Table of Contents

BUS SYSTEM TROUBLESHOOTING

Subject Page

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Controller Area Network (CAN bus). . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
CAN bus topology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Troubleshooting the CAN bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Diagnosis Bus (D-bus). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
D-bus topology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Troubleshooting the D-bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Information and Body Bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
I/K bus topology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Troubleshooting the I/K-bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Peripheral Bus (P-bus). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
P-bus topology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Troubleshooting the P-bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

Motor Bus (M-bus). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
M-bus topology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
Troubleshooting the M-bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

Initial Print Date:2/01/01 Revision Date:1/23/01

BUS SYSTEM TROUBLESHOOTING

Model: All Models Using Bus Systems

Production Date: From Model Year 1991

Objectives:

After completing this module you should be able to:

• Review the advantages of using bus systems in a vehicle.

• Review the various bus systems used in most BMW vehicles.

• Understand the operating principle of a serial bus.

• Review methods of troubleshooting bus lines using the diagnosis program.

• Recognize how to distinguish a correctly operating bus line on an oscilloscope.

2
Bus System Troubleshooting
Introduction

Up until the introduction of the E31, all information (e.g. RPM, temperature, vehicle speed,
etc.) received and transmitted from a control unit were delivered by a dedicated wire. As
the electronic systems used in the vehicle increased, so did the necessary wiring.

Wiring each device separately became a headache for production and finding space in the
body to hold all of this wiring was becoming difficult, not to mention the effect on reliability
and ever more complicated troubleshooting.

It soon became clear that a solution must be found, that solution was the bus system.
The benefits of the bus system are:

• Greater reliability by reducing the number of wiring, connectors and components.

• Reduction in wire harness size by decreasing the number of interfaces between control
units to one or two wires.

• Multiple utilization of sensors by transmitting information from one control unit to the
next.

• Flexibility in system configuration and future applications.

• Reduction in costs for components, assembly and troubleshooting.

Today’s vehicles have several bus systems that are divided according to groups of control
units which share common functionality and information. Currently the bus systems in use
are:

• CAN-bus
• D-bus
• I-bus
• K-bus
• P-bus
• M-bus

In the very near future the the complexity and number of control units in the vehicle will
change, as will the structure and construction of the bus systems. However, the principle
of operation will remain similar. Efficient troubleshooting of the new bus systems will rely on
a thorough understanding and knowledge of the systems currently in use.

3
Bus System Troubleshooting
Definitions

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 send-
ing 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:

1. Transmitter address
2. Length of data
3. Receiver address
4. Command or Information
5. Detailed description of message (data)
6. 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.

4
Bus System Troubleshooting
Controller Area Network (CAN Bus)

Introduction

The CAN bus is a serial communications bus in which all connected control units can send
as well as receive information. Data over the CAN operates at a rate of up to 500 K/bps
(kilobits per second).

The CAN protocol was originally developed by Intel and Bosch in 1988 for use in the auto-
TM

motive 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 was originally introduced on BMW automobiles in the 1993 740i/iL as a data
link between the DME and AGS control units.

Data transmitted from any subscriber on a CAN bus does not contain addresses of the
transmitting or receiving control unit. Instead, the content of the message (RPM, TD,
Temp,etc) is labeled by an identifier code that is unique throughout the CAN. All of the sub-
scribers receive the message and each one checks the message to see if it is relevant to
that particular control unit.

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 sender and then re-transmitted later when it is possible.

5
Bus System Troubleshooting
CAN Bus Topology
The CAN bus consists of two twisted copper wires. Each wire contains an opposing sig-
nal with the exact same information (CAN-High, CAN-Low). The opposing signals trans-
mitted 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.

Example of Tree structure

95-97 E38 750iL

Trunk

Branches

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 vehi-
cle.

The 1999 750iL was the last vehicle to use the shielded cable, after which the entire CAN
bus went to twisted pair wiring.

Always refer to the ETM to determine the exact wiring configuration for a specific model.

