BMW BSD (Bit Serial Data) protocol

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BMW BSD (Bit Serial Data) protocol

Post by Tivtech » Fri Jun 23, 2017 5:58 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: ... erface-BSD ... 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: ... 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)?
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

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Re: BMW BSD (Bit Serial Data) protocol

Post by victor2k » Thu Jun 29, 2017 4:28 am

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.
Last edited by victor2k on Sat Mar 16, 2019 8:53 am, edited 1 time in total.

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Location: Denmark

Re: BMW BSD (Bit Serial Data) protocol

Post by KimAndersen » Thu Jun 29, 2017 6:34 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.

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


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Re: BMW BSD (Bit Serial Data) protocol

Post by mdover519 » Thu Dec 21, 2017 12:32 pm

I have seen this problem before, the answer has nothing to do with your patterns. There are grounds on the top of the valve covers (short ground cables by the valvetronic motor on each bank). Someone at some point left those out and ran the engine. The DME, oil condition sensor, and alternator need to be replaced, based off of what I've seen in the past. All three are on the bus and they are all taken out when this happens. Is this a N62 engine?

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Re: BMW BSD (Bit Serial Data) protocol

Post by blurrrache » Tue Oct 30, 2018 7:15 pm

Did anybody managed to repair this issue, without changing the DME? Can it be repaired?

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Re: BMW BSD (Bit Serial Data) protocol

Post by TLK914 » Wed Mar 06, 2019 12:48 am

I have fixed that fault.

NOW, if you get a bit serial data failure code, 1st check would be the network line wherever easiest to grab on your given car. Alternator or coolant pump or radiator blinds depending on vehicle. Line for BSD starts at the ECM, when stable it will waver 8-10v on a scope or meter. KOEO its usually 9v or so. if it goes to 12 or close to zero, one of your bit sensors is pulling it down or its shorting toi power or feeding back from poor grounds. They are OIL LEVEL, SECONDARY COOLANT PUMP (IE - intercooler or DME), Radiator shutter blinds (not all cars have them), Alternator regulator feedback, and Intelligent battery sensor.

As with most networks, wide opens will go no change, or dead shorts will kill the whole line, you may be able to decode the comm strings, though why would you want to? thats like doing lower end inspection disassembly on siezed engines. Whatever you think youll know or can surmise from what you record on a scope, it will end up to the same part to fix at the end.

For reference, the oil level sensor is a constant update. It will code the moment it is unplugged. The ALTERNATOR is NOT constant, due to the fact it is disabled cranking and doesnt actually come online or get commanded for a few seconds after startup. If the alternator is failed, it can take over a minute of idling before the ecm will fault the component because it simply doesnt require it to comm more often than that They can both talk simultaneously, they do it in data blocks. if network is receiving a block from one module, when the block ends the next module sending in line will send its block.

As posted, the grounds on the head being loose or left off entirely will caused bad comm and fixed 12v. 3 bolts per side, near the coils. they must be tight and connected.

low voltage (under 7v on down) is a short. start unplugging the listed components while tapped in with a meter and the voltage should jump up when a bad component is disconnected (remember, its a very short message block comm, so they simply morse code their messages then back away, if they are dragging voltage down on a full time basis, they arent working properly)

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Re: BMW BSD (Bit Serial Data) protocol

Post by TLK914 » Wed Mar 06, 2019 7:34 pm

last paragraph cut off? ahwell. If the ME cant handle all sensors plugged in, its not getting adequate ground OR its simply lost it. I replaced one DME this week, shorted oil level sensor wiped it out and made a hardcode. No other codes but BSD line down. Found fixed .7v on alternator line. Disconnected stuff till found 9.9v when Oil sensor disconnected. Replaced sensor, now line working, can comm and control BSD devices again, but BSD code still present after 2 key cycles. replaced DME and all seemed well.

HOWEVER, After DME replacement, car is fine for 1 hour running usually, THEN it will code for ALTERNATOR COMM. Prior to that it doesnt code, controls alt fine, and BSD line looks identical when coding or when colkd and working fine. Alternator quits responding on BSD after it gets hot(Running voltage also drops from 13.3ish varying to 13.1 fixed), and it also fails load test (maxes out at 60 amps or so on vat when hot). UNPLUG the oil level sensor, alternator code will go away (oil level will then code) but still charges lower than when cold. There are random service messages and posts on the web about BSD faults mean replace everything (DME, ALT, IBS, OLS). Im gonna lob just an alternator at this one and Im pretty confident that will be all it needs. IBS and Cooler pump work and arent coding, Im not lobbing parts that dont test bad because other cars needed them.

I gave up using network waveforms for diagnostic purposes. It was making me try and out think a module that isnt telling me what it wants ANYWAY. a crank sensor or a cam sensor is one thing, but comm lines have reasons for saying things differently. And if you dont know all the reasons they know right now, youll never know why the message was different.

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