Hi first post! I've been having issues with my 2005 Mercedes E500 4Matic. A few weeks ago my battery and alternator would not charge, would give me the red Braking Light and turn off all functions aswell as limp mode and the brake not working, having me to press it fuilly to even stop for real. It would turn off and the insturment cluster would turn completely black, with the car still on before it fully shuts off. After that it would need a charge and if I was to take out a jumpstarter/ or jumper cables off the main battery it would do the same thing or even shut off immediately. Looking at the forms I assumed it was the alternator and the battery so I replaced them both and at first it was working like regular... then it would start to do that again, as if the issue wasn't fixed however the car wouldn't turn off completely, And would just drive on limp mode though turning it off would have me required to jumpstart or charge again.

The voltage would be around 11v and under after the alternator fix.I'm not sure if it's the Aux Battery messing up, the BCM or whatever, I'm very baffled on what's going on with my Car and I'm trying hard to figure it out. I had someone ironically that was supposed to scan my car with Xentry but they never came though. :/ Any ideas?

 

If voltage is 11 volts and under after the "alternator fix", the alternator ain't fixed. The car ran as long as the 900-1000 Cold Cranking Amps battery could provide power and then the car began it's scheduled "graceful degradation" of shutting off systems one-by-one until you either park the car as soon as possible or crash it into a ditch because the auxiliary battery (emergency power to the SBS braking system) runs down, and the computer actuated brakes fail.

Could be the "new" alternator. Could be the Battery Control Module. You either need the money to pay a good independent shop to diagnose or have the ability to do so yourself. I don't know your background so you need to decide for yourself.

Do you have a diagnostic system that will work with your W211? If not, get one. If you cannot afford the diagnostic system, sell the car. Seriously. Not being a dick, just telling you what I have learned over 12 years of ownership of two AMG Mercedes. The last five weeks cost me almost $1500 in parts for my W211. That can be frustrating because of the money and my time invested. Inspected my W210 today and it looks like I will be out a few hundred dollars in parts and a day of my time to make it safe and reliable again.

Assuming you have the ability (not slamming or disrespecting you, I genuinely do not know your skill set), download the WIS (Workshop Information System) from Charm.li. You can read it online or download the .zip file and install on your computer. There is a rather lengthy section detailing how can diagnose the charging system end-to-end exactly as the MB techs would do so.

Be advised the .zip file is 1.2GB in size and Win11 will choke at 1.0GB. You will need an aftermarket downloader for the .zip file if you want to keep it on your computer. If not, just read it online.

You can pull up the voltage on the electrical bus on your instrument cluster. You will find several videos of how to do so on Youtube.

EDIT: By the way, if you have Keyless-Go or not, do NOT have the key fob in the ignition switch or within six feet of the car when you connect jumper cables or install a new battery. Doing so risks blowing fuses or damaging digital modules. Connect jumper cables or install new battery, then bring Keyless-Go close and/or insert key fob in ignition switch.

Best to you.

 

Yeah I’m already knowing to get the Diagnostic System, the person who was supposed to scan it was also giving out his codes for the software funnily enough.. and I also got that 11v from the instrument Cluster. I don’t have any keyless go luckily. And I noticed that my key fob doesn’t lock or unlock th car since replacing the battery and alternator.

saturday I’m having another person look into my car to see what th issue is before I get to throwing money and or parts. It was already an hassle to get th alternator in, that damn bottom bolt… :^(

but before I got the car I was doing research about the car and all its problems just to know what I’m getting myself into, as majority of my car issues I can manage myself, and even after and during having the car even now I still research, but yeah nonetheless I’ll keep the thread updated on what it could possibly be. Like a month or 2 ago I did a scan with a Icarsoft that allegedly did “coding” but that wasn’t true so I refunded it. But I still have the codes if you guys want to see.

 

Trying to diagnose problems without having full power (min 11.7 v engine off, and 13.6 v engine on) is like checking your oil with you car on a 45 degree slope. Youll never get a real reading. As noted above, spend some money on a decent scan tool - it will save you 1000s in the long run.

