Bus Line
A bus line is a group signal line that transmits serial data in both directions. It may consist of one wire or two. All control units are connected in parallel in current bus systems, this means that the information sent can be heard by all of the connected units.
Bus Subscriber
Any control module connected to a bus line. e.g. DME, IKE, GM, etc.
Gateway
A Gateway module provides a link between different bus lines to provide a means of sending information from a subscriber of one bus line to the subscriber of another. The Gateway module recognizes from the receiver address whether a message is to be routed through the gateway or not. e.g. IKE, KOMBI.
Master Controller
A Master Controller of a bus system provides the operating voltage and wake up signals to the subscriber modules. This task may also be performed by several Standby Masters within a bus system. e.g. GM, LCM.
Serial Data
Serial means one event at a time. In data transmission, the technique of time division is used to separate bits of data sent. The messages sent over a bus are configured serially.
Each message consists of
- Transmitter address
- Length of data
- Receiver address
- Command or Information
- Detailed description of message (data)
- Summary of transmitted information (check sum)
All of the connected control units will receive the information but only the unit in the address will accept and react to the data.
Note. This message format is not used for the CAN Bus.
Topology
In the context of communication networks, Topology describes the configuration or arrangement of a network.
CAN Bus Topology
The CAN bus consists of two twisted copper wires. Each wire contains an opposing signal with the exact same information (CAN-High, CAN-Low). The opposing signals transmitted through the twisted wire serve to suppress any electrical interference. Early CAN bus wiring included a grounded shield around the two wires, later vehicles discarded the shield in favor of the unshielded twisted pair wiring.
Due to the linear structure of the network, the CAN bus is available for other modules in the event of a disconnected or failed control unit. This is referred to as a "Tree" structure with each control unit occupying a branch.
Identifying Tree Structure (95-97 E38 750iL). Scheme 40
As previously mentioned, the CAN bus initially was used as a high speed communication link between the DME and AGS control units.
With the introduction of the E38 750iL (95 M.Y.), the CAN bus was expanded to include the EML and DSC control modules. The 750iL made exclusive use of the "star coupler" to link the individual CAN bus ends to a common connector.
The 1998 model year introduced new users of the CAN bus. The instrument cluster and the steering angle sensor were linked to expand the signal sharing capabilities of the vehicle.
The 1999 750iL was the last vehicle to use the shielded cable, after which the entire CAN bus went to twisted pair wiring.
Identifying Instrument Cluster CAN Bus. Scheme 41
On models that use twisted pair, the wire color of the CAN bus is uniform throughout the vehicle with: CAN-Low GE/BR and CAN-High GE/SW or GE/RT. Shielded wiring is easily identified by the black sheath surrounding the CAN bus.
Failure Of The CAN Bus Cables
The following faults can occur to the CAN bus wiring
- CAN-H/L interrupted
- CAN-H/L shorted to battery voltage
- CAN-H/L shorted to ground
- CAN-H shorted to CAN-L
- Defective plug connections (damaged, corroded, or improperly crimped)
In each instance, the connected control units will store a fault due to the lack of information received over the CAN bus.
The voltage of the CAN bus is divided between the two data lines: CAN-High and CAN-Low for an average of 2.5V per line. The voltage measurement is taken from each data line to ground. Each module on the CAN contributes to this voltage.
The fact that 2.5V are present does not mean that the CAN bus is fault free, it just means that the voltage level is sufficient to support communication.
Identifying Voltage Display. Scheme 42
Terminal resistors: are used in the CAN bus circuit to establish the correct impedance to ensure fault free communication. A 120 Ohm resistor is installed in two control units of the CAN between CAN-H and CAN-L. Because the CAN is a parallel circuit, the effective resistance of the complete circuit is 60 Ohms. On some vehicles there is a jumper wire that connects the two parallel branches together, others have an internal connection at the instrument cluster.
The resistance is measured by connecting the appropriate adapter to any of the modules on the CAN and measuring the resistance between CAN-L and CAN-H. The resistance should be 60 Ohms. The CAN bus is very stable and can continue to communicate if the resistance on the CAN bus is not completely correct; however, sporadic communication faults will occur.
The terminal resistors are located in the ASC/DSC control unit and either the instrument cluster or in the DME.
Early 750iL vehicles that used the star connector have a separate external resistor which connect CAN-H and CAN-L together.
Modules which do not have the terminal resistor can be checked by disconnecting the module and checking the resistance directly between the pins for CAN-H and CAN-L. The value at these control units should be between 10k Ohms and 50k Ohms.
Identifying Terminal Resistor Diagram. Scheme 43
If there are CAN communication faults that use the term "Timeout" this refers to a module not being able to communicate with another on the bus. Each module on the CAN bus will attempt communication several times. If unsuccessful, the module will store a "Timeout" or "CAN bus" fault and determine that there is a problem with either the bus line or the module that it is trying to communicate with.
