Mercedes S Klasse diesel starting issue

Whoa, that sounds rough! I had something kinda similar with my S Klasse a while back. Battery kept draining, and it was a pain to start. Did you check the voltage regulator? That was my issue. Also, you mentioned the wiring harness... any signs of rodent damage? Those little buggers love to chew on things.

Regarding the voltage regulator, what specific symptoms did you observe before replacing it? I've visually inspected the wiring harness and saw no damage, but acknowledge subtle degradation is possible. Was your issue also intermittent?

Yeah, it was totally intermittent. Some days it would start fine, others it would barely crank. Eventually, it wouldn't start at all. The workshop guys said the regulator was faulty and messing with the power draw even when the car was off. Honestly, with electrics, it's a nightmare. If you've checked the obvious stuff, maybe get it properly diagnosed. Could save you loads of time and stress.

I sought professional assistance as you suggested. The diagnosis confirmed a defective battery and a parasitic drain, costing 290€. Your shared experience helped speed up the resolution. Thank you.