6
Bus System Troubleshooting
 INSTRUMENT
CLUSTER

80 3 4
60 100

MS 43.0
120 140 2 1/min 5

GS 20
100 160 x1000
80 180 120
40
12 60 200 1 6
11 40 220
0
20 20 240 140
UNLEADED GASOLINE ONLY 0 5030 20 15 7
km/h 12
MPH

miles BRAKE ABS

1 2 3 4 5

CAN BUS

SPLICE CONNECTIONS
FOR TWISTED PAIR CAN

On most current models the CAN bus provides

data exchange between the following
modules:

• DME
LEW

• EML (750iL)
• AGS/EGS
• ASC/DSC
• IKE/KOMBI
• LEW MK 60

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.

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)

7
Bus System Troubleshooting
The voltage of the CAN bus is divided between Print Change End Services Help

the two data lines: CAN-High and CAN-Low BMW Test system Multimeter

for an average of 2.5V per line. The voltage Freeze image

measurement is taken from each data line to 2.35 V 2.65 V Minimum

ground. CAN does not utilize a Master

Maximum

Controller, each module on the CAN provides 10

Measurement Voltage Resistance
0
Temperature
10
System voltage
Rotation speed

operating voltage.
Function V Ohm o
C

Current Current Current Diode test Pressure

2A 50A 1000A -|>|- bar 2nd
measurement

Measurement MFK 1 MFK 2 Current probe Pressure Temperature

The fact that 2.5V are present does not mean

Connection Sensor Sensor

that the CAN bus is fault free, it just means that

Measurement
Kind = Effective value

Stimulate

the voltage level is available to support com-

Measurement automatic ± 10 V
Range

munication.
Multimeter Counter Oscilloscope Stimulators Preset
setting measurements

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 resis-
tance of the complete circuit is 60 Ohms. On some vehicles there is a jumper wire that con-
nects the two parallel branches together, others have an internal connection at the instru-
ment 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.

12400003

8
Bus System Troubleshooting
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 mod-
ule 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.

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BMW Test system Oscilloscope display

Freeze Image
Cursor 1 Memory Cursor 2
A [V] B [V] V

T
Channel B
CAN-L (low)
5.0 +++ 4.0
4.0 +++ 3.0
r Zoom Flat line should average 2.5
i
3.0 +++ 2.0 g
volts with signal line being
pulled low for communica-
Amplitude

2.0 5.0 1.0

g Channel A

e tion.
1.0 4.0 0 r
Amplitude
Channel B
0.0 3.0 -1.0
l
-1.0 2.0 -2.0 e
Time value
1.0 -3.0
v
-2.0
e
-3.0 0.0 -4.0 l

-4.0 -2.0 0.0 2.0 4.0

CAN-H (high)
3.0 -1.0 1.0 3.0
ms Stimulate
Flat line should average 2.5
Multimeter Counter Oscilloscope Stimulators Preset volts with signal line being
setting measurments
pulled high for communica-
tion.

Example of correctly operating CAN bus

Correct communication on the CAN bus occurs in sporadic bursts with short periods of
steady voltage.
9
Bus System Troubleshooting
Examples of Defective CAN bus signals
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BMW Test system Oscilloscope display BMW Test system Oscilloscope display
Freeze Image Freeze Image
Cursor 1 Memory Cursor 2 Cursor 1 Memory Cursor 2
A [V] B [V] V A [V] B [V] V
Channel B Channel B

3.0 +++ 4.0 T 4.0 16 4.0 T

2.0
r Zoom r Zoom
+++ 3.0 3.0 12 3.0
i i
1.0 +++ 2.0 g 2.0 8 2.0 g
Amplitude Amplitude

0.0 5.0 1.0

g Channel A
4 1.0
g Channel A
1.0
e e
-1.0 4.0 0 r 0 0 0 r
Amplitude Amplitude
Channel B Channel B
-2.0 3.0 -1.0 -1.0 -4 -1.0
l l
-3.0 2.0 -2.0 e -2.0 -8 -2.0 e
Time value Time value
1.0 -3.0
v -3.0 -12 -3.0
v
-4.0
e e
-5.0 0.0 -4.0 l -4.0 -16 -4.0 l