These types of faults may indicate a problem with the bus wiring, interference, missing data or failure of the communication module of an individual control unit.
Checking the CAN lines is carried out just like any other wiring. Perform continuity tests between the connections of different modules (all modules disconnected) without forgetting to make sure that the two CAN lines have not shorted to ground or to each other. It is recommended to use the "Wire Test" in "Preset Measurements" which is more sensitive than just a resistance check.
If Voltage level and the wire test are O.K, then looking at the communication signal may be useful.
The following are some examples of scope patterns that may be observed when checking the CAN bus.
Identifying CAN Positions. Scheme 44
Example Of Correctly Operating CAN Bus
Correct communication on the CAN bus occurs in sporadic bursts with short periods of steady voltage.
Identifying Defective CAN Bus Signals. Scheme 45
Rapid Constant Fixed Duty Cycle For 10 Seconds
This example represents the output signal produced by an AGS module that is isolated from the bus. This pattern times out after 10 seconds and remains a flat line at 2.5 volts until the key is cycled and the event is repeated.
Identifying Defective CAN Bus Signal Rapid Constant Fixed Duty Cycle For 10 Seconds. Scheme 46
Constant Fixed Duty No Time Limit
All of the other control units with the exception of most current AGS modules will continue to try and send information even though the control unit has already stored a "Timeout" or CAN fault. This type of signal may only be seen if a section or all of the CAN bus is disconnected.
Identifying Display Constant Fix Duty. Scheme 47
Flat Line At 2.5 Volts
If a continuous flat line is present at one or both CAN lines of a particular control unit, this may indicate that the CAN is open to that particular module. The module may have timed out and is waiting for a signal from another control unit. Check the CAN bus at other points to see if communication is occurring else where on the bus.
Identifying Display CAN High Shorted. Scheme 48
CAN High Shorted To CAN Low
If the CAN bus lines were to become shorted to one another then the signals would cancel each other out and effectively be a flat line.
Failure Of One Of The Control Units Attached To The Can
Each control unit connected to the CAN has an integrated communication module that makes it possible for that control unit to exchange information on the CAN. Failure of a control unit normally triggers a fault code in the other control units connected to the bus.
There are instances where failure of a module may paralyze or take down the entire CAN bus. This scenario would be evident by CAN faults stored in every control unit on the bus.
In order to isolate the defective control unit, the control units can be disconnected one at a time while monitoring the status of the CAN using a Voltmeter or oscilloscope. This can be further reinforced by clearing the faults of the remaining control units and then reading them again. If the disconnected control module is the defective one, the faults will only point to communication with that interrupted module and no one else.
As a quick check on vehicles produced after 9/97 (3/98 for the E39 528i) that have the CAN connection to the Instrument cluster, the indicators provide visual indication of whether communication is restored.
If for example the tachometer and temperature display are plausible then communication is occurring between the DME and IKE/KOMBI. Other indicators such as transmission range or the DSC light may give clues to the communication status with those control units.
Once the module has been replaced and coded or programmed, perform the CAN bus Test Module in each control unit to ensure that communication is OK.
Identifying Display E38/39. Scheme 49
Identifying Display E46. Scheme 50
Failure Of The Voltage Supply To Individual Modules
A slowly dropping battery voltage or a vehicle with discharged battery can lead to sporadic communication faults in various control units on the bus. The reason is that not all control units will switch off communication at the same voltage level leaving some modules still trying to communicate. Always verify a properly charged battery and charging system before beginning troubleshooting on the CAN.
Interference In The CAN Bus Cables
Interference will have a similar effect to shorting or disturbing the CAN bus wiring. Excessive interference created by a defective alternator or aftermarket devices such as cell phones or amplifiers may induce a voltage into the CAN bus line and disrupt communication. This type of interruption may be intermittent and faults may only be stored in some modules and not in others. These faults are often difficult to reproduce. Begin by eliminating any problems with the CAN bus wiring itself and verify that the generator is operating fault free. Isolate any aftermarket wiring in the vehicle and see if the fault returns.
Programming
During programming it should be noted that the module being programmed will not be communicating and therefore the other control units on the bus will store faults. These faults stored during programming should be deleted and then the fault memory should be read again to verify that they do not return. An incorrectly programmed module results in CAN faults that are not able to be cleared. Remember to always verify the correct Programmed Part Number after programming.
D-Bus Topology
The D-bus (TXD) is connected to various control units that are diagnosed using DIS or MoDiC. Earlier vehicles also used a second diagnosis line called RXD to allow the test equipment to establish communication. RXD is not a bus line but a one way communication link used to wake up the diagnosis of the connected control unit.
