of the car. CP7 is in the middle of the car and unlikely to have had moisture although not impossible. I would start at the gauge.

The fuel gauge has 12V connected and the sender hangs off the gauge. So the sender is just a variable resistance to vary the current through the gauge.

Disconnect the sender wiring in the boot. Pull the cluster and check the screws in the back of the cluster first if they are all tight and no corrosion. Put a resistor in the back of the connector to ground, the gauge wiring is Light Green / Orange. This will simulate the sender and rule out wiring to the back if the gauge still acts up.

 

Warren
For the electrically challenged, does this mean the 12 v feed to the sender comes from the gauge, and then the rheostat part of the gauge varies the resistance and from there to ground. So the gauge is in effect measuring the variable current being allowed through it by the sender?

 

Yep you have it correct.

It is a little more complex as the gauge has 2 coils, 1 coil pulls the needle down and the other is connected to the sender that pulls the needle up. This is why a bad ground on the instrument cluster can cause erratic gauge behaviour.

 

May be considered an extreme solution, but my fuel gauge problems...and all other problems with all other gauges...went away when I put the foil instrument panel "circuit board" where it belongs (in the trash) and hard-wired the entire instrument cluster.

If you don't want to take on that much work (and it really was not all that difficult), you could just hard-wire the instruments. Yes it takes some effort, but having the gauges operate properly is quite a pleasure after wondering what they were trying to tell me during the first five years of ownership.

John
1987 XJ-S V12

 

Yep that'll fix all the issues with the cluster and has been on my todo list for a few years now.

The biggest issue being the connectors. The copper traces always detach from the plastic backing. The way the connector mates is not very good as the pressure on the terminals relies on the spring in the plastic/copper bond.