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Discussion Starter · #1 ·
Hello, all you wonderful and knowledgable GLA enthusiasts!

This morning, I got the "Auxiliary Battery Malfunction" warning message. It was a cold morning (maybe around 30 degrees F). Everything functioned as usual, but this prompts me to address another issue I was ignoring for a while: my eco start/stop hasn't worked for months. I have mixed feelings about the Eco S/S, so I didn't do much about it except ask my husband to check the voltage on both batteries and he said they were fine (just hooking up his multimeter to each battery after the car had been sitting for a while---no load-testing or anything). I just filed it away as something to mention when I have to take the car in for service next (dreading dealing with dealerships).

When I think back to when my Eco S/S probably stopped working, I believe it was after I had let the battery die by leaving the key in accessory mode and leaving the radio on...for hours (I know, it was stupid...using my car as a boom box...I got distracted and when I came back to the car the battery was dead). I started the car with a jump box, got it home and left it on the charger for the night. It took the charge just fine, and I've had no trouble with the (main) battery ever since (well, it always starts nice and strong anyway). However, when I look back I think---but cannot be positive---that's when the Eco Start/Stop stopped working.

Fast-forward maybe 5 months and today I got that Aux Battery message this morning. I read a lot about the batteries in posts and links from this board. I am perfectly comfortable replacing that aux battery and have found them for sale at $50-150 (price variation depending on whether or not they sport that 3-pointed star!). But, I wonder: should I do more troubleshooting first to make sure the issue isn't with the main battery or something else, and the aux battery failure isn't just a symptom of the real problem? Is there any harm in just replacing the aux battery and seeing what happens?

Somewhat related question: am I going to get crap about warranties or something if I put an aftermarket aux (or main) battery in this thing? MB does like to keep you in a co-dependent relationship with their super-special parts (not), super-special tools (not), super-special mechanics (haven't found a decent one yet), and super-SUPER-special prices. I'm all for aftermarket and freedom of consumer choice, but not if it will negate my whole CPO warranty!

Thank you in advance for your advice!

Christine in Philly
 

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No harm in first just replacing the Aux battery. In any troubleshooting activity it's always prudent to reduce variables and replacing the Aux [with a known good] is such. Related, we often criticize dealer service for "just throwing parts at a problem" but in fairness they are often first trying to reduce variables.

In-warranty you do want to be careful not to give your MB dealership service department any excuses for denying future work. "I'm sorry Ma'am but there's an aftermarket battery in here and therefore we can't ...." You spend a lot on mod's so pony up for the Benz-authorized part. ;) Once you're out of warranty get whatever you want.
 

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I have a 2018 and I got the aux batt fail about a month ago. There does not seem to be much troubleshooting for that battery. The voltage regulator for that battery is built in to the battery from what I was able to find, so replacing the battery takes care of that, too.
I went to batteries plus, other auto stores did not have it. It was $22 and they replaced it for me. Very simple process. The batteries plus employees were outstanding.
The only issue that I noticed was that the cruise control would not set immediately after the change. I have not tried it since.
 

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Discussion Starter · #4 ·
I replaced the aux battery with a genuine Mercedes part to the tune of $160 (ouch). The old one was original, so maybe 6 years is the life expectancy anyway?

Sadly, it has not resolved the problem. I was sure to drive the car enough at the right ambient temperature to be pretty confident that all the conditions needed for Ecostart should be in place. Nope. Still no Eco Start. Ugh. What do I try next?
 

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I don't know if MB ever made eco start work. This was a big complaint years ago.

From what I know the Aux battery is used in the system although this is nebulous IMHO. For five years our eco stop has only worked after long trips ... the Aux battery needs a good charge and this doesn't happen around town.

Some have reported an MB firmware update that can fix the problem. But I'm not sure this held in the long term. MB doesn't seem consistent from one dealer to another regarding any update.

Most get annoyed by the motor shutting off and figure it is a bonus when it doesn't work. I've joined their camp figuring resistance is futile.
 

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That would be me. ;)

Although ours still working on OEM Aux. I'm all for anything we can do to preserve air quality (my vehicle is an EV) but our retiree driving is rarely in much stop & go traffic, so ...
...
Most get annoyed by the motor shutting off and figure it is a bonus when it doesn't work.
 

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Low usage, our 2019 GLA250 is now 32 months in use. In the past year, we have driven around 5k. Sometimes it is only driven once or twice a week. A trip could be 5 miles or as much as a 40-mile round trip. My stop-start works perfectly and never had a problem. We had a 2016 GLA250, and it was a problem as, in the first year, the dealer replaced both batteries twice and ended up doing a software update and never had a problem with the stop-start after that. Our 2016 used and driver the same as the current 2019. Both, at times, could sit in the garage for over a week and not driven. Find it interesting that there are so many problems with the stop-start.
 

