Troubleshooting the 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 a CAN bus resistor.
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).
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. CAN does not utilize a Master Controller, each module on the CAN provides operating 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 available to support communication.
Scheme 1
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 10kOhms and 50kOhms.
Scheme 2
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.
Scheme 3
Example of correctly operating CAN bus.
Correct communication on the CAN bus occurs in sporadic bursts with short periods of steady voltage.
Examples of Defective CAN bus signals
Scheme 4
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.
Scheme 5
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.
Scheme 6
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.
Scheme 7
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.
Scheme 8
Scheme 9
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.
Troubleshooting the D-Bus
The failure of communication with one or several control units via the D-bus can be caused by
- Failure of the D-bus cable or its individual connections.
- Failure of the IKE/KOMBI control unit.
- Failure of the I/K or P-bus or its individual connections.
- Failure of the voltage supply or ground to individual modules.
- Interference in the D-bus cable.
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
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.
Scheme 10
D-bus test module.
Troubleshooting the I/K bus
The failure of communication on the I/K 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 to individual modules.
- Interference in the bus cables.
The I/K bus is active when KL R is switched on, it remains active until 60 seconds after the last message. If the key is switched off (KL30) the bus may be activated for a time by individual users via a "wake-up" message.
Unlike the CAN bus where each control unit (subscriber) provides voltage for communication, the l/K-busses use only determined Master or Stand-by Controllers to supply B+ for communication. The voltage level on the I/K bus must be above 7V. The nominal value should be close to the system voltage of the vehicle.
Just like the CAN bus, the fact that voltage is present does not mean that the bus is fault free, it just means that the voltage level is sufficient to support communication.
Scheme 11
Control units that provide operating voltage to the I/K bus are
On E38 and E39/E53 High version vehicles
- The LCM is the Master Controller of the l-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.
Scheme 12
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.
Scheme 13
Flat line at 12 volts.
No communication is currently taking place. The bus may be temporarily offline or shorted to B+.
Scheme 14
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 l/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 l/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.
Failure of the voltage supply to individual modules.
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.
Troubleshooting the P-bus
The failure of communication on the P-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 or ground supply to individual modules.
- Interference in the bus cables.
The P-bus may be active at any time following a wakeup call. The GM provides the voltage necessary to support communication. The voltage level of the P-bus is 12V.
The Diagnosis of the central body electronics is carried out via the K-bus. The GM converts diagnosis request from the DIS plus into diagnostic mode messages and transmits them the peripheral modules over the P-bus.
Automatic testing of the P-bus connection is carried out every time the GM communicates with the diagnosis program (not during a short test).
Scheme 15
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.
Troubleshooting of the P-bus network is carried out the same as the I/K bus.
Troubleshooting the M-bus
The failure of communication on the M-bus can be caused by several sources
- Failure of the bus ribbon, e.g. open or shorted.
- Failure of one of the stepper motors attached to the bus, e.g. shorted to B+ or B-.
- Failure of the voltage or ground supply to the IHK control unit.
The M-bus is active at any time following KLR on. The IHK module provides the voltage necessary to support communication. The voltage level of the M-bus is 5V, but because status communication occurs at an average 50% duty cycle the observed voltage is approximately 2.5V. The presence of 2.5V means that communication is occurring.
Scheme 16
Checking the M-bus ribbon is carried out just like any other wiring. Perform continuity tests between the connections of the stepper motors (all motors disconnected) and the control unit without forgetting to make sure that the data line has not shorted to ground or power.
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 is an example of a scope pattern that may be observed when checking the M-bus. Notice the very high frequency of the signal at approximately 20 kHz.
Scheme 17
Example of correctly operating M-bus
Communication on the M-bus occurs continuously with an average Period duration of 50 mus. 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 18
- Fault/symptom driven test modules
- Diagnosis request (flap position)
- Component activation (flap activation)