Yesterday on my way home the convenience feature disabled message came up, shortly followed by the ALTERNATOR/BATTERY warning (VISIT WORKSHOP). Of course, car put itself into the fail-safe setting 1 where it disables things such as AC, seat control, etc. Got the car home and after looking at the aux battery in the car, it looked a few years old so figured Id replace it to start. Went to Advance and bought an Autocraft BS-14 battery (main battery was replaced 2 months ago - Autocraft Gold). After installing the Aux last night, nothing changed. I drove around the block and the error didn’t go away.

Fast forward to this morning (after figuring out how to access car voltage on cluster screen), turned the key to position 1 and UB Voltage was showing 11.7-11.6V (low). After firing the car up on cold start, it went down to 11.2-11.1V. I had 2 errands to run, and after driving the car 15 minutes, it sat for another hour, and then I went to start it and it struggled starting. I then drove it over to a clients shop nearby where we pulled both batteries to throw on the tester and although both tested fine, they were a bit low on juice. Charged both batteries to full in a few hours and went to put them back in the car and it went up to 12.6V in position 1, and then down to 11.5V at idle (higher than before but still low). I turned on the headlights and other things that were still available, and the voltage starting going down near 10.9-11.1V - all while still at idle.

So, at this point (without SDS), is that a dead sign that its the entire alternator, or can a faulty voltage regulator cause this? Also, where exactly does the live "UB" voltage reading originate from thats shown in the cluster? Is it a direct reading from the alternator regulator itself, or is it a combined voltage from all of the components together? Or from something else? Either way, it seems most of the issues are associated with either a faulty voltage regulator, alternator, battery control module, or even the front aux battery relay?

Also, am I missing another check feature to read separate voltages in the car from each of the individual components? Like I said, I thought that would be something only SDS can access. Even if I could see the different voltages in the front and rear batteries that would at least tell me there is a weird fluctuation which would lead me more to believe a faulty Aux Relay or BCM....

Thanks for any and all input.

 

With the engine running and a volt meter to the battery it should read over 13.5...

 

its not the bcm or the aux battery relay. its either alternator or regulator. no real way to tell. throw a regulator in to confirm alternator or regulator. if its high mileage id just put a new remanufactured alternator in.

 

Agreed. Tomorrow is voltmeter test. Aside from testing both batteries, is there another way to test just the regulator....or entire alternator while its in the car?

 

The dash already told you what you need to know, it's not charging. Buy a bosch regulator for 50 bucks and try it.

 

I wouldn't put anything but the OE alternator back in. I always try to save some cash where I can, but from having seen a friend go through 3 aftermarket alternators none of which seemed to work quite right, I will likely be putting an OE part back in. Unless of course someone here has real world experience using an aftermarket alternator?

 

^^^ correct! Get a remanufactured OEM alternator through mercedes.

 

Before you even try the alternator
If you know someone with STAR get it diagnosed.
I had same issues with my car (now C55asleep?) I replaced both batteries, voltage regulator, the relay in the back, the relay under the windshield cowl, and had Yachtmaster do a once over and i gave up. I had a tech at MBFTL scan it and it showed the front driver SAM (Signal Aquistion Module). I had it replaced for just over $100.

Issue went away or became C55asleep?s problem.
Otherwise you will just throw money at it until your guess is correct and its resolved.

 

Wanted to give this thread closure. Ended up being a faulty alternator, in fact the internal plastics were beginning to melt. No surprise for a car that sees 90 degree year round and 600+ hp. I replaced it on sunday with a bosch unit and all is fine. The install was actually quite straight forward and I only unhooked one small coolant line. 3-4 Hour job total in my driveway.

With that said, I now have both a brand new and used & functional Bosch voltage regulators for sale if anyone is interested in a cheap spare or quality replacement.