Hi All.
i know there are a gazillion threads and postings on this and I HAVE read them all.
I need some expert tech advice to go to the next level, please read below.
My battery drains in as little as 10hrs, its a brand new 850cca battery [ in the trunk]
I recently had the front engine compartment SAM module replaced at the stealership.
I have fully charged the battery and one by one pulled all the fuses in the trunk SAM module -- no effect, battery drains at same rate. [ recharging the battery after each fuse pulling before moving onto the next ones]
I have pulled all the fuses one by one in the drivers side dashboard SAM module - no effect, battery drains.
I have disconnected the wiring harness from the trunk mounted BCM -- no effect, battery drains.

So what next? I tried eliminating all the "known issues" from the threads on here [ seats, doors, BCM, etc] by pulling fuses but it continues.

Can you "check" for a faulty BCM by unplugging it like I did or would that have no effect?
Is there a way to test the BCM?
could it be my aux battery in the engine bay and would disconnecting that for a day whilst the car is on my driveway troubleshoot the problem?

im now at a complete loss as to which circuit could be causing the drain so from the above list where did I go wrong or what could I have missed?

After my nightmare experience with the front SAM mod at the dealership I really really dont want to go back there as their electrical knowledge seems less than mime [ anyone interested in the story who needs a good laugh let me know and I will post it LOL]

thanks in advance.

 

There are 2 things that could cause the battery drain from my experience, try to disconnect the alternator overnight, it could be a faulty diode which is draining the battery, and if that doesn't work it can be an alarm/immobilizer causing the problem, try not locking the car overnight

 

10hr it has to be pretty good drain.
Just yesterday, while working on the car I left the key in ignition overnight.
By the time I finish the job the key was in ignition for 24 hr and the car started just fine.
Meaning even without going to sleep, the normal draw is very small.
Seek if you can find wiring schematics.
It is always "Christmas tree" configuration, so disconnecting legs should narrow and narrow the issue.
Usually alternator is hooked up to starter cable that often is hard to get to.
Good luck.

 

thanks folks I appreciate the replies. I know its got a huge draw. my battery is a 50AH so to drain flat in 10hrs its pulling about 5 amps an hour draw which is huge. -- Tried the leaving unlocked idea already. I know disconnecting circuits should identify the drain, but ive pulled everything I can and it had no effect.
Havent tried the alternator though. if i can get the battery cable off it easily enough its worth a try just to eliminate it.
I should mention that im a fully licensed tech with over 30 years experience [ BUT im heavy trucks not cars, so I have technical knowledge as engines are engines and wires are wires regardless of whether its on a Benz or a Mack. ] but i dont have access to Benz diagnostic software or an intimate knowledge of how everything is hooked up so I could be missing something. thats why im hoping a member on here might have walked this road before and could point me at things i might have overlooked

 

As an alternative to removing the fuses, it is also possible to disconnect the control units from the bus.