This is a thing I read about on the internet, so I'm trying to find out if it's as easy and free as it sounds. A guy who goes by 'Rob in the UK' says disconnecting the negative rail battery sensor will do two things: it will disable the Eco Start/Stop and it will permit the car to fully charge the battery (instead of the 80% it's normally limited to). Now, I don't know why the software and hardware of the car would restrict the charging of the battery, and I don't know what effect charging it fully might have -- although I can't think of any, and I'm not completely uneducated about cars. He claims it will throw no codes and not activate the check-engine light.

I tried it today, since I strongly dislike the Eco Start Stop feature. The car behaved exactly as it always had, with the exception of the light on the Eco Start Stop button going out and the car NOT stopping the engine at idle. Rob in the UK cautioned that it might disable the brake-hold feature -- but it did not change anything like that on my 2015 C300 4Matic sedan. He also warns that it will disable the cylinder reduction feature, which my little 4-banger definitely does not have.

So, I'm asking: does anyone out there know of a downside to this trick? It literally takes two minutes, which I'll document here:









That's it. No tools needed at all. Two minutes if you're taking it slow.

To reverse it, just plug it back in.

But there might be someone here who has a solid knowledge of the W205 charging system and the computers that control it. Or there might be some other benefit to this thing being connected. If that's the case, please sound in.

I'm getting a voltmeter for the cigarette lighter on Tuesday to see if I can detect any difference in the car's charging routine with the plug in or removed.

 

Interesting. Curious to see what others have to say.

 

Leads to asking why the battery charge would be limited to only 80%?

 

So let me get this straight ... you are "hacking" the electrical system of your C300.. and asking for the unknown consequences that you don't know about... all to avoid paying $145 for the proven no risk OBD module:

https://www.ebay.com/itm/MERCEDES-ca...d/252811119026

Ia that a accurate summary of your post ?

 

Makes perfect sense to me to want a free, safe, and easy way to delete the eco start/stop.

 

Exactly. And that's exactly why I'm posting these questions. It either has a negative effect on the car or it does not. If I find out it does, I'll spring for the fix-it plug (which, of course, is also an electrical hack).

If unplugging this plug has no negative effect, then the only person I see getting any benefit from the $145 OBD fix is the person selling it.

 

For what it's worth, found this explanation of auxiliary battery function...but you can't always believe the internet

What does a Mercedes auxiliary battery do?

There are 2 types of auxiliary battery for Mercedes-Benz cars. Some cars have mini battery, very similar to primary (main) battery just much more smaller in size and other models auxiliary battery is completely different. This new shape battery aka voltage converter has been fitted to new C class W205 S205 C205 E Class W213 S213 GLC X253 C253 and S W222 class cars.

New shape auxiliary battery is completely different if you compare them to the first generation auxiliary battery and it has second name for it - voltage converter or convertor.

The auxiliary battery runs some or the most of the'cars additional functions or accessories such as PDC sensors, multimedia, various radars, stop start function and so on.

 

This is not the auxiliary battery, as far as my understanding goes. The aux battery is actually a voltage converter (capacitor), and is located in the passenger footwell (at least on my W205).

As I understand it, the car's energy management unit (EMU) monitors the battery's state of charge so that when the battery is at 80% or so it can decrease voltage from the alternator in order to (modestly) improve fuel economy by reducing the alternator's load on the drivetrain. It also disables start/stop functionality when the battery is below a certain point so that Eco start/stop doesn't cause the battery to lose sufficient charge. But all of this comes at the cost of an additional layer of electronic complexity -- and a shorter life for the very expensive battery (as well as more wear on the starter and engine mounts. etc). If the fuel savings from Eco start/stop and slightly reduced alternator output are important to you, then you probably shouldn't disconnect it. But you might be losing the value of that fuel in expensive AGM battery replacement intervals.

But again, I'm hoping someone who knows more about this might either confirm this or set me straight.

 

If battery charging is cut off at 80% of full voltage, it would be a waste of battery weight and capacity. Hard to understand doing that. Or perhaps it only disables the auto-off function below 80%? Has anyone checked with a voltmeter? Since chargers usually reduce charging voltage as the battery nears capacity to prevent heat build-up, this may be another reason for it. I'm reticent to disconnect until I'm sure it's harmless because disconnecting is probably logged in the ECU and could void the warranty if it causes any unforeseen problems.

The auto-stop may save gas if one encounters many long stop lights, but it's nothing but a nuisance in my driving environment with a lot of momentary stops for round-abouts and stop signs. And it's another nuisance to have to turn it off manually every time I start the engine.

 

I have noticed in both cars if the battery is low eco is disabled... The E550 sat for 4 month's recently and glad to report it started right up but ECO didn't function for awhile, took it for an hr ride to charge it back up....

 

Thank you @Jack Olsen for this post including detailed photos. I've now disconnected the connector on my '16 C300 and confirm the brake hold still works fine for me as well. Only thing that appears to be gone is the annoying start/stop!

My warranty ran out last month and I bought the car with an aftermarket battery already installed (AutoZone/AutoCraft H6-AGM) so I didn't really have anything to lose at this point.