 

I went to autozone to get my batteries tested. My auxiliary battery was shot completely bad, while my starter battery in the trunk is good albeit cranking health “fail”. Do you think this possibly could be a culprit? The people at th desk was telling me how the alternator was in perfectly working condition however because the charge was at 100% even. So the alternators out the picture…

 

The modules in these cars can be very sensitive to fluctuations in voltage. Even small variations can impact one, which in turn impacts another where they are reliant on each other. The Aux battery is meant to be for the Tranmission shifter module so you can change to neutral when the battery is flat - but I believe it might support other functions as well.

Change your auxiliary battery and then start again. But you need a scan tool. Until then, everyone will.be guessing.

 

Best advice is to get Star Diagnosis done to help pinpoint the problem. First things first: check the battery ground connection to chassis and make sure not corroded or loose. Check the output voltage at the alternator stud with engine running and compare to the voltage at the main battery. Volts at alternator should be > 14.2 Volts at the alternator stud. The voltage at the battery should be > 13.5 V or so depending on the load and quality of connections. This test is a good check for any loose or corroded connections between alternator and battery. You could turn high beams on and off and look for major changes in Voltage at the alternator which would also point to a bad voltage regulator. Be SAFE and connect the test lead at the alternator stud with the ENGINE OFF and ensure the test lead wire is free & clear of rotating parts before starting the engine for the test. A fluctuating or relatively low voltage at the alternator (<13.5 V or so) could indicate a bad voltage regulator on the alternator (not likely for a new alternator but possible).

Any RED messages with battery symbol on the instrument cluster could be due to main battery, alternator, aux battery, aux battery relay, battery control module, front or rear SAM module, etc. A good scan tool will indicate where to look before changing out components wholesale.

 

The Auxiliary battery is intended to provide power to the SBC computer-controlled braking system in event of a main bus failure. I've posted on this before and attached the technical documentation to support this. The main battery is the main battery, not just a "starter" battery. It provides power to all of the modules on the distributed digital bus. If it fails, the Auxiliary battery is engaged to provide 15-20 minutes of power before complete system failure.

It is possible the deep discharge of the battery after electrical system failure caused the BCM to go into a "safe" mode. If so, it can be reset by disconnecting it from the harness for five minutes.


If the BCM actually failed, it will default to a (IIRC) charge voltage of ~12.7 volts; not enough to keep the battery charged.

To test the charging system from end-to-end, download the Workshop Information System and perform the following tests listed in "Testing and Inspection":


Description and function of the dual-battery system is attached to this post. Save it. Read it.
SBC system tech documentation is also attached. It explains what happens when the main battery and then the Auxiliary battery fail; two solenoids fall open when power fails and you now have unassisted hydraulic braking to the two front calipers. At that time, I recommend stomping on the parking brake and then standing on the main brake pedal like King Kong while downshifting the transmission into lower gears as fast as you safely can. The car is literally a two-ton mass of metal with very little to slow/stop it if the brake-by-wire system fails. Plan ahead and practice emergency procedures "just in case".

 

Very helpful. Thanks!

 

Just fyi, a failing AUX battery will cause the Red VISIT WORKSHOP error to pop up on your dash. If you are based in the US, just letting you know the SBC brake system is covered under a 25 year warranty. 25 years! So if any brake related error message pops up, might be worth visiting a dealer for a free fix. In my 05 E320, I've had the SBC pump and the pressure reservoir replaced under warranty.

If you are having a charging issue, the brushes on the voltage regulator are a common wear item on the alternator. You should be getting above 14 volts when the car is running with a healthy alternator. The voltage regulator can be changed somewhat easily if you have some basic skills. Just need to get under the car to get access.

The AUX battery is basically a small motorcycle battery. It doesn't cost that much, but most auto parts store do not carry it.

Lastly, highly recommend a CTEK battery charger to keep both batteries fully charged. These German cars do not like having weak batteries.

 

^ +1 to what F1Fan said...

 

I found th issue! Whole time it was the alternator that still wasn’t working… I had a hunch and took a few steps back and took out the alternator I bought in Janurary and tested it. It didn’t work completely. AutoZone gave me a terrible alternator and I just went to AutoZone to replace it and now it’s working all this in intended. They said there was an oil leak, but the car was also sitting for three months so gun probably went into it and I also didn’t torque the studs properly which probably had a hand into it. Thanks guys! Crazy to see how simple problems can be..