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I have a 1993 4.0litre facelift XJS and just recently the fuel gauge reads the tank contents but then swings over to full and this is whatever contents may be in the tank.

I have been told that there is a relay in the boot area that may be breaking down and causing this. I can see something in the left hand buttress (viewed from the rear) that could be a relay that seems to be fixed to what may be a small unit that looks like a small electronic control unit (approx 100mm x 10mm x 150mm), but have no idea what it is!

Can anybody advise where this relay is, what it looks like and if this could be the problem? Anyone know of replacement costs or if this may not be the cause, of any other causes; could it be the fuel tank sender unit and is there any way of checking this?

The gauge has previously worked fine and does not always do this!

FRANK

Frank,
I dont have a circuit diagram for the fuel tank sender, but IMHO, I dont think a relay would be included in the sender circuit, as its just a simple potentiometer circuit.
It appears your sender has gone a bit flaky, (going open circuit).
You'll need to access the sender connections, remove spare wheel and the trim at the forward end of the boot. Behind that is a steel brace which goes across the width of the boot, and that holds the fuel tank in place. I honestly cannot remember if the sender unit is accessible with the brace in place. If its not, you'll need to unbolt the brace. The sender is about 2" in diameter with two wires attached (spade connectors).
With the tank full, the resistance across the two terminals should be about 20 ohms. Tank empty, resistance should be about 200 ohms. Dont worry too much about the values, other people have reported readings of 80 to 900 ohms.
If you get an open circuit reading, then the sender is goosed. Fitting a replacement is pretty straight forward, and they are relatively cheap (£25),
http://www.northeastbritishparts.co.uk
is one source
Good luck

Cheers Nigel

Thanks for your reply. Just read it and printed it off. I'll have a look this weekend.

Regards

FRANK