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My 2014 GL350 just failed on me in the Oregon heat. No power. When I jump start it and remove the external battery pack it stalls and shuts down. I had a chance to take battery terminals Volts measurements before it shuts down - around 9 volts.
I assume my alternator dead... Begore i get a diagnostic results from the local dealer and they propose to replace it, at 2100 USD just for alternator!, is there anything i can suggest to them to try? Could that be a stretched/failed belt? Or a "voltage regulator"? Parts adviser says he can see a "voltage switch" in the catalog. Is that the same thing? I spoke to few people and they say there is no such thing as replacing voltage regulator on these vehicles. I really really hope it's not gonna cost me 3k in parts and labor to resolve this.
I just did this, but it's a M278 gas engine and not the diesel so I don't know if you have the same alternator. It cost me hours of my time, but only a few bucks for the part. I'm in Oregon too.
buy your own alternator, wait for the guy to come install it. Some radiator hoses have to be moved so have a gallon of coolant handy to replace whats missing. when did you last replace the battery?
yourmechanic.com - They quote just over 2k with refub alternator being around 1,800 USD
"when did you last replace the battery" - yesterday. It didn't help. The car drove for about 30 mins and died again.
Anyway, I am not local in OR, leaving the car at the local MB shop and flying back to Vancouver, BC and coming back in a week with a refub alternator that cost around 1K USD. Surprisingly it is cheaper in Canada. At least something is cheaper there!
And regarding the idea replacing just a voltage regulator, I wish I could do this locally in OR, but MB tech says - "you need a new alternator". There is no way for me to know if he tested the regulator or not. I might bring the new regulator and ask them to install this first and see if that works...
You could do just the regulator but later in life the bearing fails anyway albeit considerably later. My voltage regulator was failing but the bearing was also shot anyway. I see what you're going through. If we had more time to plan I could get you probably repaired for 1100 or so. On the older Mercedes it was possibly by only a few people to replace the voltage regulator, it has these spring loaded contacts that maintain contact with the alternator at all times, this device eventually wears down over time or degrades in performance from heat cycles. It's mounted on the back of your alternator which these days require you to practical remove and reinstall the alternator anyway. If you're paying the labor you might as well have a new alternator. I'm sorry the quote is so expensive up there, I'd shop around I can get this deal here and I live in one of the most expensive parts of the country. I also do this service for around this much. Use Yelp, "Mercedes mechanic", sort by reviews, call the top 3 and be done. Don't necessarily compete on price but how badly they want your business and time availability they're willing to make for you. If we can chop $1000 of the bill I would feel much better.
They don't do any real testing at MB, they just "fire the parts cannon" and you're on the hook for their trial and error in most scenarios. I'm pretty sure they're right this time. Shop, call around. The alternator install on that engine isn't particularly pleasant but a good shop can get it out for $300 in labor and they're going to sell you a Bosch reman alternator for around $500. If price isn't the issue just leave it at the dealer and call it done. Making $400+ for a couple hours of work is a decent deal for any shop.
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yeah your new alternator comes with a new voltage regulator. it's job it to keep you running at 14.7 while charging and 12.whatever while youre running
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I have cancelled my original order and ordered this Valeo alternator from Europarts. It saved me $700+ CAD. 