On vehicles produced up to model year 2001 and use the 20 pin under-hood diagnostic connector, the locations of the two links are
- RXD-Pin 15
- TXD-Pin 20
Later control modules (from 1997) no longer required the separate RXD to establish communication, (DS2 protocol) so Pin 15 was removed from the Diagnostic socket of most vehicles.
To satisfy the requirements of OBD II, in 1995 a standardized connector was installed inside of all vehicles. This connector has to provide access to all powertrain modules via an after-market scan tool. TXD II (pin 17) was introduced as a separate communication line exclusive to DME (ECM), AGS (TCM) and EML. TXD II is technically identical to the D bus (TXD).
On vehicles that use only the 16 pin OBD connector in the vehicle, TXD is installed in pin 8. TXD II remains in pin 7.
The term D bus was actually coined with the introduction of the E38 and the expanded use of bus systems in the vehicle. On vehicles from E38 on (except Z3), the D-bus is directly wired to
Scheme 51
- ASC/DSC
- EDC (if equipped)
- LEW
- IKE/KOMBI (Scheme 51): Identifying D-Bus Schematic
The IKE/KOMBI serves as the gateway for the D-bus that converts the telegram format of the I/K bus to the format of the D-bus.
The wire color of the D-bus is uniform throughout the vehicle, it is a single WS/VI wire.
Failure Of The D-Bus Cable
The following faults can occur to the D-bus wiring
- D-bus interrupted
- D-bus shorted to battery voltage
- D-bus shorted to ground
- Defective plug connections (damaged, corroded, or improperly crimped)
The operating voltage of the D-bus is 12 volts. The voltage measurement is taken from each data line connection to ground. Each module on the D-bus provides its own voltage.
The fact that 12V are present does not mean that the D-bus is fault free, it just means that the voltage level is sufficient to support communication.
Minimum voltages that are needed for fault free communication are
- D-bus (TXD)/TXD II > 2.0V
- RXD (if equipped) > 10.5V
If problems are encountered trying to establish communication consider first
- Battery charge level of the vehicle. Maintain a battery charger on the vehicle at all times during diagnosis.
- Always check that the diagnosis head and connection are OK before working through a test module for lack of communication.
On vehicles that use the IKE/KOMBI as a gateway
Identifying Bus Test Display. Scheme 52
If identification of the vehicle is carried out by the diagnostics without any problems then the D-bus is OK.
If several control units are not recognized this indicates that a bus link is defective. Continue troubleshooting using the test modules for those particular bus systems.
On E38 And E39/E53 High Version Vehicles
- The LCM is the Master Controller of the I-bus. The IKE and MID/BMBT are Stand-by Controllers.
- The GM is the Master Controller of the K-bus.
On E46, E52 And E39/E53 Base Version Vehicles
- The GM is the Master Controller for vehicles equipped with only the K-bus.
- The LCM/LSZ is the Stand-by Controller.
Failure Of The Bus Cable
The following faults can occur to the I/K bus wiring
- Short Circuit to B+
- Short Circuit to B
- Bus line down (open)
- Defective plug connections (damaged, corroded, or improperly crimped)
Short Circuit to B+: Modules that send a message see that the message was not received and that the bus remains high. However, subscribers are unable to decide whether the fault is due to a shorted line or a defect in the communication interface. The module will repeat its message 5 times before discontinuing and faulting. The module will continue to operate as normal minus any commands that could not be delivered by the bus.
Short Circuit to B-: The subscribers do not interpret a low bus line as a fault but just as a bus line deactivation. The Master and Standby controllers do detect the short and enter it as a bus fault. (No communication).
Bus Line Down: The bus line may be open at any of several locations. As long as the Master or Stand-by is still connected, communication can occur with any modules still remaining. The fault situation will be the same as if the disconnected modules were defective themselves.
Checking the bus line is carried out just like any other wiring. Perform continuity tests between the connections of different modules (all modules disconnected) without forgetting to make sure that the bus has not shorted to ground or another wire. It is recommended to use the "Wire Test" in "Preset Measurements" which is more sensitive than just a resistance check.
If Voltage level and the wire test are O.K then looking at the communication signal may be useful. In order to get a signal, operate different devices on the I/K bus (e.g. MID/MFL) to stimulate conversations.
The following are some examples of scope patterns that may be observed when checking the I/K bus.
Identifying Oscilloscope Display I/K Bus. Scheme 53
Example Of Correctly Operating I/K Bus During Communication
Correct communication on the I/K bus occurs in sporadic bursts with periods of steady voltage around 12V.
Identifying Display (Flat Line At 12 Volt). Scheme 54
Flat Line At 12 Volts
No communication is currently taking place. The bus may be temporarily off-line or shorted to B+.
