Technical Training
Bus Systems
The vehicle electrical architecture of the MINI has been designed to exploit the full potential of its technological advances. Rather than having a control unit dedicated to its system and unaware of the operation of other systems, the systems around the MINI are all linked together.
The control units are linked to each other via bus-systems, allowing communication and exchange of information. Harness bulk and complexity is kept to a minimum using this method to relay information to and from the different systems. This method used by MINI is called multiplexing.
Purpose of the System
Multiplexing is a technique which uses the same wiring repeatedly for communication between all systems. The systems are all inter-connected, forming a network of communication lines. The information from sensors and switches are converted into digital signals by the system control unit and relayed serially throughout the system network, which is also referred to as a data bus-system.
MINI COOPER Bus Network
System Components
K-Bus System
The K-bus system utilized in the MINI is the proven K-bus serial communication network used by BMW in the E46. It uses a single wire colored white/red/yellow. The K-Bus used by Body related components to connect with other body electronic systems and the instrument cluster (IKE) is a slow bus system, transmitting at 9600 bits per second.
The K-Bus switches between 0-12 volts (High/Low) and has a low impedance, making it resistant to electromagnetic interference. Most users are active on the bus when the ignition is switched to KLR (Ignition Switch Acc. Position).
The K-Bus network enters sleep mode 60 seconds after the ignition is switched off. While in sleep mode the K-Bus is at 12 volts.
MINI COOPER K-Bus
CAN-Bus
The CAN protocol was originally developed by Intel(tm) and Bosch in 1988 for use in the automotive industry to provide a standardized, reliable and cost-effective communications bus for a cars electronics to combat the increasing size of wiring harnesses.
The CAN-Bus system is a high speed serial data bus-system linked by an unshielded twisted pair of wires: yellow/black and yellow/brown. The wires are twisted to minimize electromagnetic interference. Both wires carry information and for the CAN-Bus to operate, both signals must be present.
The CAN-system is the fastest of the bus systems used in the MINI, transmitting at 500,000 bits per second. This speed is recognized as the fastest practical operating speed without shielded cable. It is used for systems where the speed of exchange of information is vital for their performance; engine management systems, automatic transmission and automatic stability control.
The CAN-system uses a linear topology which consists of the main bus length and shorter stubs. The main bus length terminates at the EMS2000 and the IKE. The shorter stubs must be as short as possible and no longer than one meter. Any untwisted portion of the bus must not be longer than four centimeters.
MINI COOPER CAN-Bus
Diagnostic Bus System
The Diagnostic Bus System is made up of two separate bus-systems which allow the DISplus to communicate with the vehicle control units via the diagnostic socket:
- The Diagnostic bus (ISO 9141-2 OBD II)
- This allows communication with the DISplus and power train (emission) related control units
- The DS2 Protocol bus:
- This connects to the control units that do not affect emissions and to the instrument cluster (IKE), which enables communication with all the control units on the K-bus system
The Diagnostic bus uses a single wire transmitting at 9,600 bits a second. The diagnostic line is used by DISplus to interrogate each control unit on the network and helps in the fault finding process. It uses a protocol very similar to that of the K-bus system and is accessed by DISplus via the 16 pin diagnostic connector located in the driver's footwell (under the dash, close to the A post).
MINI COOPER D-Bus
MINI COOPER DS2-Bus
Principle of Operation
Bus Network
The MINI network system uses a number of interconnected bus-systems. The bus systems in use on the MINI COOPER are the CAN-bus, the K-bus and the Diagnostic buses (D-Bus and DS2 Bus). Most control units are connected to this system and can transmit and receive messages on the system.
The IKE acts as the communication gateway, enabling transfer of data from one bus system to another. The IKE contains a microprocessor, which converts and processes all signals into the format required for transmission on to another bus system. Control units transmit and receive commands and information on the buses as digital messages. Units connected to the same bus use a common protocol (format) and baud rate (transmission speed (kbit/s)) for the messages they transmit. The protocol and baud rate varies from bus to bus.
All messages transmitted on the bus systems are made up of binary digits, referred to as bits (Binary digits). The bit can be either 0 or 1. A combination of bits that make up a signal is transmitted on the bus system. Each control unit converts the digital signal back to an analog value for its own use.
Each message contains coded information indicating the message beginning and ending, and an identifier or address to route the message to the correct destination. The signal also contains a check function which a receiving control unit uses to check the plausibility of the signal. The transmitting control unit monitors the return of the signal on the bus from the receiving control unit. This way the transmitting control unit confirms the signal received is the same as the one transmitted and any faults can be dealt with.
A priority system of control unit or message importance is established providing a smooth flow of information, ensuring that important messages are processed first.
Information from a single sensor is passed on to many control units reducing the number of sensors needed, (e.g. Raw vehicle speed is received at the DSC unit which uses this information for unit operation and a processed road speed is sent to the IKE over the CAN-Bus. The IKE converts this signal into a K-bus message and transmits this via the K-bus to the BC1 for the speed dependent wiper control and the radio for the speed dependent volume adjustment).
