I tried changing out my aux battery on my 2016 E400 base coupe. The aux battery failure message started to show. I couldn't get to it right away and after reading more about it I thought I try to replace it myself and due to time used a battery from a local car parts store. what could go wrong right. I took off the negative cable and while preparing to secure it, it flipped and touched the aux power relay and sparked........ I continued to hook the new batter in hoping for the best. no luck. there was no power from the aux battery and car wouldnt start. I figured a fuse was blown and prob the power relay as well. I put a new relay. in after not finding any blown fuses but still nothing.... are there special fuses from the relay and battery im missing? would've some input from anyone who knows way more than I do.... thanks.....Joe

 

Based on the information you provided, it seems like you have a C207 coupe, I recommend copying your post and posting to that sub-thread, hoping that will increase visibility of your thread and other owners of the same vehicle can help you out. I also recommend posting in the W212 forum as that is the sedan variant of your vehicle and there is more traffic there.
In the mean time, hopefully you get your issue resolved. Did you have any luck finding the paper fuse diagram that is usually stored near the fuse boxes?
https://mbworld.org/forums/e-class-c...brio-a207-141/
https://mbworld.org/forums/e-class-w212-109/

 

thanks for the info and I will try to get in in the threads you mentioned...I did find the diagrams but cant really find anything that says fuse for aux battery...

 

Hmmm, were any of the fuse labeled just battery by any chance. Actually, I think there is a different name for "auxiliary battery" for example capacitor

 

I think (if I recalled correctly) someone took apart the so called auxiliary battery and realized it is just a supercapacitor...

 

I did find a 7.5 fuse behind the battery that was fine......