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Need Technical Info, Please

5K views 14 replies 4 participants last post by  Bimbis 
#1 · (Edited)
:help: Can someone explain the function of the Auxiliary battery? What happens when it is fully discharged/dead? Also, is the charging rate of the main and/or auxilliary battery controlled by one or more computer modules? I am chasing a phantom on my 2010 E350 Sport P2. It occasionally will not start, acting like a dead battery, with loss of dash indicators. The dealer has had it for a week and a half so far and cannot determine the root cause. When I jump it off, it runs fine, and even starts fine after a relatively short period of charging with the engine running. I'm hoping there is a technoid out there that can help me. I speak technoid, having been a control system engineer for 30 years.
 
#3 ·
Yes, my car has the keyless go feature. I am not assuming the auxilliary battery is the problem. Rather, I am trying to determine if the loss of dashboard function could be related to the failure of the associated SAM to communicate on the CAN, even after the car is jumped off and running, and if the auxilliary battery is there to prevent this loss of dash function.

According to two different people at the local dealer, the dead battery/will not start problem on the W212 series is known to MB, and the status of the correction remains "under investigation", and has been for some time. They both indicated "...we are seeing a lot of dead battery calls on W212s..."

I'm a retired control system engineer that is trying to understand a bit more than I do about how the W212 CAN, SAM, and power systems are supposed to work, but don't. Any insight would be greatly appreciated.
 
#11 ·
Yes, my car has the keyless go feature. I am not assuming the auxilliary battery is the problem. Rather, I am trying to determine if the loss of dashboard function could be related to the failure of the associated SAM to communicate on the CAN, even after the car is jumped off and running, and if the auxilliary battery is there to prevent this loss of dash function.
I asked about "Stop-Start", not the KG. Anyway, you got the battery function clarified, indeed it is not needed to keep up any cluster settings.

My Bluetec does not have an auxillary battery. In over 40K miles, I have not had any battery related issues.
I'm sure your car has the additional battery as long as you have the steering column shift (like all 7G-tronic cars should have, excluding the AMG but it must have electric D/N/P control too).

The AUX battery is nothing like the one on W211 models with SBC brakes but a small, even smaller battery than the W211 AUX battery.

Cars with Stop-Start again have an additional battery that is a bit larger than the gear selection backup battery (serves the same plus some additional functionality).
 
#4 · (Edited)
The dealer explained the function of the auxilliary battery is to ensure there is sufficient power to put the transmission into Park, even if the main battery is dead. I got may car back, but no one knows if the dead main battery problem will recur because thay cannot determine what caused it. Time will tell. I give the dealer marks for trying, but I sure wish the MB design team had done a better job on the electric system.:crybaby2:

Apparently, the dead main battery problem is sometimes resolved by turning off the A/C after run fan feature. Other times it is resolved by replacing a faulty light switch that fails to tell the CAN system the headlights are still on. Still others with Keyless Go have been found to not fully turn off everything on the CAN when the car is shut off. Mine, unfortunately, remains a mystery. Fortunately for most W212 owners, this malady does not affect all W212s, but there is no indication that is is solely one variant.

I have little doubt that the dealer, and MB, would fix it if they could, but it is a bitter pill to swallow nonetheless.
 
#8 · (Edited)
My bad - I mistook the specs for the E550 for the Bluetec. My data only says the 350 has a 70 amp-hour battery and the 550 has a 95 amp-hour battery, although they both have the same alternator. Sorry.

So I guess the better question would be - I wonder is any E550s have had the battery drain problem?
 
#9 ·
The dealer should be able to detect the source of the power drain. Of course, never rule out a battery going bad. It happens. I once bought a car that started having weak battery issues. I hooked the battery to my charger and let it fully charge. The battery never had another issue while I owned the car. When I first brought my MB home, the first night I hooked it up to my charger. I was surprised to see it still charging the following morning. Once a battery discharges to a certain point, the alternator will maintain it but not fully charge it. If you have a charger hook it up and fully charge the batteries and see if the issues go away. You can get a pretty nice charger/maintainer for about $60. Nice to have around.
 
#10 ·
Yes - I need to get a charger that does not harm the MB battery - an Absorbed Glass Mat (AGM) type. My cheap old charger will not do. The dealer checked my old battery three times with a load tester and it always checked OK, once they recharged it. We will see if a new battery holds up better.
 
#12 ·
DB - is the Stop-Start something that shuts off the engine when the car is stopped in traffic, and restarts it when you are ready to go? I've not heard of that in the USA cars. Is it a European feature? In any case, no - I don't have that feature.
 
#13 · (Edited)
Fingers Crossed

I have my car back and so far it starts every time.

I was told that the charging system is under the control of the rear SAM. An algorithm in the SAM considers the drain rate on the battery and then sets a recharge rate that should recharge the battery, while at the same time optimizing the fuel mileage by limiting the charge rate. Supposedly, MB may have cut it a bit close on how long the average drive might be in the algorithm in an attempt to stretch fuel mileage out more. They are speculating that cars, like mine, that routinely drive short distances, may over time never fully charge the battery. This is thought to be injurious to the battery's ability to hold a charge.

The dealer has already replaced the main battery, and (hopefully) installed a different program in the rear SAM that will let the alternator charge at a higher rate to compensate for short drive times. Time will tell.

As an engineer of 30 years working on networked computer control systems, I can attest to the maddening nature of intermittent problems such as this. Hat's off to the folks at Long Mercedes Service department for never giving up on this elusive problem. It is easy to grumble about the cost of dealer service, but they sure bent over backwards to try and solve this problem.
 
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