Hi:

I have a 1999 XK8. While driving last week just after filling the gas tank, a yellow warning light came on the dash telling me of a 'range' failure. I noticed the fuel gauge read empty. It looked as if the electrics had turned off. Everything in the cluster was working. After turning off the car and restarting it, the gauge once again read full. For the next day or two, the gauge exhibited this behavior from time to time when turning the car on and off.

I stopped by the local shop and they suggested the issue was with the fuel tank sender assembly. It's possible that's the case. I tested the resistance in the sender tank in the car and it provided values like 100 ohms at 3/4 full and 250 ohms at 1/2 full. So it seemed to be working.

I haven't figured out how to test the gauge itself. I've read if one can bridge the plug terminals with 1000 ohm and 100 ohm resistors the the gauge reads empty and full respective, the the gauge is working correctly and the problem lies in the wiring or the sender assembly.

Any suggestions on what to do next. I'm trying to avoid spending $500 to have the shop replace the sender assembly just to find out the gauge is the issue and the cluster needs replacing.

Thanks

 

The gauge will be controlled by the car ecu, and the fact that the gauge and the trip computer both report an error with the fuel level, I'd suggest that the sender, or wiring, is at fault.
I don't know what the resistances should be, so can't qualify your test results.

If the test for the gauge is to put a 100ohm resistor at the sender plug and get a 'full' reading, then the fact that you have 100ohms at 3/4 of a tank would be incorrect, wouldn't it?

 

Thanks for answering.

The multimeter values were approximately in line with what I found on the Web - 1000 or 900 for empty, about 80 for full.. My guess is that the range error occurred when the ECU wasn't getting any fuel level data to calculate range equals something like fuel level / ave mpg. I'm tending to think its the sender. There's an electronic shop near by and maybe they can suggest how to put various resistance across the plug spades.

 

See if this link helps.

Link http://www.jagrepair.com/FuelTankSendingUnit99XK8.htm

Gus
www.jagrepair.com

 

Thanks Gus

The link provide a great explanation and the picture were very helpful.

I visited the local electronics shop and purchased 1000, 470 and 100 ohm wire resistors. Then I tested whether the gauge was working following the link's procedure precisely. All the values checked out perfectly. So it looks like it's the sender. I'm going to give it some time to see if it was possibly a loose connection where the wires plug into the sender. If it isn't then I'll have to decide if I should tackle the replacement. It looks really easy, but I'm hesitant to open a partially filled tank and reassemble. If I do, I'll use a new gasket rather than take a chance with the old one.

BTW: I purchased resistors with a 2 watt power capacity. They worked perfectly fine. Apparently, resistors come in higher power carrying capacities but those weren't needed.

 

Not a problem! As a side note when I removed the unit I did it with the tank at 1/4. Also I did reuse the old gasket but checked it out for cracks.

Gus
www.jagrepair.com

 

Hi,

I had a problem with my fuel gauge readings when I bought the car. The gauge's readings didn't match the miles I was doing (I always zero-ed the trip computer when I filled the car). However sometimes the gauge would fall to empty while I was driving. If the gauge is working, then the next best thing is to swap out the sender unit which is a cheap part and not complicated to swap out yourself. As said before wait until the tank is virtually empty to avoid siphoning out the fuel from the tank (which isn't too complicated either using a bit of garden hose down the filler cap).

Access is easy via the back of the trunk behind the removable panels. I bought the Jag sender part with new gasket and sealing ring and there was no special tools required. When I pulled out the old sender, it was a Ford part with a much shorter steel length to the float. Once it was changed out to the Jag part, things are back to normal.

Nothing to worry about and not worth garage fees.... but don't smoke during the operation!

 

Quick pic of the fuel sender (LHS Jag part, RHS Ford part)

 

Thanks all...

Ordered parts today. Will replace when they arrive and the tank is 1/4 full or less.

 

Hi all:

The new fuel sender assembly is in the car. I wondering if this behavior is unusual?

My understanding is my tank holds 18 gallons. After consuming the first 4 gallons, the gauge still reads full. When the tank reaches 8 gallons consumed, it reads 3/4 full or the number I'd expect when 4 or 5 gallons have been used. When 10 gallons are consumed, the gauge reads 1/2 which is closer to what one would expect. As the cars consumes more fuel, the gauge seems to read accurately until close to empty.

I've tested the dash fuel gauge using 100, 470 and 1000 ohm resistor tests recommended and the gauge reads spot on what the test predicts.

Any insights at this point would be very helpful. I'm out of ideas.

Thanks

Thom

 

I have exactly the same behaviour on my OEM part Jag part. After filling the tank full, I get 80km before the gauge starts going down from full. The warning light comes on at about the right time when I'm down to approx. 10L.

With the immersed fuel pump, I've seen warnings on this forum, that the fuel in the tank effectively cools the pump, so you should avoid driving enthusiastically on low fuel.

 

Hi all:

I have more information and would like to ask one more time for your opinions. Recall I tested the dash gauge by bridging the plugs with 100, 470 and 1000 ohm resistors. The gauge read 3/4 full, 1/4 full and empty respectively. Next, I filled the tank until it was full.