-4.0 -2.0 0.0 2.0 4.0 -4.0 -2.0 0.0 2.0 4.0
3.0 -1.0 1.0 3.0
ms Stimulate 3.0 -1.0 1.0 3.0
ms Stimulate

Multimeter Counter Oscilloscope Stimulators Preset Multimeter Counter Oscilloscope Stimulators Preset
setting measurments setting measurments

Rapid Constant fixed duty cycle for Flat line at 2.5 volts
10 seconds. If a continuous flat line is present at one or
This example represents the output signal both CAN lines of a particular control unit,
produced by an AGS module that is iso- this may indicate that the CAN is open to
lated from the bus. This pattern times out that particular module. The module may
after 10 seconds and remains a flat line at have timed out and is waiting for a signal
2.5 volts until the key is cycled and the from another control unit. Check the CAN
event is repeated. bus at other points to see if communica-
tion is occurring else where on the bus.

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BMW Test system Oscilloscope display BMW Test system Oscilloscope display
Freeze Image
Cursor 1 Memory Cursor 2 Freeze Image
A [V] Cursor 1 Memory Cursor 2
B [V] V A [V] B [V] V
Channel B
Channel B

5.0 +++ 4.0 T T

5.0 +++ 4.0
4.0
r Zoom r
+++ 3.0 4.0 +++ 3.0 Zoom
i i
3.0 +++ 2.0 g +++ 2.0
Amplitude 3.0 g Amplitude
2.0 5.0 1.0
g Channel A
g Channel A
2.0 5.0 1.0
e e
1.0 4.0 0 r 1.0 4.0 0 r
Amplitude Amplitude
Channel B
0.0 3.0 -1.0 0.0 3.0 -1.0
Channel B
l l
-1.0 2.0 -2.0 e 2.0 -2.0 e
Time value -1.0
1.0 -3.0
v v
Time value
-2.0 -2.0 1.0 -3.0
e e
-3.0 0.0 -4.0 l -3.0 0.0 -4.0 l

-4.0 -2.0 0.0 2.0 4.0 -4.0 -2.0 0.0 2.0 4.0
3.0 -1.0 1.0 3.0
ms Stimulate ms
3.0 -1.0 1.0 3.0 Stimulate

Multimeter Counter Oscilloscope Stimulators Preset Multimeter Counter Oscilloscope Stimulators Preset
setting measurments setting measurments

Constant fixed duty no time limit CAN High shorted to CAN Low
All of the other control units with the If the CAN bus lines were to become
exception of most current AGS modules shorted to one another then the signals
will continue to try and send information would cancel each other out and effec-
even though the control unit has already tively be a flat line.
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.

10
Bus System Troubleshooting
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.

E38/E39 Style diagnosis test module E46 Style diagnosis test module
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BMW Diagnosis CAN CONNECTION BMW Diagnosis Test information

Note: CAN interface CAN-Bus
Data transmission on the CAN bus is Causes
The failure of communication on the CAN-bus (i.e fault code entries relating to
disturbed. bus communication in the individual control modules may be caused by the
following:

X18836 - Breaks in line (open circuits) or short-circuits in the communication lines

Possible causes of fault: - Interference voltages in the vehicle electrical system caused for instance by
defective ignition coils or ground connections
-Short circuit between wires CAN-H
- Failure of the communication modules in the individual control modules
0.5
CAN H and CAN L GE/SW
- Failure of the voltage supply to individual control modules. A slowly dropping
X18832 X18832
battery voltage when the battery is almost discharged can also lead to fault code
CAN H and ground
CAN-H CAN-H
CAN L and power supply B1214 M0CAN/Message 08
0.5 0.5

Defective or incorrect input circuit in

GE/SW GE/SW
Signal from instrument cluster control
X11176 3 X1658
24 module absent or disturbed.
ABS/DSC control unit A2 R33
Check line to instrument cluster control
Incorrect coding of control unit Instrument cluster Steering angle sensor
module and
continue troubleshooting at instrument
Instruction: 11 X11176 cluster control module.
If the CAN bus is O.K. , replace the
ABS/DSC control module

Note Note

Function Documents Test Schedule TIS Measuring Control unit Function Documents Test Schedule TIS Measuring Control unit
Selection System Functions Selection System Functions

07400001 07400002

11
Bus System Troubleshooting
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 try-
ing 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 communica-
tion. 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 elimi-
nating any problems with the CAN bus wiring itself and verify that the generator is operat-
ing 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.

