To solve problems with huge electricity consumption, for BMW E6X/E8X/E9X and following models “intelligent” alternators are used. They only visually resemble a “simple” alternator! The fundamental difference of these alternators – they don’t maintain an exact (fixed) onboard voltage of the car but can be managed (with max required voltage: 16.0V) via a serial interface.

These alternators provide a max current of 120 .. 180 A and are managed via BSD (Bit Serial Data) interface.


What is BSD?

Checking TIS data, you can found connection schemes of BSD units, but more detailed information is not available. Actually – it’s quite logical. Why the user should go deep in such technical nuances? Even more – most probably, there is a lot of commercial secrets and innovations!


Additional information regarding BSD.

BSD is a network, based on LIN principle. Passive level of signal (“0”) – close Ubat (12 V); active (“1”) – close 0 V. More detailed description regarding LIN interface here. Architecture: master/slave. Master – DME; slave – peripheral units. Unlike the “usual” LIN, BSD ensure two-way data exchange, it means: peripheral units “respond” after inquiry of master – send data, but in case of BSD master also can send to peripheral units different management commands. What commands should be sent? For example, order the peripheral unit to perform self-diagnostics; identify itself (send ID or model, configuration data); send status information (error logs), etc.


More exact about the alternator.

In addition to commands, mentioned before (self-test, error log, ID), alternator receives the command, what voltage should be maintained, in what mode to work (in addition to a “regular” mode there is also a mode in case of discharged battery, etc.).

Instead, the alternator sends to DME following data:

  • a voltage on its clips and status (has/hasn’t succeeded to maintain requested voltage, how much time it took to reach);
  • current, consumed from the alternator;
  • induction winding current;
  • the temperature of management chipset;
  • the temperature of the alternator’s stator.

From these data DME additionally calculates:

  • the alternator’s mechanical load to the engine;
  • relative load of the alternator;
  • monitors plausibility of data, sent by the alternator, it’s “health”, controls allowed current (also – limitations of temperature/overload), manages electrical balance (don’t allow voltage drops in case of low SOC of the battery), etc.


So the live data lookalike – data, received from the alternator (part of data) and data, calculated by DME:


Here, technical basic information from BMW AG regarding alternators, used in E6X/E8X/E9X:

Short derogation. Alternators, used by BMW cars, are VERY powerful – in idle they are able to develop even 50 % of max current (or 80 .. 90 A for powerful versions of 160 .. 180 A).


Returning to the image – please, pay attention to the line with underlining. DME checks (controls), exactly what kind of alternator is connected to the BSD network! How does this control happen? DME asks the alternator to its ID, checks it and decides, is it “own” or a “stranger”.

ID contains two kinds of information:

  • identification data;
  • configuration data.

Identification data describe – to what (what model/engine type/car manufacturer) this alternator is intended. Configuration data describe the more exact specification of the exact alternator (max allowed voltage, max allowed temperatures of stator and chipset, etc.). Only in case, if the data of identification and configuration will correspond to FR of the car, DME will start to manage the alternator! In the opposite case, the error messages will be recorded in the error message memory of DME, and the alternator will not be managed!


Manageable alternators are used by all leading manufacturers. To make it easier, manufacturers of alternators produce “physically” identical alternators for all partners (car manufacturers). Even their management modules (they are not simple voltage regulators or “tablets” anymore) looks like from outside. They are alike – as much as it refers to the electronic part. The software makes a difference. Yes, exactly – the alternator management module is a small computer, which operation is defined by software. Here, BMW AG decided, that data exchange speed for BMW cars will be 1200 bits (bods) per second, defined their own (described only in the documentation, intended for internal use only) protocol, named it BSD and – done! Another manufacturer can use other (or even the same) data broadcasting speed, define their own (different) data content and – named it in another way, use the same models of alternators, only with corresponding software. Software defines – for example, this alternator will suit only for BMW petrol engine car with N52 series engine, or this – only for Mercedes with the exact type of diesel engine.

