Demon1234

Its located on the passenger side in the trunk, just aft of the wheel arch (top left in the pic). Sorry, i have no better pic to show you at this moment.

Gfernando15

ZZGlk

The relay seems to be a frequent issue after a few years. Unfortunately, if it goes bad, and doesn't give you notice, it will usually pull the Aux battery down and hasten it's demise.

The ECO function has a lot of different inputs that all need to be at the correct values before it will work. Battery voltage is just one of them. The owners manual has a description of the logic for ECO.

My 2013 original Aux battery still tests good, but I turn off ECO every time I start, so their isn't much load on it. Your dealer or shop should be able to test the Aux battery and tell you if it needs replacement. I think they are around $80-100 for a comparable AGM battery. No point in replacing it if it's still good. If it is good, then you can assume the relay is working and keeping it charged.

Gfernando15

ZZGlk

Your main battery could have been the problem, but its not the only condition. See page 145 of the owners manual for all the conditions that ECO needs to work.
Unless your code reader gives you Mercedes specific codes, you probably wouldn't have any generic codes to tell you about ECO.
The green ECO indicator says that all the conditions to use ECO have been met and it should work next time you stop.

Gfernando15

I replaced the main battery and now eco start stop is functioning!

pippa sky

so would this battery being dead effect my transmission or the smooth, or not smooth shifting of gears??? My 2013 GLK 350 shifts hard

ZZGlk

I doubt the Aux battery would do anything to change the shift points or smoothness of your transmission. My 2013 will sometimes get clunky between 2nd and 3rd gear. When that happens I just use the paddle shifter to ease it in gear before it clunks. After doing that for a while, it seems to lose the clunk. You can also reset the learning mode of the transmission and force it to learn you driving habits all over again. There was a post on this forum a while back that told how to do that. I don't remember the exact sequence but it might help your shifting.

Could also be time for a transmission fluid change, depending on your mileage. The service interval for the 2013 is at 70k miles. That seems a long time to go without changing fluid. If you've had a fluid change, be aware that a lot of dealers won't drain the torque converter when they do a tranny fluid change. That means there is still about 4 quarts(litres) that doesn't get flushed.

PACIFICMAN

Got It. Thanks for the replies so far. I checked the manual list, I believe the car meets all the criteria while running / driving. The main battery is a fresh-ish Bosch AGM. It shows 12.8v at rest, 16.3v running on the terminals... just over 2 years old with a 48 month warranty, still kicks ***. I'm going to have it tested at the vendor. I had been letting the ECO do its thing, with no apparent problems(93,000 Mi.) but I'm not married to it nor am I annoyed by it, so... It was the Battery message that spurred me on and now is the lack of function, YELLOW ECO that's distracting. I drove the car a week with no Aux. battery and did not miss the stop / start.