BMW BSD (Bit Serial Data) protocol

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BMW BSD (Bit Serial Data) protocol

Postby Tivtech » Fri Jun 23, 2017 7:58 pm

Hello all,

We have this BMW 750i with weird behavior. Summarizing, it seems that the digital signal from the alternator regulator (a replacement, non OEM unit) is corrupting the signal from the Engine oil level sender and vice versa. Both signals use the same communication bus to talk with the ECM. When both of these devices are connected to the bus, the engine oil level cannot be read and it is displayed "greyed out" on the multimedia screen in the vehicle. If the alternator regulator connector is disconnected, oil level is read and displayed correctly and the alternator still charges the battery as checked both by battery voltage (about 14V) as well as a DC current clamp probe (in the range of 50-150 Amps), so it seems the regulator is working in fail-safe mode without being controlled by the PCM. At the same time, a diagnostic trouble code is set in the ECM (and can be read with a diagnostic scanner) related to the regulator being disconnected.The regulator fitted to the alternator is non-original AFAIK, yet the the ECM detects it as well as when it is disconnected from the bus as described above.

The protocol must be bi-directional since it delivers data from the ECM to control the alternator charge current (while the ECM monitors battery voltage at the battery terminal with the IBS, the intelligent battery monitor sensor), while also delivering data from the oil level sender to the ECM to be displayed on the vehicle multimedia screen.
Also, it might be controlled by the ECM (serving as bus master, polling the slave devices on the bus).

Looking at the signals on this bus with a scope reveals a variable duty cycle 12V square wave with a period of 830uS.
When only the oil level sender is connected the signal looks normal, but when the alternator regulator is connected to the bus too, signal contention can be seen in the form of shorter than normal spikes occurring at random places in the waveform as can be seen in the attached pictures.
It is suspected that the replacement non-OEM alternator regulator is causing this signal contention by not adhering to the specified protocol and sending data at random times, thus causing the signal contention with the oil quality sensor.

Searching the web we have found descriptions of this protocol:
https://www.scribd.com/doc/91834548/Bit ... erface-BSD
http://www.bmw-planet.net/diagrams/rele ... 62_BSD.htm
So it seems that this bit-serial data interface is a single-wire data bus with a data transfer rate of 1.2 kBd.
Indeed, this info matched our own measurements (12V signal level, 830uS = 1200 baud).

Moreover, we even found an article here on the PICO website that describes this signal and it's functionality as we concluded by our own measurements:
https://www.picoauto.com/library/case-s ... tion-error

Thus, we would be glad to find answers to the following questions with the help of knowledgeable forum members:
1. Can anyone verify our conclusions regarding BSD bus contention caused by using a non-OEM alternator regulator?
2. Does anyone have info regarding the BSD protocol and methods to decode it?
3. Do the oil quality sensor and alternator regulator constantly transmit data and somehow synchronize to avoid contentions or is it only in response to ECM requests (more probable)?
Attachments
2017-06-21_22-14-30_557.jpg
DTC's when Alternator is disconnected from BSD bus
BMW good BSD signal 2.jpg
Seemingly good BSD signal, only Oil sensor connected
BMW good BSD signal.jpg
Seemingly good BSD signal, both Alternator and Oil sensor connected
BMW BSD signal contention.jpg
BSD signal with possible signal contention, both Alternator and Oil sensor connected
BMW 750i DTCs.jpg
DTC's when both Alternator and Oil sender are connected to the BSD bus
BMW 750i error message on multimedia display.jpg
BMW 750i Oil level display on multimedia display, when error occurs greyed out
Tivtech
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Re: BMW BSD (Bit Serial Data) protocol

Postby victor2k » Thu Jun 29, 2017 6:28 am

Hello,
You must replace your alternator/regulator with a working one for finding the answer.
Many of these non-OE spare parts not work/work bad on BMW.
Looking at the" BSD signal with possible signal contention, both Alternator and Oil sensor connected" photo I think the regulator keep too much the BSD line shorted to ground,so oil sender can't talk with ECU.
Regards
victor2k
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Re: BMW BSD (Bit Serial Data) protocol

Postby KimAndersen » Thu Jun 29, 2017 8:34 pm

Hi there

I found some informations regarding voltage regulators that might interest you and others.

Its from a website called Motoplat, where there are a description of the different BSS systems and how they are compatible between different automotive makers and respective alternators and voltage regulators.

Look under the menu BSS Connection. http://www.motoplat.nl/techinfo.html

After have read the section - I clearly see how things can get complicated !. :wink:

Regards
Kim
KimAndersen
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