12
Bus System Troubleshooting
Diagnosis Bus (D-bus)

Introduction

The D-bus is a serial communications bus which can transmit data between the BMW DIS
or MoDiC and the connected control units. The control unit to be subject to diagnosis is
selected by sending a diagnosis telegram to the control unit address. By request from the
tester, the control unit can transmit information and the contents of the fault memory or acti-
vate a control unit output. The D-bus is only active when the DIS or MoDiC is connected
to the diagnostic socket and communicating.

The D-bus is actually the oldest bus system used in BMW vehicles, it was introduced in
1987 as TXD which provided communication between DME and the Sun 2013 Service
Tester. The D-bus is still referred to as TXD in the ETM.

Data over the D-bus currently operates at a rate of up to 9.6 Kbps (bits per second).

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 communica-
tion 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 com-
munication, (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 exclu-
sive 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.

13
Bus System Troubleshooting
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:

• ASC/DSC
• EDC (if equipped)
• LEW
• IKE/KOMBI

12400005

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.

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.

14
Bus System Troubleshooting
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:

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If identification of the vehicle is car-
BMW Diagnosis Test information ried out by the diagnostics without
X19527 OBDII socket Bus test

F15
Test preconditions: Always check that the diagnosis cable and tester are OK
any problems then the D-bus is
OK.
30 F25 15 before working through the test module. If necessary, the diagnosis procedure
should be carried out on another vehicle or the diagnosis cable checked by
7.5A 7.5A means of a self-test. The self test-diagnosis cable (Selection:
administration) checks whether the diagnosis cable and the diagnosis interface
in the tester ar OK. For this purpose, the diagnosis cable must be connected
to the test socket at the rear of the tester.
50 X10015
X10015
Test procedure : The bus systems (I/K and D-bus) are tested in the test
module. The following control units are addressed as part of the test:

If several control units are not rec-

DME/DDE, ABS/ASC/DSC,KOMBI (insrument cluster), IHKA/IHKR/IHR, ZKE,
RADIO, BM/MID and LCM/LSZ. The identification of the control unit is read out
for testing purposes.
30<25 15<15
0.5
0.75

ognized this indicates that a bus

RT/SW
GN/WS
X183 Bus test E38/39/46/52/53/Message 01
Data transfer from the tester to the D-bus,
TXD1
K-bus and I-bus is tested in the following
16
1 test module. link is defective. Continue trou-
4
Take note of the information provided in
the functional description "Bus system bleshooting using the test modules
and bus test"!
for those particular bus systems.
31L<3
0.5
BR

Note

Function Documents Test Schedule TIS Measuring Control unit

Selection System Functions

07400100

D-bus test module

15
Bus System Troubleshooting
Information and Body Bus (I and K bus)

Introduction
The I and K buses are a serial communications bus in which all connected control units can
send as well as receive information over one wire. The I and K bus are technically identi-
cal, the only difference is their use by model. From this point forward they will be referred
to as the I/K bus and differences will be pointed out separately.

The I bus was originally introduced in the E31 to provide a standardized, reliable and cost-
effective communications bus for a cars electronics to combat the increasing size of wiring
harnesses.

The E38 expanded the use of bussing in BMW vehicles by adding three more busses (K,
P and M) and adding more control units to the network.

The data transfer rate is approximately 9.6Kbps (bits per second).

The I/K-bus is always active when terminal R is switched on. If the bus line is quiet more
than 60 seconds, all of the control modules will go into Sleep Mode.

When receiving messages over the bus line, the control unit first determines if the message
is error free before accepting it.

The information sent over the bus is configured serially. Each message consists of:

1. Transmitter address (8 bit address)

• The senders name.

2. Length of data (number of following message bytes)

• How long the sender will speak.

3. Receiver address (8 bit address)

• Whom the sender wishes to speak to.

