www.jec.org.uk

Under normal circumstances when my engine is at the proper operating temperature the temperature gauge needle is exactly on the ‘normal’ position on the gauge. Recently, however a fault with the gauge has developed: When I was sitting in the car and before I put the key into the ignition I noticed that the gauge needle had not returned to the zero position on the gauge, as usual, but had, when last switched off , only returned to a position about 1/8th of an inch up the scale from the usual zero position. I started the engine and watched as the temperature increased, with the needle passing the ‘normal’ position on the scale and stopping at a point about 1/8th of an inch above it. The needle had, in fact, travelled exactly the same distance that it normally does but between two different places on the gauge. The engine was not overheating.
When I turned off the engine the needle again failed to return to zero, but, this time, stayed even further up the scale, in fact, just below the ‘normal’ position. The next time I started the engine the needle climbed to almost the top of the scale when the engine reached it’s proper operating temperature and again, the needle travelled the same distance on the scale but between different places. Apart from minor discrepancies in the needle position, when the engine is running and when the ignition is off this is the way things are, now, with the temperature gauge on this car. I cannot help feeling that as this is not a temperature problem, only a temperature gauge problem, there would be little point in investigating the heating/cooling side of things. Replacing the temperature gauge is not an option.
Can anyone make any suitable suggestions?

The gauge is driven by a stepper motor and should re-calibrate its position every ignition cycle. In a kind world there's just been a glitch and a hard reset might help. To do that, ensure the ignition is off and then disconnect the ground/earth/negative connection to the battery. Touch that ground wire end to the positive battery terminal for a few seconds - this will not complete any form of short circuit, but will reset various modules and hopefully your gauge.

Reconnect the battery to see if that has helped. If not, then I see no option other than to start on the instrument panel and its connections. The first of these two videos includes how to remove the instrument panel and the second gives better detail on unclipping the connectors.

https://www.youtube.com/watch?v=Af3YRj60Yro
https://www.youtube.com/watch?v=Sh5AsKND3pM

Again it would be fortuitous if re-making the connectors was sufficient to resolve the problem. If not the internals await you if you want to go further. These panels can suffer from dry solder joints as they age.

There are also a couple of TSBs that while not actually addressing your exact problem, might be worth checking out

http://www.jagrepair.com/images/TSB/XK8/413-03.pdf
http://www.jagrepair.com/images/TSB/XK8 ... ailure.pdf

After all that you might have to consider changing the panel -not straightforward - or changing the stepper motor - probably for the advanced class! - or paying a recognized instrument panel repairer to get to work on it

Thank you for your very comprehensive reply. I have already carried out a hard test to no avail. I am about to look at the videos but would be happier for a professional to carry out any repairs than to try myself. I have tried one instrument repairer - Actronics, who can't help but know of no others. Perhaps you do. I'll keep you posted.

This outfit looks to be the sort of place that you might phone or email and see if they can help

https://www.googleadservices.com/pagead ... Qx6BAgHEAE

I have contacted Cartronix about this problem and they have told me that they can repair the stepper motor for £150 including vat and
£12.95 incl. for return postage of the instrument cluster if I send it to them; or alternatively they would fix it on a while-you-wait basis if I take the car down to them, for an extra £40 incl. I shall choose one of these options once I have decided if I feel confident in removing and refitting the instrument cluster myself.
Thanks again for your help so far.

I have now looked at both of the utube videos that you recommended and it is a great pity that while the second one is perfectly clear to both see and hear the first one is not. It is so indistinct in both respects that I would not be able to use it to feel confident in removing the dash cluster. I don't suppose you have access to another option?