Hey everyone!

Long time lurker, decided to create an account today as I'm quite desperate. I’m from Paris, France and I’ve got a 2004 SL350 (Japan import by the previous owner, 3.7 V6 M112). Bought it in December 2023 at 90,000 km (about 55k miles) and I’ve driven it up to 127,000 km (79k miles) now. I really love this car, engine has always been smooth, SBC always worked perfectly, never had any issues with the braking system until now.

Earlier this year I started noticing the engine had just a tiny bit more trouble starting when turning the key - still fired up instantly, but not quite as strong as when I first bought it. I figured the battery was starting to get a bit tired, nothing crazy, but I kept it in mind.

Fast forward: 2 months ago I moved countries and I gave the car to a garage near me to put it up for sale on consignment. The car was running perfectly at the time - no warnings, SBC working fine, everything. Unfortunately, they basically let it sit for 2 months without starting it even once. Also from what they told me, they jump-started it at least once. When I came back today to pick it up (as I'm back home for ~10 days), the dash as expected shows a red battery defective message - still was able to turn the car up and drive it back home miraculously. When trying to use the SBC function of pushing the brake pedal at red light to auto-brake, it didn't work: it showed a grey (not red) error message on the dashboard about the brakes. Then once I got home, I tried starting up the car again, but it wouldn't start, as expected. So I went and bought a new Bosch battery, came back and replaced it, and while the car now starts instantly, there still is the big red battery error on the dash screen.

Then, I scanned it with an OBD Mercedes special tool and got error B1829. From what I’ve read here and on other forums, that’s usually the F52f1 100A MIDI fuse blowing and cutting power to the SBC since that fuse is responsible for "charging" the trunk battery (or the main battery? - sorry for my limited knowledge, trying my best here). At least I hope so, because it seems like if it is a problem with the BCM, it would be extremely expensive to repair.

So tomorrow I’m going to swap that fuse and see if that removes the red error message and brings the SBC back. If it doesn’t, then my next step will be checking the auxiliary trunk battery, because I guess those can go bad if the car sits too long. Worst case scenario it’s the SBC pump itself, but honestly at 79k miles it feels way too early for that to be the case, right?

So I guess my questions are: has anyone had this exact combo (B1829 + SBC inactive + battery defective) and it was just the fuse? Or was it something else I'm not aware of?

For context, the car has a Japan-fitted facelift (2010+ style) bodykit, bumper’s slightly misaligned but otherwise it looks like a facelift SL. I already did sway bar end links and bushings earlier this year to sort a front end noise, and overall the car is in very good condition. I was planning to tidy up a couple other small things before listing it for sale again, but now I’m stuck with this SBC and battery issue that wasn’t even there when I dropped it off. Pretty frustrating the garage didn’t even bother to start it once in 2 months.

Appreciate any input - I’ll update once I change the fuse tomorrow!