4. Command or Information
• What the sender wants done.

5. Detailed description of message (maximum 32 bytes of data)

• How the sender wants it done.

6. Summary of transmitted information (check sum)

• The sender summarizes everything said.

16
Bus System Troubleshooting
The sender of the message then waits (100ms) for an acknowledgement that the message
was received.

All of the connected control units will receive the information, but only the module
addressed will accept and react to the data.

The rules for communication on the bus line are:

• Only one module speaks at a time.

• Everybody speaks at the same speed.
• Messages are acknowledged by the recipient.
• The message is repeated if the addressed module fails to respond.
• The Master Controller has priority.
• Quit sending message after 5 failed attempts.

Communication between busses: On vehicles equipped with an I-bus (E38, E39, E53
High) messages to be sent back and forth between the K-bus and I-bus have to be trans-
ferred via a Gateway. This Gateway is the IKE. The IKE determines by the address of the
message recipient whether the message needs to be passed along to the other bus. The
D-Bus and CAN-Bus also utilize the IKE or KOMBI as a gateway.

Polling: Each module on the I/K bus is informed by a message from the Master Controller
as to the ready status of all of the other connected modules. The modules polled are
according to the coding of the Master Controller. Every 30 seconds after KL R is switched
on, each module on the bus line is polled.

A message concerning bus subscriber status is updated continuously based on the results
of these polls. If a subscriber fails to respond with “device status ready” the Master will try
again after 1 second.

If the module fails to reply again, the Master will assume that the subscriber is defective and
send the message “subscriber inactive” to all connected modules. The inactive module
will continue to be polled until the key is switched off in case the module resets itself.

17
Bus System Troubleshooting
I/K Bus Topology
MRS III EWS 3.3 RDW
4
HIGH
80 3
60 120 140
100
2
1/min
x 1000
5
INSTRUMENT
IHKA
100 160
80 180
40 60 200 120 6
220
1
CLUSTER
40
½ 20
240

0 20 140 0 7
km/h
MPH 40 0
ELECTRONIC 20 15 10
70 0 F 640 F !

123456
miles
122 4 +72 0F PRND S P ! ABS

CHECK OIL SERVICE

AUTO REST - +
A

K-BUS K-BUS
ENGINE INSPECTION 20 DIGIT READOUT 432
DIAGNOSIS BUS

I-BUS
M-BUS

SZM LWR BMBT

INFO
1 4 MENU
On-board computer GPS-Navigation

Antenna
2 5

CAN BUS
DSP Aux. Ventilation
3 6 TONE SELECT

Amplifier
Code
FM AM

M FZV DWA
Emergency

GM III MODE Set

11.13.2000 Thursday
Monitor off

10:17
MENU

P-BUS
MK 3 NAV MFL
M B
M

W
SRS
AIRBAG
POWER
GPS NAVIGATION SYSTEM

Driver's Door
Switchblock/ Passenger's
M Module Door Module
RADIO
PM-BT LEW
PM-FT/SB LCM
M CHECK
ENGINE

Seat Memory Sunroof CMT 7000

Module Module
DSP
interface box
PM-SM PM-SHD DSC III

CDC PDC
AGS
obd ii STANDARDIZED
FAULT CODES

r
ne
an
Y

Sc
1 2 3 4

DME
N

OBD II
CONNECTOR
TXD II
DIS
BMW

{ DIAGNOSIS BUS
BMW DIS

CHECK
ENGINE
20 PIN TXD
DIAGNOSTIC
BMW DIS

CONNECTOR Example:
up to 9/00
E39 High Version with I and K
bus network.

KOMBI
RAIN DIAGNOSIS BUS (TXD)
EW
S3

80 3 4
SENSOR
.3

60 100 1/min 5
100
120 140
160
2 x 1000
80 180
40 60 200 120 6
AIC 0
½
20
40
20
220
240

140
1
0 7
km/h
MPH 40 0

DSC III
ELECTRONIC 20 15 10
!
HC

MR 012345 km 1224
miles
PRND S321 P ! ABS
SO
B

S3 CHECK MP
MP + 88 8 L/100 KM/H
KM/L
OIL SERVICE INSPECTION
!

