KiloWatt

Well, we had a really hot summer day and some slow inner city traffic on top. And after the hot vehicle had been parked for 10 minutes ... similar faults messages came up again.

But it was not exactly the same behavior. For example the ABS etc. .etc. messages cleared all by themselves every time after a couple of meters of driving without a quick restart of the engine as before. Sometimes there was just a quick flash of these messages and they disappeared immediately in a fraction of a second after startup after parking during that hot day.

So yeah, very interesting to connect Xentry and check for faults once again. Hmmm, NO faults from the new speed sensors on the font axle.

BUT a stored unplausible speed fault from the left REAR speed sensor. OMG what is this, will it never end???

Or maybe reseating the (possibly to some degree corroded) big N47/5 ABS module connector made the difference in behavior and the old front sensors were all good?

Because there were some stored messages that the N47/5 could not be reached (but maybe this could be caused by a bad sensor too that confuses it?), I did a couple of CAN checks. It was all good (as can be seen in the attached screenshot, other not present module RFK is backup camera btw.) except that the BNS (also called BSG or in English BCM - Battery Control Module) was not present even though it should be according to the setup. So, I thought voltage issues like faulty batteries can cause these strange effects and maybe this module is not doing what it should do.



But this is a one battery generation car and as far as I could do research until now, the BCM was only present in the older two battery cars (N82/1 module above aux battery). So, did Mercedes just forget to eliminate this module from the CAN setup for this (early) one battery generation? Or is there a different BCM module in some other location that is not working properly?

Two more things I did now just to make sure was to locate and reseat all the fuses to the N47/5 ABS module and perform a successful recalibration of this ABS module.

Big question is what to do next:

Replacing rear speed sensors too?
Replacing N47/5?

Or is it the just two year old small backup battery in the dash in the end? Would be weird though, because the 2007 320CDI (that I haven't had the opportunity to continue working on so far) with quite similar issues doesn't even have that backup battery but the big aux battery in the rear...

BOTUS

the rear ABS sensors usually fail first

suspension movement can break wires in the cables and the IPC chips used to amplify the magnetic field they see, are made of chocolate
the rear reluctor rings corrode and fall off
and disc and pad detritus builds up on the inner front wheel bearing seals

then like most brands the ABS module will die (its just they must have spec'ed the ESP motor brush guides to a normal clearance rather than the fail almost immediately spec BMW buy in on theirs)

if the car was driven on smooth roads vs another that got a rougher ride then snapped wires comes first - otherwise with similar treatment u are only in the hands of the engineers and accountants design to fail magic where they spent 20 years building a method to make deliberate failure come in a clever, more random manner than the moral police have cottoned on to


.

KiloWatt

Tested the small backup battery that seems fine with 13.1V after 2 days of resting vehicle (but nobody knows if voltage will break under load of course). And checked + reseated the connectors and the hidden 5 amp fuse of that battery.

We will see if the faults come back.

But can anybody tell me if a 2009 pre facelift S600 with single battery actually has that mystery "BNS" module on the CAN bus or if the power/battery management module was eliminated without correcting it in the config of the single battery generation?

KiloWatt

Haha, a German w221 pre facelift S600 (don't get confused by the exterior mods) with exactly the same issue it seems. Yes, parking brake going crazy because the car thinks it is moving with parking brake.
And ABS ESP faults disappear after a quick burnout. Here you go...

KiloWatt

Come to think of that maybe it has been the rear sensors' fault all the time. If the rear sensors report a speed even though the car is not moving but the car is programmed to "trust" the rear axle more, it might interpret the 0 speed from the front sensors as unplausible speed, store the fault and shut down the ABS module etc.

Interestingly, parking brake management (rear axle!) goes berserk and the car often does not want to pick up power until the fault clears, probably thinking one of the rear wheels is slipping badly...

BOTUS

my original sensors failing made the park brake go odd before the rear ABS sensors died completely - I was getting extra bings and bongs at pull away for 6 months and at times the park brake was off when I knew I had put it on, and then testing sometimes the auto park brake release on pull away wouldn't operate

a new OEM sensor on the opposite rear wheel died inside 4 years - with a totally different one off, dead as a door nail failure mode - then the gearbox went metal as well - that side is back on its original 17 year old sensor

KiloWatt

Yes, very interesting - I will replace the rear sensors and long for a hot day to test things thorougly ...

KiloWatt

Replaced both rear axle speed sensors and had the opportunity for a long 8h trip in warm weather including some stop and go city traffic and multiple short stops and restarts with a hot car.

No issues whatsoever fortunately. Maybe one of the rear sensors was the problem from the very beginning, and the module just "thought" the front sensors must be wrong.

Spoke to the AMG certified expert at one of Mercedes' own gigantic German service work shops btw. He said that there is no reason to change both sensors on one axle. They only change the one that is faulty and leave the rest until they stop working. Well, in my case I would have saved a lot of hassle if I had changed all sensors on both axles.

There was one nerve wrecking moment after the rear axle sensor replacement. I had the idea to perform an ABS module drive test according to the Xentry procedure.

I seemed to not be able to perform the drive no matter what I did, but module got stuck in drive test mode. Even a restart of the engine, Xentry, whatever, did not help. It was stuck in test mode with the ABS lamp glowing.

Then I had the idea to NOT perform the drive procedures when the ABS module is accessed by Xentry in diagnosis mode (and you actually see the instructions). Don't think this makes sense at all because you have to remember the test sequence EXACTLY without instructions on the Xentry computer screen. But then I finally managed to successfully finish the drive test and everything was fine.

Moreover, the Xentry instructions for the drive test are not really correct for the w221. There is an ABS lamp in the dash, but not the ESP lamp that is mentioned and crucial for the test. Instead you have to look for the yellow exclamation mark triangle on the screen to know what's going on. Oh well, I guess they forgot to update the instructions for cars with the screen.

Just so that you guys think twice before doing the drive test and keep the above things in mind to not get a stuck ABS module...

Will keep you updated when I have chance to replace the sensors of the our S320CDI.

BOTUS

in my experience both points are normal

trying to get the car to behave during test procedures is almost impossible
I think the instructions are deliberate lies - I can only guess certain stuff is just to trip up non trained personnel

I wonder if there is a list of lies to trip up non franchised shops, that they train you to ignore - and critical steps they tell you about on dealer training courses - if its not this they are just totally incompetent

KiloWatt

Scary stuff, I guess one should only do these things, including updates etc., when it is absolutely unavoidable.

BOTUS

many third party tools especially high end ones can do a lot - but seem to struggle...

I have a 4k US$ snap on one that on current software (at the time) had

the exact gearbox TC adaption menus as Xentry
and a certain set up for engine adaption clearing

then I got the update to 2023.4 and both had changed from being one way for years - to a simple wipe and never try to reset a new set up - whats that about ???

KiloWatt

Wow, weird. I gave up on 3rd party stuff after some frustrating first steps years ago. I had hoped that at least their own Xentry system is kind of solid, but no obviously...