When I bought the car the temp gauge read low - just above C which I guessed was a stuck thermostat. I fitted a new one and the gauge then read between N and H and fluctuated.........sound familiar Michael and Brian?
Laser thermometer indicated spot-on 88 degrees C at the thermostat housing and top right hand of the rad, so temp all OK.
The temp gauge on my black car always reads a rock solid low N so I swapped both of the sender units over (both red-top units).
Black car now reads solid high N. New car now reads mid N and fluctuates a little to high N.
From this it would seem that the sender unit can affect the reading (presumably as a result of age?) but is not responsible for the fluctuations.
Interestingly the fuel gauge reads accurately initially but rapidly falls. It reads accurately again at the start of the next journey but then falls away again. I have checked consumption and it is spot-on 25mpg so no problem there.
I remember from my Rover days that there was a voltage stabilizer connected to the back of the fuel and temp gauges and when it failed both gauges would do very strange things - is there a similar item on the XJS?
Also the battery voltage gauge kicks when a load is applied eg apply brakes - to me this would indicate some kind of earth fault? Is there a separate earth for the instrument panel and if so where is it located?
I would appreciate any suggestions from you Forumites as whilst these are minor snags they are a little annoying!
I will post photos of the mouse nest when I have taken them..............
Regards
Tony