K-BUS
ENGINE

K-BUS
K-BUS

RADIO CAN BUS

IHKA RECEIVER
MODULE
- BASS +

- TREBLE +
DOLBY SYSTEM FLAT
FADER

BALANCE

70 0 F 640 F
AUTO
FBZV GM III DWA B
M
W

0 0:
WH
228
390
AUDIO
A m
14
1 7 0 0:W
3W H

IHKA
MB

TEL
AUTO REST - +
BC SRS
AIRBAG
P-BUS

K-BUS MIDK-BUS DIAGNOSIS BUS

M-BUS

B MFL-CM
M

Telephone
Driver's Door Psgr's Door Interface Box
Switchblock/ Switchblock/
M-BUS

K-BUS

Module Module DME AGS

M FBZV GM III
PM-FT/SB
DWA
PM-FT/SB LCM III
GR II
P-BUS

RADIO
StarTAC

M
BMW

Seat Memory Sunroof AMPLIFIER

Module Module
PM-SM
Driver's Door PM-SHD SRS
AIRBAG

M Switchblock/ Passenger's MID PDC

Module Door Module MFL-CM
M PM-FT/SB
PM-BT

Seat Memory Sunroof

Module AMPLIFIER
obd ii STANDARDIZED
Module
LCM
LWR

FAULT CODES

PM-SM 1 2

Sc
an
3

ne
r
4 Y
PM-SHD ABS/ASC5
N
OBD II - DLC
(diagnostic link connector)
EHC

TXD II
AGS
CAN BUS

BMW
DIS
obd ii STANDARDIZED
FAULT CODES
{ DIAGNOSIS BUS
BMW DIS

n er
an
Sc
1 2 3 4

TXD
20 PIN
DIAGNOSTIC Example: DME
CONNECTOR
E53 Base Cluster with K bus
BMW DIS

network .
iC
oD
M

DIAGNOSIS BUS
18
Bus System Troubleshooting
EWS II MRS
The I/K bus consists of a single copper wire. The wire color of the I and K bus is uniform
throughout the vehicle with: I-bus WS/GR/GE and the K-bus WS/RT/GE (Note: 2001 E39s
with base Kombi have changed K-bus wire color to the same as the I-bus, WS/GR/GE).

Due to the linear structure of the network, the I/K bus is available for other modules in the
event of a disconnected or failed control unit. Just as the CAN bus, this is referred to as a
“Tree” structure with each control unit occupying a branch. The I/K-bus provides the diag-
nostic connection to the control units located on those busses (except IKE/KOMBI).

Always refer to the ETM to determine the exact wiring configuration and color for a specific model.

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 indi-
vidual users via a “wake-up” message.
Print Change End Services Help

Unlike the CAN bus where each control unit BMW Test system Multimeter

(subscriber) provides voltage for communica-

Freeze image

tion, the I/K-busses use only determined 10.15 V DC Minimum

Master or Stand-by Controllers to supply

Maximum

B+for communication. The voltage level on the

20 0 20
System voltage
Measurement Voltage Resistance Temperature Rotation speed
Function V Ohm o
C

I/K bus must be above 7V. The nominal value Current

2A
Current
50A
Current
1000A
Diode test
-|>|-
Pressure
bar 2nd

should be close to the system voltage of the

measurement

Measurement MFK 1 MFK 2 Current probe Pressure Temperature

Connection Sensor Sensor

vehicle. Measurement
Kind = Effective value

automatic ± 500 mV Stimulate

Just like the CAN bus, the fact that voltage is

Measurement
Range

Multimeter Counter Oscilloscope Stimulators Preset

present does not mean that the bus is fault setting measurements

free, it just means that the voltage level is suffi-

cient to support communication.

19
Bus System Troubleshooting
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 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 defec-
tive 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 recommend-
ed to use the “Wire Test” in “Preset Measurements” which is more sensitive than
just a resistance check.