Edit: I also find it interesting that this particular part, 'Battery Sensor', p/n A0009050454 is/was supplied by Hyundai Mobis, which is the parts supplier arm of Hyundai Motor Company. Hyundai parts in a Mercedes 😖

 

The OBD module is not a "electrical hack" - it re-programs how the ECO start-stop operates - the module reprograms the start-stop operation - if you turn your MB off with ECO disabled - then when you restart your MB your ECO stays off - and your ECO remains off if you change to another driving profile as well. Vice versa if you turn off your MB with ECO activated...

Well... how are you Smart Guy's who have just knocked out your ECO Start/Stop - try this - feel free to post a YouTube video of you driving at a dumpster at 30mph and show us what happens now that you have knocked out your Collision Prevention Assist (and/or CPA+) - oh - that's right - you asked AFTER you hacked your electrical ... I guess life IS a box of chocolates for some of you guys... gee's....

 

Or you could say it normally

 

Say what "normally" - based on a obscure UK post from a Brit-diot - on a feature that just takes one press to deactivate on start-up (ahhh, I have to do that every time I start the car - I'm soo soo disappointed in Mercedes blah blah blah ) - or takes a proven $140 OBD module to re-program eco s/s BETTER - this dude hacks into his electrical - chops a whole wire - and then AFTER he cuts the wire - he asks the forum "I am so happy with my free hack - by the way does this wire do anything else - because I don't know really what this wire does"...

As a Mercedes enthusiast let me tell it the way this really is ... this is Forest Gump... stupid is what stupid does... report back on the Dumpster experiment with that wire cut

 

Wow dude... might want to lay off the caffeine for awhile. Relax and let other people do what they please with their vehicles. Nobody here is suggesting that you must unplug this connector (not "cut a wire" as you stated) on your own car. If you want to use the $140 option then you do you boo boo

 

Yeah, fabbrisd1, you seem really angry; I'm not sure why. I didn't cut any wires. I disconnected a sensor plug -- already with a pretty clear idea of the layer of electronics I was taking out of the loop and what that would mean for the car. What I did is effortlessly reversible -- about as easy as plugging in or unplugging a phone charger. I posted here to see if someone more familiar with Mercedes electronics knew something I didn't. Instead, I got you shouting at me about Forrest Gump. I'm not sure why.

I unplugged the sensor again for a drive today with a voltmeter in the cigarette lighter and I can report that it does not do anything to disable the collision avoidance system, or throw any OBD codes, or change any other function that I've been able to detect. Collision avoidance works on the sides and also in front.

What unplugging the sensor does do (I'm told) is to put the car into an unmodified charging routine, which is to say the alternator charges the battery until it's fully charged. That's pretty much what cars have always done. Mercedes has it go into the lower voltage mode to put less load on the engine and make some tiny improvement in fuel economy.

Most importantly, unplugging the sensor disables the Eco Start/Stop nonsense, which might cost me some barely-detectable amount of fuel economy, but is going to be kinder to my battery, starter and engine mounts. I'm going to use today's voltage readings as a baseline to see if there's any difference in charging patterns for the type of driving I do.

I'm sure Mercedes corporate would suggest I not unplug the plug, just as they'd suggest you not use the OBD-based hack to change the code they wrote. But that's not a surprise. They're a big corporation and they're not concerned with how irritating people might find the Eco Start/Stop.

As of now, I'm very happy with what I did. If anyone else wants to do some research on their own Mercedes models and report back, that's fine. If not -- that's fine, too.

 

take a chill pill dude lol

 

Lighten up, Francis.

 

Actually, it is a result of the EDICTS of the unelected bureaucrat eco-nuts in the EU and their counterparts in the USA CAFE etc. to "improve the overall gas mileage of each manufacturer" and thus "reduce reliance on fossil fuels." Soon pano roofs will be solar cells - are sails next?

 

I'm going to do it after a little more reassurance that it doesn't mess anything else up. Thanks Jack Olson for posting.

 

https://media.daimler.com/marsMediaS...ml?oid=9361921

Short read

 

Nice read and interesting on the longer life starter..

Still hate ECO though and always turn off after starting car.... heck I am so trained to turn it off now that when I get in NON MB cars I am looking for the ECO button after I start the car.

Let my ENGINE RUN....
(to Quote Moses or was it "let my people go" have to watch the movie again and see what Chuck Heston says)

 

If the auxiliary battery is anything like a standard LiPo or NiMh battery, the charging restriction to 80% makes complete sense. These batteries operate at peak efficiency and last the longest when they're stored between 50% and 80-90% of full capacity. Charging all the way to 100% over and over makes a small, but noticeable change in the battery longevity. Same goes for your smartphone batteries btw. Again, we don't know how the auxiliary battery was designed, so I could be way off base, but this is definitely a possibility.

 

it makes sense, Jaguar eco start/stop works the same way. It's just an add-on to a system that was designed a decade or so ago.

 

This is my second Mercedes that has had Eco, and I like it. I stop at a light, HOLD comes on, the engine shuts off, and all you hear, besides the radio, is the silence of your engine not using any premium fuel.