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I have recently purchased a 2011 SLK250 in Hong Kong which despite the ECO button showing a green light, it fails to show the ECO icon on the instrument panel even when the engine has reached operating temperature. Needless to say the start/stop function never kicks in. The previous owner reported that the ECO Start/Stop has worked erratically for him.

After researching this matter on this and other forums, I decided to take a punt and change the Aux battery on the SLK250 in the hope that I can restore the ECO Start/Stop feature. I had a choice to source the replacement battery from the one and only one Mercedes dealership in Hong Kong or source an OEM replacement from FIAMM via a reseller in Hong Kong. Sadly, sourcing the FIAMM replacement proved difficult as no sooner they restocked, a local garage had bought the entire stock!

Having sourced the replacement Aux battery from the local Mercedes dealership, I found it trickier than expected to install the new battery due to the limited access behind the passenger seat. The one-piece trim proved difficult to remove entirely, so access was limited by removing the trim only partially, barely enough to gain access to the battery location.

Here is how I replaced the battery; Unclip and remove the net. Then unscrew the two trim buttons behind the passenger seat. Gently prise the trim away from the door seals and pull back to expose a black metal plate and part of a white rubber foam covering the aux battery. Remove the black metal plate which is secured by two 10mm hex bolt at each end, then remove the white rubber foam to expose the battery.

Once the Aux battery is exposed, remove the +ve/-ve battery terminals which are secured by two 10mm hex bolts. CAUTION: the +ve/-ve leads are not colour coded so please make a note of which is which otherwise refitting them incorrectly would be dangerous and your car will suffer sever electrical damage! I also took the precaution to insulate the +ve terminal lead to prevent it from shorting with the -ve lead or against the metal bodywork!

Once the leads are removed from the battery, it was quite difficult to pull the battery out due to the tight fit coupled with the limited access - loosening the rubber battery tray behind the battery aided the removal of the battery.

The original battery fitted at the factory (part number A0009827008Q1) is a 12V 12Ah battery manufactured by FIAMM in 2011. The replacement battery sourced from the dealership in Hong Kong (part number A2115410001-Q8) is also a FIAMM battery manufactured in 2018.

After installing the new aux battery and refitting the rubber cover, metal plate and the trim, the ECO icon finally showed up during the test run. The Start/Stop is also finally working once again - success!

Here are a few photos (credit to my assistant) of the steps I took which I hope will help others to tackle this otherwise simple job. It took me (and my assistant) about 40 mins from start to finish requiring only one 10mm socket wrench.
 

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cool thanks for sharing. I can confirmed it's the same location and steps as the above on r172 55amg. For me, I didn't replaced my battery but place CTEK charger on the aux battery for 1hr before my long vacation.
 

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cool thanks for sharing. I can confirmed it's the same location and steps as the above on r172 55amg. For me, I didn't replaced my battery but place CTEK charger on the aux battery for 1hr before my long vacation.
Curious to find out if recharging the Aux battery (which should be AGM type) with a CTEK charger can restore the aux battery to support the Start/Stop feature?

I have recently ordered a CTEK Start/Stop charger but wouldn't be picking the charger until I return to the UK for my Christmas vacation.
 

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This is a superb tutorial. Take a bow!!!
I followed it to the letter, and changed my auxiliary battery yesterday. One tip...give the new battery a ‘clean’ with Mr Sheen furniture polish: it helps the replacement battery slide in easier. Stop/Start was back working immediately. As in the above post, my battery was made in August 2011(no wonder Start/Stop had packed-in!), and the new one in December 2018.
 

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This is a superb tutorial. Take a bow!!!
I followed it to the letter, and changed my auxiliary battery yesterday. One tip...give the new battery a ‘clean’ with Mr Sheen furniture polish: it helps the replacement battery slide in easier. Stop/Start was back working immediately. As in the above post, my battery was made in August 2011(no wonder Start/Stop had packed-in!), and the new one in December 2018.

Did you know the actual on the road fuel gains according to Honest John between the petrol R171 and the R172 200 SLK are an amazing 1.9%, the improvements to the R172 are, direct fuel injection, turbo, stop/start and energy regeneration.
Is this worth the cost of a new battery, how much fuel savings are actually down to stop start, this is UK driving not Hong Kong.
 

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IMO, personal preference. If you must have ECO, then consider that batteries generally last 3-? years. If your system works/doesn't work, it may not be the battery. Many things must be in sync for that system to operate, not just battery quality.

Your money, your choice, your back (being a contortionist with small hands will greatly assist you in this endeavour if you choose to go down this route).
 

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I know I don't have a ECO button on the dash, so guessing my USA 2012 doesn't have it.

Main battery hasn't been spinning the car over as fast as I would like, so I plan to check/replace that battery soon.
 

