With the headlights and sidelights off, the instruments are all reading normal and the dash lights are on and there are no warning lights. As soon as I turn on the sidelights or headlights, all the gauges fall to zero and the dash lights goes off and a number of the warning lights come on. No speedo, petrol gauge or anything. As soon as I turn the lights off, all goes back to normal. I replaced the blue relays with new ones. As the dash works when the lights are off, I would assume the fuses are ok.
Any thoughts on where to start ? Have an MOT in a week.
What year and model do you have, and which engine do you have in it? Jaguar made the XJ-S/XJS from 1975 to 1996 and there were lots of changes to the car during that period. Knowing which version you have will help others to respond.
Paul
Sorry forgot riot to mention. 1993 v12 facelift coupe.
Bad ground. Everything in the dash is grounding through the cluster lights and back through the headlight filaments to their ground. When you turn on the headlights, this ground path is energized at 12V and everything gets messed up.
You need a chart showing where all the grounds are located on your car, and there are MANY! I have such a schematic, but it is for an older model. Might be helpful, though…
PM me with your e-mail address, and I can send it your way.
I had the almost identical problem on my XJS and, as Kirby mentioned, it was a bad ground/earth.
Most XJSs benefit from removing the cluster to clean all connections and contact points. I suggest you do so…and add a redundant ground wire while you’re at it.
Cheers
DD
Thanks for that. Any idea where that ground would be ? the one on the A pillar where most of the dash cables seem to be or one of the grounds in the engine bay ? I found one black cable dangling from the steering wheel with an eye connector, and I cant find where it had been connected to.
I Might need to dislocate my thumb to be able to reach the earth on the A pillar. Which earth solved it for you ? the A pillar ?
Don’t know; didn’t care 
I cleaned the contacts points at the back of the cluster and the brass tabs in the harness connectors, and added a redundant ground at the same time…all of which is fairly standard procedure.
Problem solved.
Cheers
DD
Just an update. I removed the dash, at the back were two connectors which plugged into a big, flexible printed circuit on the back of the dash. I scratched each ribbon copper connector with the end of a small screwdriver and could see that is was oxidized and glazed over. Plugged it back in and all problems solved. I also cleaned the earths each side of the radiator, firewall and on the a-pillar, but ai think it was the oxidesed connectors on the back of the dashboard. Thanks everyone for the tips. I did a bit of what everyone said and seems to have worked.
Excellent! Another happy owner!