Fuel Gauge Problem

[Mike B]

Hi, can anyone help with a fuel gauge problem?
Series 2 2.0 turbo exclusive.
The fuel gauge works for about a minute on switch on, then swings to full with the empty light flashing.
So far I have checked the sender and plug on top, swapped the entire instrument console from my other XM which works fine in the other car, soldered in a new green plug which connects to the instrument console, cleaned the plug at the side of the drivers carpet and cleaned as many earthing points as I can find. No success with any of the above. All gleaned from other advice on here and elsewhere.
All I can think of now is some fault with the trip computer? I thought it was part of the instrument console?
Any ideas before I plant a bomb in it!
(Fuel gauge has worked fine for 6 years!)
Cheers
Mike

[Peter.N.]

Strange, showing full with the empty light showing. You have tried just about everything I can think of. I wonder if there is a separate voltage regulator for the fuel gauge which isn't on the instrument panel.

I should check the voltage to earth from both connections on the level sensor and see if they change when the fault occurs, on should I think be at permanent earth potential anyway. If you short the connections together it will normally read full.

Never had one with a fuel computer, mine were all 2.1s so can't comment on that.

Peter

[Mike B]

Thanks Peter,
I thought there might be a short making it show full and tried to check the wiring as much as possible.
What's puzzling is it works normally for the first minute or so before going wonky?
I'll check more with the multimeter as you suggest when the fault occurs.
Cheers
Mike

[xmexclusive]

You have sorted the green socket so how about the plug?
Check the wires that connect to the green plug.
These are IDC push fit connections and can go intermittent with age.
First check the wire tightness in the contact blades then if needed ease the blades a bit tighter.

John

This post has been edited by xmexclusive on November 02, 2015 11:03 am

[Mike B]

Hi,
It was another plug I soldered in, in desperation I cut the wires one by one and soldered in a well checked plug.
I had already swapped the instrument console from a known working car. Also the original instrument console works fine in the other car.
I had read about the IDC connections going wonky.
I'll have another go with multimeter this afternoon.
Thanks
Mike

[xmexclusive]

Hi Mike

I have had a quick look at the Mk2 wiring diagrams.
I checked a couple of the diagrams but these may not be specific to your XM.
These show two options for item 650 the fuel sender.
The wiring from the sender to the dash display is shown as 3 wires.
There is a multi pin plug where these wires fit into the top of the tank sender.
At the other end on the green plug these are wires 1,4 and 3.

On the first variant the full tank resistance value is seen between pins 1 & 2 of the tank sender plug. Pin 4 of the tank plug gives the variable (tank quantity output).

On the second variant the variable resistance (tank quantity output) is seen on pins 1 & 2 and no signal is connected to the other two pins.

According to model the dash display is set up to process only one of these signal variations.
I suspect the model difference is petrol/diesel but cannot be sure at present.

A check with a meter should show which type of sender you have fitted.
Remove the sender and test well away from the tank if petrol.
Meters use voltage while resistance testing and might generate a spark.

The wrong type of sender fitted or the wrong type of dash display might well give the symptoms you describe.
Otherwise it needs wire continuity checking between sender and display.

Put up your full model, engine type and build date and I will see if I can find and post the relevant wiring diagram.

John

This post has been edited by xmexclusive on November 03, 2015 10:28 am

[Mike B]

Hi John,
The wiring diagram would be very helpful, the one I have found isn't much good.
The Orga no is 6653 and it is a 2.0 turbo exclusive with the XU10....JTE engine.
The fault has slowly appeared, only doing it occasionally to begin with. It would happen if you switched the ignition on without starting the engine for a few minutes. Now it does it after about a minute regardless of starting the engine.
I don't think the two variants are petrol or diesel related as I have a 2.1 td with Epic pump that has the trip computer also. The instrument consoles can be swapped between the petrol and diesel cars. Both consoles work in the diesel car but not in the petrol one.
Is it possible that the two variants are for cars with and without the trip computer I wonder?
My 2.0 turbo petrol has 5 wires from the sender, 2 for the fuel pump in the tank and 3 for the gauge.
The time delay of about a minute from switch on for the fault to appear is what is puzzling?
I will have another go with the meter.
Many thanks
Mike

[xmexclusive]

Hi Mike

I think the trip computer is a red herring as far as your problem is concerned.
To run the trip computer the car needs an engine fuel injection ECU.
These ECU's supply fuel used information as a signal on a single wire to the dash.
No fuel injection then no trip display.

The resistance sender in the tank is a semi-circular track of elements with a metal contact.
These track elements can wear out which can put very strange variations in the signal sent.

Will try to sort out the correct diagrams.

John

This post has been edited by xmexclusive on November 03, 2015 10:32 pm

[Mike B]

Hi John,
The trip computer works fine apart from the approx distance on the remaining fuel function which needs an input from the fuel gauge or tank sender. That only works for the first minute when the gauge is functioning.
Got another sender unit from a friend today but it only has 4 wires so it is probably the other type you described earlier. I'll give it a try though, you never know
Cheers
Mike

[Mike B]

Hi John,
Finally got to the route of this problem. Turns out it is the fuel gauge sender in the tank.
I took the one out of my diesel car and connected it up in the petrol one and the fuel gauge worked normally! Yahoo. (Both cars have mpg trip computer)
Petrol one has a fuel pump in it of course so they are not interchangeable but the gauge sender unit is the same.
Endured weeks of tracing and checking the wiring from sender to dash - ah well.
Now all I need is another sender unit for a 2.0 turbo exclusive with trip computer.
The sender has 5 pins instead of 4 which the non trip computer cars have.
Cheers
Mike