Advantages of multiplexing
- Harnesses are smaller and less complex
- Harnesses are cheaper and lighter
- Improved reliability, reduced wires and connections
- New systems can be added easily 'Plug and Play'
K-Bus
The K-bus system is primarily an event-driven system. Messages are sent after a request has been made for component operation or status change.
Each of the K-bus users are continually monitoring messages on the system. When a user needs to transmit, it waits until there is no activity and then transmits its message. The sender then listens to the system for its own message to be returned by the receiver. If it doesn't receive its own message, a user with higher priority has also transmitted and its message was ignored. The lower priority user then has to wait until the Bus becomes inactive and re-transmit. When it receives its own message back (correctly) successful transmission is confirmed.
Arbitration on the K-bus is controlled by the sender of the message. Each sender has an allocation priority. The body control module (BC1) has the highest priority on the K-bus system and messages from the BC1 override any other K-bus message which has been transmitted simultaneously
All control units on the K-Bus receive all messages, but only the module addressed in the message will accept and react to the data.
The K bus remains operational 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.
The K-bus provides the diagnostic connection to the control units located on the bus (except for the IKE).
CAN-Bus
The CAN-Bus consists of a twisted pair of wires. One line is CAN High (CAN-H, yellow and black) the other is CAN Low (CAN-L, yellow and brown). Both Can-H and CAN-L have a standing voltage of 2.5 volts. CAN-L is pulled low to 1.5 volts. CAN-H is pulled high to 3.5 volts.
With CAN-H and CAN-L both at 2.5 volts there is no potential difference (voltage) between them. This is known as the recessive state and is equivalent to logic 1. With CAN-H pulled to 3.5 volts and CAN-L pulled to 1.5 volts there is a potential difference of 2 volts between them. This is known as the dominant state and is equivalent to logic 0. CAN-H and CAN-L always switch together. These two states (0 or 1) are the only two possible.
When a control unit transmits a signal, it is made up of a series of dominant and recessive states. The signal is a combination of the two possible states, in effect 0 and 1, hence a digital signal.
For correct operation CAN-Bus must be terminated at both ends (EMS2000 and IKE) with a control unit resistance of 120 ohms, connected between CAN-H and CAN-L. These terminations ensure that bit errors due to signal reflections are avoided.
Data transmitted from any subscriber on a CAN bus does not contain addresses. Instead the content of the message (RPM, TD, Temp, etc) is labeled with an identifier code unique throughout the CAN. All of the subscribers receive the message and each one checks the message to see if it is relevant to its own operation.
If the message is relevant then it will be processed, if not, it will be ignored. The identifier code also determines the priority of the message. In a case where two control units attempt to send a message over a free bus line, the message with the higher priority will be transmitted first. The protocol of the CAN ensures that no message is lost, but stored by the Master Controller and then re-transmitted later when it is possible.
CAN Bus Terminal Resistors
Terminal resistors used in the CAN-Bus circuit 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. (Ohms' Law of a Parallel Circuit)
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.
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 10k0hms and 50k0hms.
D-Bus System
The D-Bus is a serial communications bus that transmits data between the DISplus and the connected control units. The control unit subject to diagnosis is selected by sending a diagnosis telegram to the control unit address. By request from the DISplus, the control unit transmits information and the contents of the fault memory or activates a control unit output. The D-bus is only active when the DIS or MoDiC is connected to the diagnostic socket and communicating.
- D-Bus is connected to power train (emission) related control units.
- DS2 Protocol Bus is connected to control units that do not affect emissions and the IKE. Through the IKE communication with the K-Bus is possible.
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. D-Bus operation and signals are similar to K-Bus.
Diagnostics
K and D Bus Faults
The failure of communication on the K or D bus can be caused by several sources:
- Failure of the bus cable.
- Failure of one of the control units attached to the bus.
- Failure of the voltage supply or ground to individual modules.
- Interference in the bus cables.
- Bus interrupted
- Bus shorted to battery voltage
- Bus shorted to ground
- Bus line open
- Defective plug connections (damaged, corroded, or improperly crimped)
If problems are encountered trying to establish communication with a control module 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.
Since MINI COOPER uses the IKE as a gateway.
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.
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 1/K bus (e.g. MID/MFL) to stimulate conversations.
Failure of one of the control units attached to the 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 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 the volume must change accordingly.
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 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.B's) and coded, perform the K bus Test Module in the Diagnosis Program to ensure that communication is O.K.
Failure of the voltage supply to individual modules.
Slowly dropping battery voltage on a vehicle with a 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.
Flat line at 12 volts.
No communication is currently taking place. The bus may be temporarily offline or shorted to B+.
Flat line at 12 volts.
No voltage on bus line.
Bus may be shorted to B-, open or control unit defective.
CAN Bus
The failure of communication on the CAN bus can be caused by several sources:
- Failure of the CAN bus cables.
- Failure of one of the control units attached to the CAN.
- Failure of the voltage supply or ground to individual modules.
- Interference in the CAN bus cables.
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.
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.