Wonky fuel readings

My brother has a problem with his 1998 X308. The fuel gauge is returning incorrect readings. He has replaced the sender with a known good item from the X308 that I purchased to get my XJR on the road.
The tank has to be low on fuel to facilitate replacement of the sender so we know the reading is going to be low. I removed 20 litres from the tank when I removed the sender so there cannot be over that amount in the tank when he replaced the replacement sender.
The display started at 20klm then went to o and back to 19klm.
Any thought out there where to go from here?

I know nothing about the X308, but on the last models of the XJ-S there was a black box called an “anti-slosh module” in the wiring between the sender and the gauge. I believe its intended purpose is to prevent disconcerting swings in the fuel readings. Whatever, it does fail, and surprisingly often, leading to weird symptoms such as you are experiencing.

There are some XJ-S owners who have dealt with a flaky anti-slosh module by heaving the POS over the hedge and connecting the sender directly to the gauge.

Thanks I’ll see if I can see anything on the wiring diagram when I (well the son) get my PC back up and running

Are you talking about the trip computer, when it gives you the distance you can travel according to the amount of fuel you have left in the tank? If so, there was a TSB about erratic readings many years ago. I can’t remember the exact wording , but it was somewhere along the lines of having to drive the car for a certain time/distance before the computer gives accurate readings. I think it may have been TSB 413-01 Aug 98.

It will be computerised in these for lagging.

I think There are 2 sensors in one and it does averaging I guess.

If the wire to the dash it broken for one it could have odd effects

I believe its is both the trip and the fuel gauge itself that is reporting unreliable readings.
Andy where do you think there are 2 sensors?

The X350 and X200 have SADDLE TANKS and use two sender for the average fuel level.

The X308 and X300 have similar tanks that are a single unit above the rear axle. (only one sender)

Here are some TSBs I got when I worked at the dealer.

413-01 Trip Computer Operation.pdf (45 KB)
413-04 Erratic Operation – Instruments.pdf (40.6 KB)
413-S703 Fuel Gauge Reads Low.pdf (101.4 KB)

yeah sorry bad memory.

They have 2 wires between the dash and sensor.

Ohm readings are:
900R when empty
80R when full

Thanks, I have forwarded these to my brother, hopefully it solves the problem, I’m thinking its going to be the ribbons on the dash cluster.

Thank you Gentlemen for your technical expertise on my Brother Robin O’Connors queries. I will look into your answers and hopefully it will solve my problem.
Regards Russell O’Connor.