The alternator was faulty when I purchased the car (battery light on) I mention this but thinking it may not be connected with the fault.

A week later I have a fault condition (nearly always) when the lights are turned on (even through the auto light setting)

When the lights switch on (even the side lights). It almost seems like there is a temporary loss of 12v supply in the instrument panel. The instrument cluster and the dials all close down and then eventually come back on and the outside temp gauge defaults to degrees Fahrenheit. Once the fault clears after 29 second or so, the traction control light remains on.

That sounds very much like you have a fault with the alternator smart charging.

Using a multimeter measure the voltage at the battery terminals with the engine running at a fast idle and the lights off. What voltage do you get ?

Repeat the test and switch the lights on. Does the voltage at the battery terminals drop and then recover ?

Hi unofix - many thanks for the quick reply on this. I probably didn’t make it clear that I replaced the faulty alternator with a new one.

I have tested as you suggest and the battery is reading 12.7v. The alternator at high idle with the lights on or off is reading between 14.47 and 14.5v

The fault is intermittent and I cannot replicate it at the moment. It almost seems to be like a bad connection with the 12v supply to the dash being dropped momentarily - but only happens when the light are turned on!

Yes I understood the alternator had been replaced, but you would be surprised at the number of faulty replacements. There is quite often a problem with the small 'Smart' connector that plugs in to the alternator but with those voltage reading I think it unlikely.

Check that the negative battery lead is nice and tight to the battery post as this can cause similar problems to yours if it is loose. The instrument cluster has it's own built in voltage regulator  to maintain a steady voltage so as gauges remain steady. They can tolerate quite a wide input voltage before they are unable to provide a steady output.

My thoughts are this is an earthing problem, maybe a loose earth strap between the engine and the chassis. Can you perhaps create the same fault with the instrument cluster by switching on the Heated rear window, or using all the electric windows at the same time? 

Thanks once again - yes I tried turning on absolutely everything but it it only the lights going on (even if just the sidelights, that cause the problem)

will try your suggestion of checking the engine earth

Most of the lights are controlled directly by the Body Control Module (BCM) which is why you don't have fuses for sidelights (parking lights), dipped beam headlights, brake lights, etc.

Your car seems to be to new to be having issues with the BCM, but it has been found before that the connectors can become dislodged and make intermittent contacts causing problems with lights.

The BCM is part of the passenger compartment fusebox located in the passenger side footwell

The earthing points are a good start. If they look good then check the wiring loom and connectors going into the headlights themselves. Sounds like it is trying to pull too much current, so could be damaged wiring. Double-check that someone hasn't added any modifications that are tapped into the headlight loom (maybe they added extra 10000W driving lights?).

Thanks both - have gone through everything you suggest (the engine earth was not tight!)

The fault is not showing at the moment so I think it may have been resolved!!

Just one question - the dashboard LED display section dims marginally when the front heated screen is turned on - is this normal?

Yes thats quite normal 

Still don’t quite understand why the blip in power always ended up leaving the traction control light on when the instruments all restarted