Hey All,

Got the Red Battery "Check Owner's Manual" message last week. immediately went to the Engine Data Screen and voltage was showing 12v fluctuating to 11.7v. that should be around 14 with a healthy alternator.
Hooked up my smart battery charger and it too was reading no charging.
For those that don't know, the voltage regulator on most cars is a wear item. Unlike many other cars where it is internal to the alternator, Mercedes, and I think BMW, mount them on the outside of the alternator.
Its actually the brushes that wear out. and from my usage statistics of having multiple Mercedes Models over the years, they are good for between 150k and 175K miles.
sorry I don't have pictures, it is a pain to replace but can be done without fully removing the alternator from the car.

If you have basic wrenching ability, own a jack with some jackstands, or even some death ramps. You can do it in about an hour with a lot of cursing and some scrapes and bruises.
Buy a new regulator(004-154-50-06), it was available next day at my local MB dealer for $159+tax. a remanufactured Alternator from MB, was $1300 Hindsight tells me to go ahead and order a back cover (000-154-70-04-64) for the alternator, mine disintegrated when I just touched it, and I didn't replace it, probably going to regret that.
1. Remove and protect the negative battery cable, I slid mine through the vent hole of the battery cover to keep it from falling back onto the battery.
2, Remove the engine covers, both inlet tubes to the filter boxes, right side (passenger in US) turbo intake tube. I also removed the bolts to the aux pump behind the right headlight so I could move it out of the way to easier get to the turbo hose clamp.
3. I am making this its own step because it is extremely brittle and easily broken and costs $135 at the dealer. REMOVE the Expansion Tank Bleeder line that goes from the engine, above the center top pully (or for fancy people, the swaybar pump) to the radiator and the expansion tank.
4.remove the Radiator Fan, one clip on each side then you wrestle it out passed all the hoses and tubes, and you don't have to worry about braking the bleeder line, because you removed it and placed it in a safe place, right?, RIGHT!?!
5. Remove and inspect the Serpentine belt, I didn't fully remove it as it was fairly new I just removed it from the Alternator and then pulled the tension up and out of the way. to release the tension initially you put a breaker bar with a I believe 14mm socket on it, though I could be wrong I don't recall. push the breaker bar toward the alternator to release tension.
6. Remove the 2 bolts on the front of the Alternator. shimmy it around (I used a small prybar) to get it released from its mounts. Once it is loose and dangling you can down do all the undercar stuff.
7. Jack up the front of the vehicle and put it securely on jack stands.
8. Remove the front and center belly pans.
9. Fiddle around with the alternator until you can, remove the rear cover, which is press fit on (or in my case it just disintegrates in your hand when you touch it), and get to the plug and the 12V cable, and remove.
10. You should be able to R&R the Voltage regulator. it is held in place by 3 bolts, you will get all new bolts with the regulator.
11. The regulator comes with some bad Ikea style instructions, that tell you to push down the cover of the brushes until you hear a click, it basically releases the brush into their working position, if you don't click it, it won't make contact.
12. The alternator also has sliding lugs on the back side that the bolts screw into, It is tough and requires more manipulation to get to the lugs, but I recommend using some channel locks or something similar to push them out, as it will make it harder to get the alternator to line up to put the bolts in.
13. reinstalling the alternator is pretty much the reverse of the previous steps, though I did put the bottom bolt in while under the vehicle, and didn't put the belly pans back on until everything was fully buttoned up.

Here is a youtube video that shows some of the steps, he didn't remove the radiator, or the aux pump, but I just found it gave me more room to move the alternator around.

Hope this helped.