These received/sent data for different manufacturers are completely different! This is a free interpretation of current car manufacturer’s development team – to decide, which data should be sent first, which – later; how to describe each of parameters, etc. Then the car manufacturer sends these data concepts to the manufacturer of alternators, which introduces in the management module of the exact aternator model. Alternators, intended for different car models and different manufacturers will not “agree” with DME of the inappropriate car!



Structure of the alternator actually is very simple, I won’t write a lot regarding repair. I will mention only some nuances, which are related to the replacement of their management module.

I will immediately add – I don’t know EVEN ONE CASE when after repair, done by the third side, these intelligent (manageable via BSD) alternators would continue to normally “communicate” with DME. Respectively, in ALL cases after replacing management module, fundamental problems appeared. These problems can be divided into two large groups.

1. After repair, the “old type” management module, which doesn’t provide anything like intelligent management, is installed. In this case the alternator “behaves” in the following way:

  • it paralyzes all data exchange of BSD interface: nor coolant pump, nor oil quality sensor and other peripheral systems are able to communicate with DME;
  • after starting the engine, the alternator doesn’t turns on – it doesn’t starts charging. To turn it on, it requires to increase RPM for several seconds close to redline. At this moment the alternator start to work and continue to maintain fixed voltage till next time of starting the engine;
  • alternator maintains fixed voltage: 14.0 V or 14.4 V, depending from management module releases (“regular” or “cold climate”).

Such alternator can be used as a “regular alternator”- wire, intended for BSD connection connect to bus 30 via a resistor (or control lamp, which will indicate charging problems), but – serious problems are guaranteed (what kind of problems – read below).

2. After repair, some kind of “universal” management module is installed. In this case:

  • typically – other customers of serial interface are able to communicate with DME, but regarding the alternator, there are active and continuous error messages;
  • the alternator works in emergency mode – maintains fixed voltage (typically: 14.3V), it is not manageable via serial interface.


Of course, in both cases IBS system of BMW will not work, the battery is not charged correctly, energy management system doesn’t work. With the probability of 99 %, in wintertime you will stay with your car in some remote place and will not be able to start it again. If IBS system doesn’t work, also warning system (regarding small amount of energy left in battery) won’t work, all energy economy modes (both with turned on engine – disconnecting most “edacious” consumers – and locked mode) will be turned off, the car will not be able to go to sleep correctly – discharge of battery will continue even in time, when the car is locked and as if “sleeping”. Additionally, without management of IBS, the diode bridge of the alternator will be damaged quickly – it will be overheated or diodes will get damaged, when overreaching the max allowed current.


Why correct performance of the IBS system is so important (including correct registration of battery), read here, here, here and here.


I have made a definite conclusion for myself – for these intelligent alternators, after repair (replacing bearings, collector,  diodes – if necessary), ONLY OEM management modules, intended for the EXACT model of car, should be installed! Other “alternatives” will not work!


At the end.

The problem, which quite often can not be solved by even as if “specialized” alternator’s repair centers: how to indicate a damaged diode?


  • when starting the car, the alternator starts to heat up, in short, its temperature reaches over 100 oC;
  • in worst cases: alternator heats up also with turned off engine, onboard voltage drops rapidly.

Diagnostics sequence:

a) disconnect the alternator; take off the decorative cover of the alternator;

b) turn on the multimeter in range of 200 Ohm;

c) one clip of multimeter connected to the winding of alternator’s stator;

d) other clip of multimeter connect sequencly to plus and minus output of the alternator.


In both measurements, multimeter should indicate OL. If even in one of the cases multimeter indicates low resistance (several Ohm or less), at least one diode, which is connected to this output clip (accordingly – plus or minus) is damaged (short-circuit).

Disconnect the stator windings, check each diode separately.


When diodes are disconnected from stator windings, switch multimeter to a range of “Diode” or 2K Ohm. If the diode is in working order (in temperature of 20 oC):

  • direct connection (+ clip of multimeter connected to its anode) has to indicate 200 .. 400 Ohm (a drop of mV);
  • in opposite direction (+ clip of multimeter connected to its cathode) has to indicate OL.


If even one result of measurements differ from mentioned above – the diode is damaged and has to be replaced!

Attention! Diodes, connected to plus and minus outputs, are different (they have different “polarity”)!