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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
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
Joe, you've done all the right things besides shorting your trunk power supply. It was an easy mistake.
Now you're ready to tackle your main 850CCA AGM
There are a couple of protections involved near the AUX:
1- The main power cable is protected up front by the prefuse box.
2- The R-SAM cable has a built-in fuse link on the BCM end.
3- Hopefully the short did not travel up to fry a R-SAM output (use a scope) - Take the PCB out to look at burnt traces...
I would have honestly guessed your BCM replacement should have fixed it... Now it's time to scan and probe your way up the lader.
Last edited by CaliBenzDriver; Oct 15, 2021 at 11:38 PM.
You sound like the guy I need......I saw the profuse box but didnt see any fuses... do I have to lift the Battery cover ? lifting the cover of the refuse box didnt help... Is the fuse in the R Sam cable accessible?
You sound like the guy I need......
I saw the profuse box but didnt see any fuses... do I have to lift the Battery cover ? lifting the cover of the refuse box didnt help... Is the fuse in the R Sam cable accessible?
the fuse links are concealed inside the Prefuse enclosure. Normally it's designed as a non serviceable blackbox but you can source damaged links. Before you decide to take Prefuse apart, first see if your feed to the AUX BCM as gone cold - if not leave your Prefuse alone is a good idea.
You need to TEST around AUX to discover what's gone missing. The less things you take apart, the faster will be your repair.
Test to figure what is good vs. what is bad! What voltages do you read on your new AUX BCM?
[Carefully don't short circuit your main battery... it can weld things up and burn down your ride]
Sandy,
I am still on vacation, but can write a bit to assist u with what I have on me.
Vacation means no PC, no PC = no Mercedes WIS , no data support.
Find attached excel file. Line 111 and up, see the F32 prefuse there.
Verify below please :
01.1 You wrote : there was no power from the aux battery and car wouldnt start.
Explain how do you measure NO POWER from AUX battery ?
If you measure direct at AUX battery post, Plus and Minus and don't get any voltage reading........hhmmm , you have totaled loss the battery.
This is NOT likely though.
02. Define : car wouldnt start.
No power to crank engine, or you can't even get instrument cluster warnings to light up at ignition key position 2 ?
Can you turn ON headlights, sound system and the like ?
Ignition Position 1 (accessories), position 2 and CRANK engine to start are the 3 stages.
03.1 Measure voltage at main battery please. Direct at battery post Plus & Minus. Tell us.
03.2 Measure voltage at positive battery post and main negative brass stud at suspension well ( not at battery negative post )
This is to make sure the battery sensor did not got damaged in OPEN CIRCUIT mode.
03.3 Measure voltage at main negative brass stud at suspension well and after PYRO fuse.
Use the video to learn which area/output of pyrofuse means AFTER pyrofuse. The video is not to copy cat pyrofuse bypass procedure.
This 3.3 step is to make sure your pyrofuse is not Blown Out ( open circuit ).
Pyrofuse feeds only the starter positive terminal, nothing else.
=========================
Assuming 03.1 to 03.3 all turn out good , proceed to check which fuse at F32 prefuse blown.
=========================
04.1 When I write NEGATIVE terminal , it means I am referring to main negative brass stud at suspension well
Negative to F32 Prefuse terminal IM1, do you get battery voltage ?
This IM1 uses hidden inside F32, a fuse number F159 of a 100 or 200 amps, depending on car model and market.
IM1 is the positive power which connects to rear AUX baby battery K114 parallel relay.
In theory this will be the fuse which will blow, when your accident occurred.
04.2 Negative to F32 Prefuse terminal MR8
Do you get battery voltage ?
This is to verify that the special V19 Q Diode ( relay , sort of ) is not blown ( open circuit ).
04.3 Negative to F32 Prefuse terminal IG1
This one is a bit tricky to test, as I do not know if this K2 relay of yours is permanently sleeping (open circuit ) due to loss of command from Front SAM or it is damaged
in open circuit mode from your accident...or simply fuse no F158 of 150 amps for IG1 is blown.
