DAmercS550

So on my 2012 S550 4matic i have been monitoring the in dash voltmeter to determine what conditions cause undercharging. The voltage shows that the alternator behaves like a smart alternator (which I believe is equipped on this model, it has the electrical attachment on the negative post). I start the vehicle and it will climb from ~12v to 13.6-14.8V when the engine is cold. When driving at highway speeds the voltage acts normal for what would be expected from a variable voltage alternator (after initial charge at 13.6-14.8V, it will drop to around 12.6v and vary slightly depending on braking and cruise conditions. The issue is in stop and go traffic, short drives, or when idling approximately 15 minutes especially under high electrical load (headlights, A/C on, etc.) the vehicle will fall below 11.6v if electrical output not managed properly (turning off A/C, etc.) eventually to the point i will get all of the dash indicators of systems shutting down (low voltage issue), which can lead to car not starting due to low voltage . I would normally believe this is an alternator issue, although i have doubts as i stated it charges normal under highway speeds. I understand there could potentially be other issues (such as voltage regulator, the fuse in passenger footwell, etc., im not really familiar with the aux battery and its effect). Also the service B and C reminders alternate showing on dash (not sure if this is an effect of issue, or if its just overdue i am prepared to do the alternator job but wanted second opinions before i begin throwing parts at it. If it is the alternator, i understand there are 180 and 220 amp alternators available for this car and im having an issue determining which one would be appropriate. I have recently purchased this car used and attempting to work a few issues out on it. Thanks in advance!

vettebk

I would charge the battery and then check resting voltage before a start up. If it is less than 12.0 volts at rest before start up, replace the battery. My 2007 S600 has a starter battery up front and a consumer battery in the rear. Your 2012 may not have the consumer battery. If it does, and it is original to the car; they typically last just about ten years and fail. BOTUS, one of our more discerning and knowledgeable members, has written about reviving a consumer battery. Alternatively, replace the battery.

On the charging system; check the serpentine belt, pulleys and tensioner. Many times a failing tensioner or bad pulley bearing will cause low charging because of belt slippage. You can end up replacing a battery, and then kill the new battery by a slipping belt.

You mentioned the other items which would normally also give you a code. Did you scan for codes?

Please report back with voltage at rest after charge before start up, any codes, visual results of belts and tensioner, and battery age (look for a label with a date). If everything checks out, I would then start to consider more aggressive diagnosis for persistent parasitic drain or bad grounds. But not until after you keep us posted.

BOTUS

yep - I read yesterday and didn't think of a sensible reply.....

however with brain part engaged - if the battery is being weird - the alternator and other interesting electronics Merc use won't know how to behave - as its simple and cheap - and odd charging lights on the cluster cause such long term hell on these (where no one every explains how it was resolved) - checking or replacing the battery is a sensible starting point

and NEVER run diagnostics on anything that matters - without additional support to keep the car at a voltage where its systems can contemplate telling the truth

DAmercS550

Thanks for the replies. Im currently out of town but as soon as i can i will check and report back. In the meantime, the battery (starter battery under hood) is about two months old. When i purchased the car it had a standard battery which i then replaced with the appropriate glass mat battery. I had the battery tested today so that does not appear to be the issue. I agree about check belt, pulleys, etc. which i will do. As far as the lights on the dash that only occurs when voltage drops down to ~10.9. When i charge my battery, or jump start the car all dash indicators are cleared. It appears to only do that as a means to protect electronics (power steering, abs, etc.) from damage. I hope this clarifies more.

BOTUS

Aux battery on these is to support the ISM (an electronic external gearshift mechanism - called the intelligent servo module) it allows it to get out of Park when the other battery is dead (I also expect it does the central locking - as it seems its just a budget get me out the **** support tool).

On other later models, car's with Stop Start snap my cam chain - the planet destroying emission harming feature - its to control the logic of when to destroy the planet - so when worn out it stops harming the car and the planet, and the world is a better place.

service counters don't do anything other than irritate the driver

if you hunt about for posts from people with the post late 2009 facelift cars - its very common they get the red battery icon nag in the middle of the cluster - many do the rounds of replacing all the batteries doesn't help, some then move to the a new alternator doesn't help, then a few brave ones (that haven't given up) get a bank loan and do the control module - then never report back if it helped..... Which makes me suspect its really all about Mercs usual tricks - so flash a software update at it and stop stressing, there was nothing wrong - just a bit of "accidental code" and your car magically found this bug after a certain time-frame


.

DAmercS550

Took it to a trusted indie shop. As of now looks like it was just the alternator, car seems to be running perfect. Will update if anything else pops up.