Identifying Display (Flat Line At 5 Volt). Scheme 55
Flat line at 5 volts
No output voltage from the Master or Standby controllers. Bus line may be open or control unit may be defective.
Failure Of One Of The Control Units Attached To The I/K Bus.
Each control unit connected to the bus has an integrated communication module that makes it possible for that control unit to exchange information. Failure of a control unit normally triggers a fault code in the other control units connected to the bus.
As a quick check for the I/K-bus, activate the four way flashers. The flash indicators must light up in the instrument cluster. Switch on the Radio, and adjust volume using the MFL or MID/BMBT, the volume must change accordingly.
On High version vehicles press the recirculation button on the MFL, The IHKA should respond to the request. This test checks the gateway link as well as the I and K bus communication.
If the tests prove O.K, this means that communication on the bus is O.K. Any faults still existing can only be related to faults specific to a control unit or a local I/K-bus wiring defect to a module.
There are instances where failures may be software related. A faulted module may paralyze or take down the entire bus. This scenario would be evident by functions not being carried out and possible faults stored.
In order to isolate the defective control unit, the control units can be disconnected one at a time. Repeat the bus test after each disconnected control unit. If the disconnected control module is the defective one the faults will only point to communication with that interrupted module and no one else.
Once the module has been replaced (observing current S.I.Bs) and coded, perform the I or K bus Test Module in the Diagnosis Program to ensure that communication is O.K.
A slowly dropping battery voltage on a vehicle with discharged battery can lead to sporadic communication faults in various control units on the bus. The reason is that not all control units will switch off communication at the same voltage level leaving some modules still trying to communicate. Always verify a properly charged battery and charging system and fuses before beginning troubleshooting on the bus. Also, do not forget to check for a proper ground to a control unit, this may not allow the bus to see a signal low (0-2V)
Interference In The Bus Cables.
Interference will have a similar effect to shorting or disturbing the bus wiring. Excessive interference created by a defective alternator or aftermarket devices such as cell phones or amplifiers may induce a voltage into the bus line and disrupt communication. This type of interruption may be intermittent and faults may only be stored in some modules and not in others. These faults are often difficult to reproduce. Isolate any aftermarket wiring in the vehicle and see if the fault returns.
P-Bus Topology
The extent of the P-bus depends on the special equipment of the vehicle.
Identifying P-Bus Topology. Scheme 56
The P-bus consists of a single copper wire. The wire color of the bus is uniform throughout the vehicle: BL/RT.
Due to the linear structure of the network, the P-bus is available for other modules in the event of a disconnected or failed control unit. The P-bus provides the diagnostic connection to the P-modules.
Always refer to the ETM to determine the exact wiring configuration for a specific model.
Example Of Correctly Operating M-Bus
Communication on the M-bus occurs continuously with an average Period duration of 50 s. When a command is issued by the IHK control unit the pattern will briefly change in period length and then return to the constant signal.
Failure Of The Bus Ribbon
The following faults can occur to the M-bus wiring
- Short Circuit to B +
- Short Circuit to B
- Bus line down (open)
- Defective plug connections (damaged, corroded, or improperly crimped)
The IHK control module checks the M-bus for continuous position feedback from the stepper motors. If the M-bus is interrupted then the control unit will store a fault for every stepper motor on the bus.
In order to determine if a stepper motor is at fault for the lack of communication, disconnect one stepper motor at a time while monitoring the M-bus signal line with a voltmeter or oscilloscope. The pattern or voltage should return to normal when the defective stepper is found. As a confirmation that communication is restored, change the setting on the IHK panel, if the remaining connected flap motors assume the selected position communication is OK.
Diagnosis of the M-bus is carried out by the DISplus/MoDiC via the IHK module. Available in the Diagnosis Program are
Scheme 57
- Fault/symptom driven test modules
- Diagnosis request (flap position)
- Component activation (flap activation) (Scheme 57): Identifying Air Distribution Display ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________
Review Questions
- What should be the voltage preset at a CAN line if checked with a multi-meter? Is the voltage the same on both lines?________________________________ ________________________________________________________________________________ ________________________________________________________________________________
- Where are the Terminal resistors located in the CAN bus network? What should the measured resistance of the CAN circuit be? How is it checked?________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________
- Explain the differences of CAN-High and CAN-Low? How can they be distinguished from one another?____________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________
- Which control units on the CAN bus contribute to the voltage necessary for communication? Describe the method to determine if one control unit is affecting communication.____________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________
- What is the minimum voltage required at the D-bus?___________________________________ ________________________________________________________________________________ ________________________________________________________________________________
- Why is checking a bus signal with an oscilloscope a practical option?_______________ ________________________________________________________________________________ ________________________________________________________________________________
- Describe some quick tests that can help to determine if a bus line is currently operating ______________________________________________________________________. ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________