To verify K2 relay is functioning , front SAM must be alive.
If you want to verify K2 relay is working okey , simply by opening the door, it will be activated.
K2 relay also feed power to terminal MRG2 or MG2 via fuse F156 of 150 amps.
So available power at MRG2 or MG2 means K2 relay is okey.
04.4 If you can access MRG2 or MG2, that means you have lifted up F32 prefuse box and you better check for voltage (blown fuses) at MR6 and MR7 terminals, as these 2
feed the engine computer (MR6 ) and Front SAM (MR7).
Becareful when inspecting F32 Prefuse Box with its side cover removed to see/exposed all its side terminals , which are all high power positive 12V source.
====================================
05.1 Go to front SAM and check Fuse 5. That fuse links Front SAM to rear SAM.
Do post the results and we proceed further from there.
If you are not confident to do all above tests, don't do it. Get professional help.
Good luck.....
Last edited by S-Prihadi; Oct 17, 2021 at 08:45 AM.
Reason: typo
Lots of great info to pour over but at this point its well over my pay grade. part of my issue was where my car was parked.....I knew it would be an issue to tow so I was hoping to fix it in place. But after almost an hour and a half of maneuvering the car was headed to the dealer.....I appreciate your insights, I have learned a lot which will be useful going forward. I will post the resolution when available but not sure how fast my dealer can get to it....another annoyance with all thats gone on....Thanks again ....
Yes in retrospect I wish I had done that but I of course figured it wouldnt be an issue. So the result was multiple issues with the rear SAM unit which was replaced as well as issues with the pre-fuse box up front. Once these were replaced my baby purred like she did before. Hard for me to understand how so much damage could have happened, I had thought perhaps an item or two shorting out would stop the issue there. In hindsight I would have protected the the cables better but what got me was the memory built into the cable which then made it perfectly touch the accessory power relay when it whipped free......def a lesson to be learned here...... in addition a big issue was where the car was parked and how to tow it out. It took about an hour and a half to finally be on our way to the dealer. I had even looked into an independent German car repair but after hearing him say he's never seen this issue I went with the dealer and came home with a much lighter wallet.. when the dealer had the car on a lift he showed me how the rear tires had cracks in the rubber. They were cracked by the tow truck pincers as we maneuvered the car. they were close to 6 years old with good tread but due to replaced soon anyway, not a big deal but an additional cost none the less. Driving a rental made me appreciate the car all that much more...thanks to all for any advise given, it truly was a learning process for me.......
Glad it is all sorted out.
The F32 pre-fuse being replaced by the dealer would probably minimum be the 200 amps IM1 blown fuse inside it.
Since you have the trunk open, the K2 relay would wake up and IG1 150 amps fuse feed the rear SAM directly too. These two could be a goner too.
Dealers wont tear open an F32 pre-fuse, they will replace complete assy F32.
That IM1 is the positive feed to the K114 relay of AUX trunk battery.
The G1/13, that is the AUX baby battery.
IG1 is the biggest power feed for Rear SAM ( 150 amps fuse ) and is K2 relay activated.
There is another small power feed from front SAM to rear SAM, Fuse 5 (7.5 amps ) of front SAM.
Sandy wrote :
I took off the negative cable and while preparing to secure it, it flipped and touched the aux power relay and sparked.
By accidentally sparking the negative cable, meaning negative cable hit a positive connection , what likely happened would be a reverse
polarity feed to rear SAM from this accident, for a tiny fraction of a second. So K114 ( aux power relay ) becomes negative feed and IG1 is still the positive.
The logic board or brain of the rear SAM is basically a small computer, it wont stand up to reverse polarity.
Your are lucky if only rear SAM and F32 pre-fuse gone bad, each and every other modules connected to the Rear SAM and active at that point
in time, could have been damaged too.
Next time, main battery negative has to be disconnected in any work related to front or rear SAM or prefuse F32 or AUX battery.
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
I'm lucky enough to have to same thing happen to me. if you don't mind me asking...how much did the dealer charge??
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, no data support.