Today I measured the fuel sending resistance and got 144 ohms with the gauge reading 3/4 full. So the gauge reading was in the expected ballpark based on the resistor test. The problem is this. The dash digital display indicated that I had consumed 8.3 gallons and I verified this by refilling the tank. It took 8.4 gallons. So the fuel consumption estimate was spot on. Yet with 8 gallons consumed, I would expect the gauge to read something like 5/8th full and it does not.

As I watch the gauge drop as I drive along, I'll develop a correspondence table of gauge reading versus fuel sending unit resistance. I expect the table will be non-linear.

Is non-linear behavior normal in 99 XK8s. The local independent shop says it is, but I've never seen anything like this in any of the 10 other cars I've owned. If it's not normal, what could the problem be.

Thanks

 

Thom. Maybe I missed this,but have you also mapped sender resistance too? Sounds like you've hooked resistor of known value to sender wire, proving the display is reading correctly. I guess I start thinking about high resistance connections messing up the gauges precision.

When you originally posted the part about yellow light and empty reading I wondered about a swinging open connection. I do remember that the guage will not display any increase/update if you fill the tank with ignition on, the guage will show near empty until you cycle ignition off/on.

As I type this I wonder if we are onto something here. As I mentioned the guage will not show any added gas until ignition is cycled. I dont remember if the yellow light will stay lit too. If yes than what if you have a swinging open?

Gas good, say 3/4 tank. You hit a bump. Connection opens, resistance shoots up to infinity. High resistance = empty tank, light goes on. Guage will show empty until ignition is cycled.

Boy that'd be something if the guage cant show increase but will show decrease. Once again as I type, I realize this is self evident. Of course it will show decrease - that's how you manage when you need gas!

Go shake the wires with ignition on. See if you can force the empty error. Oh, and thanks for sticking with me as I thought this answer through as I typed :-)

John

 

Hi John

Thanks for the thoughts.

The original problem was a message showed a yellow problem light showed up in the center in the instrument cluster with a message saying something like "range' error. When this happened the fuel gauge read empty with the yellow low fuel light on. After driving a few miles, the warning message went away and the gauge read what it read before it fell to empty. This might have been the loose wire issue you suspected.

Nonetheless, I replaced the fuel sender unit guessing that was the problem Now when I fill up the tank from nearly empty, I turn off the ignition and withdraw the key. I put fuel in the tank until full. Then I start the car and the gauge reads empty with the yellow light on. I turn off the engine and restart. Now the gauge reads full with the yellow light off.

After the refill, the gauge moves towards empty much slower rate than gas is actually being consumed. As I mentioned in my prior post, I plan create a table of gauge reading versus fuel consumed to see how non-linear the gauge readings are.

Tomorrow, if weather permits, I'll shake the plugs to see if I can induce an empty reading even though I am 100% certain the tank has at least 17 gals in it. I'll report back on what I find.

Again, thanks

Thom

 

Hi Gus, Just been looking at the link you provided, my 2002 XKR-100 has a different fuel tank, looks like the sender unit is on the top, do you have instructions to replace the fuel sender unit for the 2002 XKR. Label on the tank states dual fuel pump tank.
Thanks

 

Sorry but I do not. It has been discussed on this forum and all you need to do is do a search to find it. From what I can recall the sending unit is in the tank with the fuel pump. If you have not replaced the fuel pump it would be a good idea to do that at the same time. This link might help!

Link

 

Hi all:

I am back with more information. First, shaking the plugs at the sender had no effect on the fuel gauge instrument panel reading. Second, I monitored fuel consumption and prepared the chart below. The horizontal axis shows how much fuel was left in the tank from full on the left to 1/8 on the right. I was able to take readings at each of the eight horizontal axis points using the following procedure. Step 1, translate each of the points into gallons consumed, e.g. 7/8th of a tank would be 2.25 gallons consumed in theory. Step 2, when the number of gallons consumed reading in displayed in the bottom center of the instrument cluster reached each test point, I noted the fuel gauge reading and measured the resistance across the fuel sending unit. The blue line shows the fuel gauge reading and the brown line shows the measured resistance at the eight test points

As you can see in the picture, the fuel gauge shows full until a quarter of the tank is actually consumed. Then the gauge declines almost linearly until the tank nears empty. The resistance put out by the fuel sending unit sort of looks linear from full across to 1/8 left.

So here's my question. Is this normal behavior for a 99 XK8. The gauge reads high until 75% of a tank is consumed. While I would not expect that, especially the gauge sticking at full initially, this may have been the behavior all along before I saw the range failure fault on the dash and replaced the fuel sending unit. I never paid that much attention.

What are your thoughts at this point? I'm stumped and the local independent shop says that just a Jaguar being a Jaguar.

Thanks for any advice you can provide

 

Yep, that’s a great effort to try and understand the differences.

Your gauge behaviour is exactly the same as mine.

Not really a big deal as it’s simply correct telling me to fill her up when only 1/4 tank left. I try not to get to the stage when the light comes on otherwise people say the fuel pump can overheat

 

In an ideal world, there would be a linear inverse relationship between the fuel level, the sender resistance and the gauge response to the latter. In practice that's not going to happen: - for example, the tank isn't truly 'square'.

These devices are only meant to be indicative.