20
Bus System Troubleshooting
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.
Print Change End Services Help

BMW Measuring system Oscilloscope display

A [V]
Cursor 1 Memory Cursor 2
8 [V] V
Freeze Image
V High
Channel B 7V to B+
16 16 8 T
r Zoom
12 12 6
i
8 8 4 g
g
Amplitude
Channel A
V Low
4 4 2
e 0V to 2V
0 0 0 r
Amplitude
Channel B
-4 -4 -2
l
-8 -8 e-4
Time value
-12 -12 -6
v

-16
e
-16 -8 l Message Time
5ms to 30ms
-2.0 -1.0 0.0 1.0 2.0
-1.5 -0.5 0.5 1.5
ms Stimulate

Multimeter Counter Oscilloscope Stimulators Preset

setting measurments

61400100

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.
Print Change End Services Help Print Change End Services Help

BMW Measuring system Oscilloscope display BMW Measuring system Oscilloscope display
Freeze Image Freeze Image
Cursor 1 Memory Cursor 2 Cursor 1 Memory Cursor 2
A [V] 8 [V] V A [V] 8 [V] V
Channel B Channel B

16 8 T 16 16 8 T
16
r r Zoom
12 12 6 Zoom 12 12 6
i i
8 8 4 g 8 8 4 g
Amplitude Amplitude
g Channel A
4 2
g Channel A
4 4 2 4
e e
0 0 0 r 0 0 0 r
Amplitude Amplitude
Channel B
-4 -4 -2 Channel B -4 -4 -2
l l
-8 -8 -4 e -8 -8 -4 e
Time value Time value
v -12 -6
v
-12 -12 -6 -12
e e
-16 -16 -8 l -16 -16 -8 l

-2.0 -1.0 0.0 1.0 2.0 -2.0 -1.0 0.0 1.0 2.0
ms -1.5 -0.5 0.5 1.5
ms Stimulate
-1.5 -0.5 0.5 1.5 Stimulate

Multimeter Counter Oscilloscope Stimulators Preset Multimeter Counter Oscilloscope Stimulators Preset
setting measurments setting measurments

61400100 61400100

Flat line at 12 volts Flat line at 5 volts

No communication is currently taking No output voltage from the Master or
place. The bus may be temporarily offline Standby controllers. Bus line may be
or shorted to B+. open or control unit may be defective.

21
Bus System Troubleshooting
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 nor-
mally 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 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 para-
lyze or take down the entire bus. This scenario would be evident by functions not being
carried out and 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 con-
trol module is the defective one the faults will only point to communication with that inter-
rupted 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 try-
ing 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 prop-
er 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.
22
Bus System Troubleshooting
Peripheral Bus (P-Bus)

Introduction

The P-bus is single wire serial communications bus that is used exclusively on vehicles that
are equipped with ZKE III. When the E38 was introduced the objective was to reduce the
complexity of the wiring harness. Peripheral modules are located in areas of the vehicle
close to sensors or actuators where wiring the components separately would create an
excessively large harness (e.g. door ). In some cases (e.g. Sunroof module) these periph-
eral modules are integrated with an actuator or switch to create one unit. The peripheral
modules are connected to the GM III by the P-bus.

The P-bus is only used in the body electronics area and is very similar in communication
protocol and speed to the I/K-bus. The P-bus is not designed for rapid exchange of con-
tinuous information, rather, the messages on the P-bus are short control commands. This
limited message flow allows for fast reaction time by the Peripheral module (e.g. a door lock
or window request).

Go to sleep mode: The ZKE III goes to sleep after the key is switched off ,no messages
are being sent and after 16 minutes. The GM is responsible for sending the “GO TO
SLEEP” command to all of the P-bus subscribers.

Wake up: Controllers that have the capability of sending the wake-up call are the GM and
the driver and passenger door modules. The wake up call is simply a P-bus low.

P-bus polling: When KL R is switched on the GM sends out a P-bus poll every 5 seconds
to the modules that are coded as being installed. The P-module must respond in 5 sec-
onds. If it does not respond the GM tries two more times. If the poll is still unanswered the
GM enters it into fault memory.

Coding: The GM informs the P-modules of relevant coding data.

23
Bus System Troubleshooting
P-bus Topology

The extent of the P-bus depends on the special equipment of the vehicle.

