Archive for May, 2013

10
May
13

BMW MRSZ “1 internal ECU error” fix

I lost almost an entire day trying to figure out what went wrong when I was playing with my E36 airbag module coding in an attempt to disable passenger seat occupancy detection.

I didn’t understand why most MRSZ settings were duplicated, such as these two:

ERKENNUNG_SITZBELEGUNG_1
	aktiv
ERKENNUNG_SITZBELEGUNG_2
	aktiv

I set only one of these to “nicht_aktiv”, after which the module threw an “internal ECU error” and the FS_LOESCHEN command returned ERROR_ECU_REJECTED. Even after I restored all coding settings back to their original values, the error remained.

As it turns out, after digging through 24KB of microcode, the reason to duplicate every setting is very simple – it is a form of EEPROM error detection. The unit periodically checks to see if every setting has the same value in both copies, and when that is not the case, a static error 195 is logged. Every error code above 100 is mapped to the “internal ECU error”, and when that happens the erase function is also blocked.

Fortunately, at the same time I also found a “secret” command to write EEPROM bytes. Not all areas can be written with that command, but at least the area where the errors are stored is writable.

Attached is an EDIABAS script that attempts to “reanimate” an MRSZ unit. Note that you need to restore the coding to a working state before running the script. NCSExpert’s SG_CODIEREN command will end with an error, that is because it tries to erase errors at the end of the process, which fails, the actual coding should in fact be OK.

MRSZ_FIX.PRG (beta)

Note – the script supports only MRSZ units (MRS v06, P/N 8374799), it does not support MRSZ2 (MRS v07, P/N 8372521), MRSZ3 or MRSZ4. The “secret” command simply does not exist in these modules. I do not know if it supports the older ZAE2 (P/N 8374798) units.

The source code of my MRSZ2 attempt can be found here. It includes working support for MRSZ.

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