Hello MB experts. I have a question that I am hoping someone might be able to help me out with. Yesterday I pulled out of the local lube shop after getting my oil changed in my 2010 ML350, and about a mile down the road I got a dash indication of a red light with a battery sign in the center. There was no message accompanying the icon, and there was no noticeable loss in any electrical components. It is pretty cold here (I live in the midwest), and the car has started just fine for the past 2 days with no sign of a weak battery. My dash indication reads 12V when the engine is off, and that goes up to between 13-14V when started and under load. Everything is ok for a few miles, then the red light comes on again and the voltage under load drops to around 11V and remains there. I took it to auto zone to see if they could run a quick check on the battery, but was surprised that they could not due to the incredibly inconvenient location (under the passenger front seat apparently). I also was told that there are two batteries in this vehicle...the main and an auxiliary that are located in the same compartment. My question is regarding the indicated voltage on the dash display. Is the value that is displayed indicative of the big battery, or the auxiliary one? Also, is there a good way to diagnose which battery is the culprit without pulling them both out and having them tested? That just seems like a lot of hassle to me. I am thinking that the alternator isn't to blame here, as I would think both batteries would be toast by now if it had failed. I bought this vehicle in 2014, and have not had any issues with the batteries or anything else for that matter in the time I've owned it. Maybe it is time to have one or the other replaced? Just thought I'd come here and hope that there might be someone who can point me in the right direction of an easier way to diagnose the source of this battery light. From my experience in other vehicles when that light comes on it is usually accompanied by dimming lights or other tell tale signs that something is wrong...but nothing here yet. Hoping to get it figured out and replace the culprit before the temps drop into the -20 to -30 range this week. Than you for your help!

 

Do you have an owner's manual?

Do you have a DVOM?

Was the battery reading of 12V just that or was it 12.1 etc? Batt. voltage w/eng. off should be 12.6-7V

Using the jumper points in the diagram, test voltage with DVOM with eng. off and with eng. running. Post results.

 

AV8R1978,

A red light with a battery in the center message means that you will need to replace your auxiliary battery. The voltage displayed on the dash after pressing your instrument cluster (R) 3 consecutive times is indicative of the big battery, not the auxiliary battery. After replacing your auxiliary battery, I am not entirely sure if you will have to perform a hard reset to clear the message, or if the message will disappear by itself.

 

So if the voltage I'm looking at on the onboarding voltmeter is reading off of the main battery, why do I want to replace the auxiliary battery? It's just confusing because everything looks ok during starting and for the first couple miles, then the voltage drops to around 11V and the light comes on again? Is the red color specific to the auxiliary batter? Would it be a different color if it was the main battery? I just dont want to buy the wrong thing.

 

Not true, the WHITE light is for the auxiliary battery.

 

AV8R1978,

Take a look at this thread as a helpful reference.

https://mbworld.org/forums/s-class-w...lternator.html

 

Speaking from experience and what I encountered when I had my 09' ML350. Can't speak for a 06' ML500.

 

Thanks for all of the input. I'm thinking either the main battery or alternator at this point. Went to take it to get checked out today and it was dead in the garage. Very surprising as there was zero indication of any drop in power, no dimming of any lights or anything like that when I parked it yesterday. I had roadside come out and jump it and it fired right up with no problem. Service tech advised to let it run 10 mins or so which I did. Turned it off and went to start it again and it was dead. So...either the battery is shot and cant jold a charge, or the alternator isnt charging it. Plan is to pull main battery tomorrow and take it in for diagnostic test. When it was running after the jump the voltmeter read 12 under loading, but it obviously wasnt holding that charge. Could that 12V be coming from the alternator?

 

Gee's man - dead battery - YES !