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Before you go to that expense check to see if you do have stop/start and regeneration, lift hood and remove plastic cover fixed to the starter battery(3 clips), if the negative battery clamp, the black lead has one of these in pic below you have stop/start and regeneration, if there is no small box attached to the clamp you don`t. If the battery is original, it may be the battery, My C-Class Coupe (same engine as R172) has original battery stamped Sept 2011, it`s still going strong but I am in the UK.

If you don`t like stop/start (for me it`s a useless piece of extra tech to go wrong) just disconnect the sensor arrowed in pic, this will deactivate stop/start, there will be NO CEL on dash and the car drives normally, (you do loose brake hold but that`s all), you get an extra bonus which is far more advantages, well it is for me. With the sensor disconnected the EMU which controls the alternator goes into a pre-arranged charging mode(as it does if sensor fails) and instead of only charging battery to 75% will now charge it to 100%, at between 13.8 volts float and 14.2/3 volts charge, link below.

Edit, I just reread you post, I own 2 Merc`s a C-Class 2012 model and an SLK R172 2015, on the C Class the symbol stays yellow on dash when stop/start is not working, when stop/start is not working on the SLK there is NO symbol on dash, same as your car.

#104 Old 3 Weeks Ago

I had a bit of time on my hands this evening so I have now implemented my switch idea. I had the switch already left over from another project and some 2 core cable of the appropriate size.
I routed the 2 core to the cabin from the battery compartment using the convenient grommet just behind the battery. The sensor wiring has a red power cable connected to the positive battery terminal and a blue one which is the sensor output to the rear SAM. I’ve put the switch in the blue signal line. I’ve installed the switch in the kick panel above the pedals, so it’s in reach from the driver’s seat but out of sight.
So now with the switch closed it’s OEM behaviour and with it open stop/start and brake hold are disabled but crucially cylinder deactivation is unaffected!



https://www.slkworld.com/slk-r172-general-discussion/528785-start-stop-eco-tech-load-bull-6.html#post4542330
 

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Did you know the actual on the road fuel gains according to Honest John between the petrol R171 and the R172 200 SLK are an amazing 1.9%, the improvements to the R172 are, direct fuel injection, turbo, stop/start and energy regeneration.
Is this worth the cost of a new battery, how much fuel savings are actually down to stop start, this is UK driving not Hong Kong.
I understand the 2% to 3% typically quoted for ECO Start/Stop is the fuel savings only from regeneration (i.e. only charging the battery during barking, over-run, coasting, etc). On newer MB cars (e.g. the W246) the ECO display shows this as 'bonus miles'.

In my experience the actual fuel savings is much more from the engine being stopped completely at the traffic lights. When ECO start/stop was not working in my car, my typical mileage in my daily commute to work was around 15 liters/100km. After restoring ECO start/stop with a new Aux battery, my typical mileage to work is now 7.9 liters/100km. These are readings taken off the on-board trip computer and the actual savings will vary for each driver due to style, route and environmental conditions.
 

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Is the AUX battery something to change preventively, or wait until it fails?
I would wait until it 'fails' before replacing the Aux battery. My definition of 'fails' is when ECO start/stop isn't working - others might wait until the Aux battery failure warning signal.
 

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Did you know the actual on the road fuel gains according to Honest John between the petrol R171 and the R172 200 SLK are an amazing 1.9%, the improvements to the R172 are, direct fuel injection, turbo, stop/start and energy regeneration.
Is this worth the cost of a new battery, how much fuel savings are actually down to stop start, this is UK driving not Hong Kong.
Very little to do with fuel savings.

Lots to do with lowering emissions for European homologation where manufacturers are using every trick in the book. ( some more legal than others .... VAG group )
 

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Wong, that means you are at a standstill for almost 50% of your journey, quicker to walk or cycle.
Yes you are correct - my daily commute journey (5km) takes 10 to 15 mins in total depending on my luck at 5 (almost perfectly out of sync) traffic lights. So the standstill time is around 5 to 6 mins before i get onto the highway where I can drive uninterrupted at the speed limit all the way to the office.

Unfortunately it is way too hot to walk in Hong Kong in the summer heat and humidity, especially since I need to wear a suit for work (and sadly no showers in the office). Also walking/cycling is not allowed on the highway, so it will be a much more torturous and almost double the distance if I avoided the highway.
 

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Seems my post got lost, I said, that`s why the system should default to off, there would be no surprises at junctions when uses forgot to switch it off and those rare drivers like yourself Wong where the system actually showed some meaningful benefit could switch it on.

Rob.
 

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The smart top allows you to have the system remember the last setting. Another one of the numerous reasons to install this option.
 

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Earlier failure of timing chains in our engines with stop-start tech, might the thousands of extra starts from this technology have a bearing on these earlier chain failures.
 
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