Have looked on the forum for help but not found a similar case. Started my car no problem and drove about 2 miles.

All was well but suddenly the engine cut out and I coasted to a halt. Turned the key but no ignition lights on and starter would not turn. But fan keep on and I had only gone a couple of miles in coolish weather.
Checked car over and only electrical circuits working were the fan and immobiliser light. Fuel and all wiring seemed ok.

Had to turn fan off by taking out the fuse. Main fuse 50amp was blown so replaced it and it instantly blew again.
Clearly some major electrical fault.
Can the boffins on here help and give me some direction. Any help/suggestions appreciated.

Major electrical faults are often linked to the Rectifier, so much so that many of us carry a spare. That's where I would start.

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Your problem looks like one I have allready had. If the 50amp fuse blowns. you have a short cut somewhere and still in place. You have to find where he is. It can be long and difficult. In my case, it was just a small wire burnt in the immobiliser device. Desperate to find the problem we asked some help by phone to MMC. They asked to check the immobiliser and to try without it by switching to a non immobiliser plug in taken from a 3W in the showroom without this option. Miracle everything was operating again. Just a wire to replace and the immobiliser was ok again.

Yes almost certainly a short circuit somewhere from either cable or component Vitaloni ?? Problem is that 50 amp fuse feeds a thickish red cable which splits around 350mm further down the loom to ten different red distribution cables!

Six of these cables have secondary lower current fusing via direct feed to main fuse box, so they would have blown before 50 amp one and can be discounted unless there is cable chaffing causing a short en-route to fuse box.

This leaves the following red direct main feed cables, R23 to engine relay, R24 to start relay and R29 to ignition switch. There is also an R80 which I haven't traced yet, accounting for all ten red cables.

At this stage it would be helpful to know just when the replacement 50 amp fuse blew. Was it as soon as inserted with everything off, when ignition turned on or when a start was attempted? With this info I can be more specific in where to look.

Regarding fan staying on, with 2014 on models it's fused via one of the feeds from main 50 amp fuse. I'm guessing therefore it's an earlier model with retro fitted fan wired direct to battery via its own fuse. Perhaps you could also confirm.

yup Richard v6 , retro fitted fan direct to battery. As I said i started the car up and all was well and drove about 2 miles and it just cut out with no warning or apparent reason and I coasted to a halt. I switched off the ignition and tried to restart it was no ignition lights came on and starter did not activate. I checked car over and could not see anything wrong nor any wires amiss. But the 50 amp fuse had blown and a replacement blew instantaneously as soon as i put it in. The ignition was off.

Car is not back home yet but will start re looking for the fault when it is but really , am not sure where to start. Thanks for replies so far, it is appreciated and all suggestions are welcome.

Thanks for verification Vitaloni. As the 50 amp fuse blows instantly the possibilities unfortunately are many, a short on any of the spliced together ten red distribution cables mentioned above or an ignition switch failure even though it's off. Doubt the two relays mentioned above would be the cause as neither would be energised with ignition off and no permanent grounds on them to cause internal short.

Suggest a check for anything chaffing from 50 amp fuse to eleven way splice and on to each of the ten red cables fed from this point. If nothing found would turn attention to ignition switch as this is the only component fed directly by 50 amp fuse and permanently live to its input even when off, red cable to black connector. If you have a plentiful supply of 50 amp fuses pull off this connector and try with another fuse.

It's tempting to benefit from MTWV2 experience but his immobiliser was working with everything off and fuse only blew with ignition on if I understand correctly. For obvious reasons immobiliser requires a permanent 12 volt feed but this is fed by fuse 3 (15 amp) in main fuse box which would have blown saving 50 amp one given immobiliser short.

Sorry tracing of this fault is onerous but when you do find short it will be pretty obvious given its instantly taking out a 50 amp fuse.

BTW try as I might I cannot find the other end of the tenth red cable from main feed splice. It appears on wiring schematic for both early and late M3W coded R80. It could be a spare feed never adopted on loom. If this is the case the loose end should be securely insulated but who knows

Probably stupid , but try it in the dark. The current to blow a 50 amp fuse might produce a brief spark, if not an audible click.

Thanks all for your advise and suggestions. I have been through all the wiring I can see, and removed battery etc to get better access and cannot find any wire that appears chaffed nor seen anything amiss. I have ordered some 50amp fuses so will proceed with the investigation when they arrive. Thanks again.

I would still recommend isolating the rectifier to see whether whether the fuse still blows. Disconnect the rectifier multiway plug and also temporarily remove the thick cable from the red terminal post just in front of the oil tank. It's a quick and easy test and will either point the finger at or exonerate the rectifier.