Faulty fuel level sensor access

Please advise access to fuel level sensor[s]

Under the rear seat cushion there are access panels.

Dear Rob
Thanks for valuable information

Do you possibly have information which identifies pumps and explains logic of operation of the two pumps and level sensors

The official manual is no help



My electrical guide shows only one pump.

If it is not working, i would first suspect the relay in the boot.

The level sensors just feed information to the rear control module.
If either is not working, they could be stuck with rust, or its a bad electrical connection or a break in a wire.

I’m not sure why they need two. The pump, in addition to supplying the engine, circulates the fuel around the two tanks. The fuel tanks are like a saddle so the levels should even out.

There is a Technical Guide that has more information.

Here is a link to the Technical Guide for S Type. As Rob says, it contains a good description of the fuel system.

Shropshire, UK

Rob and Eric many thanks for your help After viewing the fuel flow diagram I regret the passing of the old one tank , one pump , one pipe system
Like you I cannot see the reason for two fuel level sensors unless they average the two signals to supply gauge . My pump works ok but my gauge has suddenly started to stop at 50% on way down , thinking that a sensor may have stuck I have just filled tank right up and the gauge is reading full ok
I will let you know if problem solved

Hi Warren,

If you said what model year your car is, I missed it but the diagram Rob included is good for around MY2000 and probably many others too. There is only one fuel pump, the other pumps that are referred to in the TG are jet pumps and work on the suction created by the fuel passing through a small orifice.

I always though the reason for the saddle tank arrangement was simply to provide capacity for enough fuel with an under-floor tank so that boot space could be maximised. I suppose the engineers designed this system with two level sensors so that that any level differential between the two sides caused by a fault would be detected and would lead to an indication as a fault code or warning.

I would start by having a look at the Rear Electronic Control Module which is located behind the trim panel on the right hand side of the boot. When you get the trim off you’ll see a metal plate and the RECM is bolted to the other side of this plate. If you undo the bolts you can pull the RECM out enough to get at the connectors.

Disconnect the battery first, then unplug CA101 & CA103. These connectors are on the side of the module with four connectors in a row. CA101 is the black 20 way connector at one end and CA103 is the large white one next to it.

First have a look for any signs of corrosion (not likely here because that module is well away from moisture) and then check the resistance between CA101 pin 15 (white wire with red tracer) and CA103 pin 23 which is a plain brown wire. You should see a value between 20 and 160 ohms and if you do the same test between CA101 pin 16 (white wire with blue tracer) and CA103 pin 23 you should see a very similar reading. You could try jacking the car up first one side then the other to see if the readings change commensurately.

If you do this with a full, half full and nearly empty tank you should see the resistance change with the range shown. If that proves out, then the problem is elsewhere.

The RECM encodes the analogue values as data and sends this to the instrument pack on the SCP bus.

In the past, I have looked at this data bus at the RECM with an oscilloscope when I was tracking down another problem but whilst it’s easy to confirm the presence of data it’s pretty well impossible to tell if it’s correct. Then you have start substituting modules.

Let us know how you progress.


Thanks Eric
Information a great help I will let you know how I get on
For the record I have a 1999 SE S Type 4.0l V8 in very good condition and a very nice and original 1990 XJ V12 and in the past have owned and enjoyed an XJ series 3 XK 6 , a number of XJ Series 3 v12’s and a XJS V12 . They have all been most reliable and I have never been stopped on the road I have also found the Lucas electrics very reliable I think at least part of this good experience is the dry weather in Australia with the bonus that salt is never used on the roads Of course regular preventative maintenance is required but that applies to all makes