2004 ML270CDI auto. Turn ignition key, cranks for about 2-secs, stops cranking/all ignition lights go out and headlights come on. Keep the ignition key in the start position and after another 2-secs, headlights go out, ignition lights back on and it cranks for a further 2-secs before repeating the episode. It can do this any number of times or for ever! But occasionally, or maybe the next day, it starts perfectly first time with no problems. Any ideas?

 

Update! It was nothing to do with glow-plugs although one was faulty (all new now) and it was nothing to do with the crankshaft motion sensor (that's new now as well!). Took the ignition key out, locked and unlocked on the buttons a couple of times and pressed the tailgate lock button a couple of times and all seems fine now. I will report any further developments. Suspecting immobiliser issue.