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I have a 2018 and I got the aux batt fail about a month ago. There does not seem to be much troubleshooting for that battery. The voltage regulator for that battery is built in to the battery from what I was able to find, so replacing the battery takes care of that, too.
I went to batteries plus, other auto stores did not have it. It was $22 and they replaced it for me. Very simple process. The batteries plus employees were outstanding.
The only issue that I noticed was that the cruise control would not set immediately after the change. I have not tried it since.
Hi Warp, do you have a part number for this? For this price and them replacing it, I'm going to my nearest Batt + store this week while my work week is quiet.

For anyone else, I'm interested in where you went and what the part number was. I see the dealer wants about $122, but throw in labor and I"m probably out nearly $200.
 

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Discussion Starter · #9 ·
Hi Warp, do you have a part number for this? For this price and them replacing it, I'm going to my nearest Batt + store this week while my work week is quiet.

For anyone else, I'm interested in where you went and what the part number was. I see the dealer wants about $122, but throw in labor and I"m probably out nearly $200.
Spend your $ on parts, not labor! This battery is really easy to replace yourself. I paid for MB genuine part just because my car is under CPO, but I sure as **** wouldn't pay for some overpriced MB tech to put it in! Let me know if you need the SM pages for how to do it.
 

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Spend your $ on parts, not labor! This battery is really easy to replace yourself. I paid for MB genuine part just because my car is under CPO, but I sure as **** wouldn't pay for some overpriced MB tech to put it in! Let me know if you need the SM pages for how to do it.
Thanks Christine, totally agree ! Just to add what I have learned thus far:

  1. I took the battery out myself, 5 mins. Once I understood how to get the plastic cabin air filter bottom off plus the back of the battery cover. It's out now, but no replacement in hand, here is why
  2. I went to Batteries Plus, there is no $22 replacement, what they have is almost the same, it's AGM and it fit, but I would have to bend the contacts in my car, and the battery was $135.
  3. Autozone and Advance Auto parts, you are probably good buying the mercedes OEM Christine....Advance is $110, and while they show dimensions, it's just slightly larger, so not sure it would easily go back in box. Plus, both aftermarket do not show a venting tube hookup, so any venting stays in our cabin? Not sure about this, these AGM batts are more meant for motorcycles, ATV's where venting is not needed.
  4. Buy online OEM but again, it's hard to tell exact same batt and places like Autohaus are $119 but after shipping/tax they are $155.
Bottom line, I am going to go to dealer tomorrow and see if it's around $160 as well and be done with this.

Has anyone tried to hook up one of these to a tender while out and try and recharge and reuse? This is likely not what I will do since the Battery is already 7 years old. I'm now thinking that since I don't drive the car much, would it make sense to try and put a tender on a new AUX batt. that is installed? It's not difficult to pull back carpet and remove the cover to access the terminals, doing this annually?
 

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I always put a new battery on a tender as you never know how long it has sat on a shelf somewhere.

My spare generator battery gets put on a tender quarterly just to be sure it's ready.
 

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Discussion Starter · #12 ·
It never hurts to put a battery on a tender, just to make sure it's got a full charge before you use it.

I don't know enough about how MB has looped in this aux battery to know if there's any real draw on it when the car sits, but a battery should hold a charge for a long while if all its cells are good and there's nothing drawing down on it. I think of my motorcycle batteries and they only need to go on a tender when the bikes are tucked in for a long winter nap. Granted, I'm talking about a '99 Triumph and a '01 BMW, neither of which have the sophistication or complexity of the electrical system of a MB, but why would something well-engineered be designed to require constant charging?

All that being said, if you did want to make the aux battery easily tendable, you won't need to access the battery itself every time you plug it in. Just add the lines for a plug-in tender and have the plug accessible up under the footwell somewhere. So, you install a line like this (photo attached) to the battery, and just plug in the tender when you think it might need some tending. We have a few of these cables attached to the motorcycle batteries, but only one tender that gets moved from bike to bike routinely all winter.

I still have mixed feelings about this EcoStart thing not working and I haven't decided if I want to call it to the attention of the dealership. Odds are they'll say that they'll just blame the add-a-fuse I put in for the phone charger (or the aftermarket door lock knobs, or the roof rack, or some other absurd claim to avoid responsibility) and try to void my entire CPO warranty over something I'm not sure I even want working!

Bicycle part Bumper Office supplies Auto part Cable
 

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So a couple updates.....Christine, I don't know if you noticed but your ECO button should not have had a green light under it when your AUX batt malfunction happened. Now that I have bought/put in (right at dealer) a new OEM batt, the green light is lit, and the malfunction no longer appears. Interesting, I opted for the Made In Italy battery vs. made in China. It was about $15 more ($177 w Tax), though they were basically exactly the same (12v, 12AH, 170amp), difference being terminals, new batt I bought was replacing a made in Italy battery and had exact same terminals. The parts guy was thinking the same thing about using a tender since I drive the car 1-2 days weekly max, and 2200 miles a year right now.

