General

General:

The BMW Diagnostic System:

BMW MoDiC:

A large number of control units with self-diagnosis capabilities are used in BMW automobiles. In addition to the control of the relevant system, these control units are capable of making available or receiving data for diagnostic procedures. The data can be read out or entered by means of the BMW DIAGNOSTIC SYSTEM.

TROUBLESHOOTING & DIAGNOSTIC PROCEDURE:

The specific engine control system you happen to be troubleshooting/diagnosing dictates what method of diagnostic procedure you must follow.

With the introduction of the DIS all of the Diagnostic Manual information was incorporated into the DIS software. Data communication with a specific control module provides an up-to-date system specific interface. The DIS provides Fault Symptom troubleshooting procedures with access to Fault codes, Status Requests, Component Activation, Special Service functions, etc.

This section provides information regarding how the control modules communicate diagnostic information and/or fault recognition to the DIS and with each other via CAN (Controlled Area Network) communication.

Additionally, Engine Control Module Variant Encoding (M1.x) or Control Module programming (>M3.x) is another topic that must be considered when diagnosing a system. If a control module has been coded or programmed incorrectly, it could possibly induce driveability complaints and or incorrect system function.

DIAGNOSTIC COMMUNICATION:

RXD/TXD::

TXD II::

DIS/MoDiC to control module communication is dependent on the integrity of the Diagnostic Link (D-Bus). The diagnostic link is a wire circuit in the vehicle that connects to all control modules designed to communicate with the DIS tester. The connection point for the DIS with the diagnostic link in the vehicle is at the 20 pin diagnostic connector in the engine compartment.

The diagnostic link has evolved through the model years as follows:

RXD/TXD: The wires in the 20 pin connector are identified as RXD (pin 15) and TXD (pin 20). This configuration is used with all M1.x and M3.x systems.

^ RXD is a one way communication link from the DIS/MoDiC to the control module which initiates the start of communication.

^ TXD is a two way communication link where the diagnostic data is transferred back and forth between the DIS/MoDiC and the control module.

TXD II: With the introduction of OBD II, the diagnostic link changed to meet the needs of the OBD II Data Link Connector (DLC). The DLC is located in the passenger compartment and provides a connection for an aftermarket Scantool for powertrain systems only. Scantool communication with these control modules is only possible when the 20 pin diagnostic connector cap is installed. A circuit bridge in the cap completes the DLC circuit.

The original RXD/TXD link is still in the vehicle for non powertrain systems. But, (control module dependent) RXD may not be used at all since later control modules communicate with the D2 protocol (different language) which only requires TXD (single wire communication).

TXD II (pin 17) was added to the 20 pin diagnostic connector to provide a separate connection to Drivetrain control modules (DME, AGS, EML) for OBD II compliance. The separate connection is for security reasons. The DIS/MoDiC share the same TXD II wire though the Scantool and DIS/MoDiC communicate with different "languages".

^ Scantool "speaks" ISO 9141 protocol
^ DIS/MoDiC "speak" D2 protocol.

If problems are encountered trying to establish communication, consider the following:

^ Battery voltage too low (connect a battery charger prior to communication).
^ Power and Ground circuits for the control module possibly open.
^ Diagnostic link circuits possibly open or shorted.

CAN BUS TOPOLOGY:

CAN (Controlled Area Network):

Star Coupler Connector:

Twisted Pair Wiring:

Introduced with the 1993 740i/iL, the Controller Area Network (CAN) provides a fast communication link between the Engine and Transmission Control Modules serving as both an input and output multiple signal bus. The CAN consists of two wires (CAN High and CAN Low) with a ground shield. In it's the early configuration, the CAN bus is a simple end user bus system.

With the introduction of the 1995 75OiL, the CAN bus was expanded to include both engine control modules, transmission control module, EML IIIs control module and the DSC II control module.

A "star coupler connector" was used to link the individual CAN bus leads from the modules into one common connector strip.

The 740i/iL models do not use the star connector. The single DME, AGS and ASC control modules are linked at a splice point in the wiring harness.

The 1998 model year brought forth the next generation of CAN Bus communicators. The
Instrument Cluster and Steering angle sensor of the DSC III system where also linked to the CAN bus to expand the available signalling capabilities.

The Star connector continues to be used for the 75OiL but wiring layout (topology) was changed to improve communication integrity in the event of a break or short in another section of the CAN bus. This was accomplished by using the DME control modules as gateways for the EML and AGS control modules.

Additionally twisted pair wiring is used for the Instrument Cluster and Steering Angle Sensor which provides EMI protection without a separate ground shield.

The CAN bus of the 1998 740i/iL and 540i has also changed. The entire CAN bus is twisted pair wiring. The EGS is linked to the DME by a dedicated twisted pair CAN bus with a common twisted pair to all other control modules.

The CAN bus must be considered when diagnosing an engine control system. Faults may manifest due to a breakdown in a segment of the CAN bus with another control module causing a fault to display in the engine control module only due to lack of CAN bus data. When diagnosing a specific system, always refer to the Electrical Wiring Diagrams for up-to-date configurations.

CONTROL MODULE VARIANT ENCODING AND PROGRAMMING:

Engine control modules are either variant encoded (M1.x) or Programmed (>M3.x). The possibility exists that an incorrectly coded or programmed control module will cause system faults and or driveability complaints with no faults found.

Prior to condemning a control module as being defective, it should always be considered as a possibility that the control module has been incorrectly coded or programmed. This can simply be checked by connecting the DIS and displaying the control module ID page.

Variant Encodable Control Modules:
^ The variant code of an M1.x control module is displayed on the control module ID page.
^ Compare the displayed 4 digit code with the label on the control module and/or from Technical Service Bulletin 13 02 90.

Programmable Control Modules:
^ The ID page provides the part numbers of the control modules. Compare these numbers to what should be in the vehicle by referring to Technical Service Bulletin 13 02 90. Also, perform a determination process with the DIS/MoDiC for correct part numbers.
^ Look into the vehicle history file for possible service actions that have been performed on the vehicle.
^ If programming has been performed on the control module recently, consider reprogramming it.