So with the tender, I've installed the new battery that was stamped up top with 30 21. Not sure what 30 is for, but 21 is likely this year battery (replacing a batt stamped 30 14). So I have not put it all back together, I am wondering, with the Ctek tender, with battery installed, do I put neg. to neg. batt terminal or should I put it to a ground down there, like I do when hooking up the main battery (and of course + to pos. terminal)? It looks like when you pull the carpet back there is a ground or two I could try???? I obviously am not an electrical engineer, just being cautious I don't do something stupid with a new battery. I'm inclined to "top off" the new batt. to make sure all is good, and then I also have learned how to perhaps do this annually. With your Pic Christine it appears the answer is use the pos. and neg. terminals of the aux. battery and plug it in? Why bother grounding to body/frame?
 

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Discussion Starter · #14 ·
So a couple updates.....Christine, I don't know if you noticed but your ECO button should not have had a green light under it when your AUX batt malfunction happened. Now that I have bought/put in (right at dealer) a new OEM batt, the green light is lit, and the malfunction no longer appears. Interesting, I opted for the Made In Italy battery vs. made in China. It was about $15 more ($177 w Tax), though they were basically exactly the same (12v, 12AH, 170amp), difference being terminals, new batt I bought was replacing a made in Italy battery and had exact same terminals. The parts guy was thinking the same thing about using a tender since I drive the car 1-2 days weekly max, and 2200 miles a year right now.

So with the tender, I've installed the new battery that was stamped up top with 30 21. Not sure what 30 is for, but 21 is likely this year battery (replacing a batt stamped 30 14). So I have not put it all back together, I am wondering, with the Ctek tender, with battery installed, do I put neg. to neg. batt terminal or should I put it to a ground down there, like I do when hooking up the main battery (and of course + to pos. terminal)? It looks like when you pull the carpet back there is a ground or two I could try???? I obviously am not an electrical engineer, just being cautious I don't do something stupid with a new battery. I'm inclined to "top off" the new batt. to make sure all is good, and then I also have learned how to perhaps do this annually. With your Pic Christine it appears the answer is use the pos. and neg. terminals of the aux. battery and plug it in? Why bother grounding to body/frame?
Yes, neg and positive cables for the tender hookup go right on the battery. Just hook + to + and - to -. The battery is already grounded, so you're just feeding the battery directly and wouldn't ground it separately. I still doubt you would need a tender if the battery is good and there's no phantom power drain, but since there's no harm, why not, I guess?

My green light for the Ecostart has remained lit through all of its malfunction, so i am still at a loss on it. Sounds like yours is good again, so congrats!
 

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I never use ECO, usually always select Sport mode which cancels it, so you can have mine ! ;)
 

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I found out that after sport mode cancels it, you can press it again and it comes on, stays on.
I think that, like everything else on this car, this button doesn't just have a start stop cancelation function. I think it has a charging aux battery via regenerative braking (probably what the "sailing" symbol is about) and it seems to have an influence on the gas mileage too.
30/21 most likely means week 30 of 2021.
 

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Christine, you need an Xentry rig to take control of the car. Meaning only ask the dealer a question once you know the answer, and therefore take control of that dynamic as well.
They're not that cheap, but if you stay with Mercedes, you continue to use them.
 

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Vladds, thanks for the interesting info. Do you use your sport mode all the time, and regularly hit the eco button again? Sport stays on but the green is now lit again? I may try this if that's the case. If it's not affecting Sport mode at all, you may be right that it's just allowing juice to charge the aux. or allowing regenerative, however it's accomplished. I'll have to check if the owners manual says anything about this.

Oh, and I'm sure you are right, first number is the week of the year.
 

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I found out that after sport mode cancels it, you can press it again and it comes on, stays on.
I think that, like everything else on this car, this button doesn't just have a start stop cancelation function. I think it has a charging aux battery via regenerative braking (probably what the "sailing" symbol is about) and it seems to have an influence on the gas mileage too.
30/21 most likely means week 30 of 2021.

That must be a change for newer versions. I just came back from a drive. I was letting the car warm up since only the coolant was showing "white" (warm) temp. I couldn't have been driving more than 8 mins. and at my first light, ECO shut the motor off for me. So my ECO is ready to go very quickly, parameters must be minimal in a 2015 as it's also only 33* here and it was already engaging. In any case I next turned on sport mode and let that go a minute and then hit ECO button and it turns sport right back off and puts in back in C with mine. :confused:
 

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Sorry to find out.
An example, where the owner's manual and even the "extended version owner's manual" available online to download, provide no information is:

For the race start feature, a pre-condition is also the outside temperature, it cannot be below 40F.

There is no chapter discussing the "sailing" feature, which requires the eco button lit up green, etc.

I never tested to see exactly how warmed up the car has to be before the eco stays on with sport.
But my behavior is: I start with C, keep it in C until the coolant and oil are white, I don't worry about the gearbox temperature turning white. Then I turn S mode on.
Last time I re-tested the Eco with S about 2 minutes later.
 
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