Servotronic GM III DWA

P-BUS
Driver's Door
Switchblock/ Passenger's
Module Door Module
PM-BT
PM-FT/SB

Seat Memory Sunroof

Module Module
PM-SM PM-SHD

The P-bus consists of a single copper wire. The wire color of the bus is uniform through-
out 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 connec-
tion to the P-modules.

Always refer to the ETM to determine the exact wiring configuration for a specific model.

24
Bus System Troubleshooting
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 volt-
age necessary to support communication. The voltage level of the P-bus is 12V.

The Diagnosis of the central body electron-

Print Change End Services

ics is carried out via the K-bus. The GM

BMW Diagnosis P BUS
Automatic testing of data transmission
to the peripheral modules.
converts diagnosis request from the Door module, driver's door:
DATA TRANSMISSION OK
DISplus into diagnostic mode messages
Door module, passenger door:
and transmits them the the peripheral DATA TRANSMISSION OK

Slide/tilt-sunroof module:
modules over the P-bus. DATA TRANSMISSION OK

Test result: OK

Automatic testing of the P-bus connection

is carried out every time the GM communi-
cates with the diagnosis program (not dur- Note

ing a short test).

Function Documents Test Schedule TIS Measuring Control unit
Selection System Functions

07400100

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 recommend-
ed 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.

25
Bus System Troubleshooting
Motor Bus (M-Bus)

Introduction

The M-bus is used exclusively between a climate control module, e.g. IHKA/IHKR and a set
of stepper motors used to move various air distribution doors.

These “smart” stepper motors have an integrated communication I.C. that allows them to
listen as well as transmit information over a single bi-directional data link.

Integrated microprocessor
and communication module

To distinguish its position in the IHK housing,

each stepper motor is programmed by the
manufacturer with its own electronic address.
64400100

Each stepper motor on the M-bus also has a unique part number that is printed on the
body of the motor. It is vital that each stepper motor be installed in its correct location for
proper operation due to the addressing.

A three wire ribbon connector is used to connect the stepper motor to the IHK control unit.
All of the stepper motors are connected in parallel to each other via the M-bus ribbon cable.

The ribbon contains:

• Power
• Ground
• Bi-directional signal wire

''SMART'' STEPPER MOTOR

CONTROL MODULE

+
MICROPROCESSOR M

POWER
GROUND

5V
SIGNAL
64400101

26
Bus System Troubleshooting
Each stepper motor acts as a “slave” , it listens to all data on the bus, but only accepts or
responds as long as the message is transmitted without errors and it recognizes it’s own
address.

M-bus protocol: The M-bus protocol differs from the CAN and the I/K/P busses in that
communication takes place on a constant basis within a framework time of 650ms.

When the IHK module is commanding a change in position of one or more stepper motors
the sequence of data is:

1. Start bit
• Informs the stepper motors that a command is coming.

2. Synchro bit
• Establishes the message as originating from the IHK control unit.

3. Data field
• The command to move a stepper motor to a particular position.

4. Address field
• The IHK control unit names the stepper motor the command is intended for.

If the message was received by the stepper motor without error (oversampling) the stepper
will carry out the command and transmit its acknowledgement:

1. Synchro bit
• Establishes the message as originating from the stepper motor.

2. Data field
• Status information from the actuator (feedback).

3. End of frame
• Closes the communication session.

Communication continues on the M-bus until the GM sends the “go to sleep” command
over the K-bus.

27
Bus System Troubleshooting
M-bus Topology
E38 IHKA (early)
REST
72 O F 72 O F

0 0
AUTO AUTO

Hardwired stepper motor

FRESH AIR INLET FLAP 64400102

The M-bus consists of a three wire ribbon. On E38 vehicles because of the large number
of stepper motors used, the M-bus is divided into two separate circuits. All other vehicles
use a single M-bus cable. The M-bus wire ribbon is not routed with the rest of the vehicle
harness but is a separate harness attached to the IHK housing.

Always refer to the ETM to determine the exact wiring configuration for a specific model.

M-bus Troubleshooting

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 Print Change End Services Help

on. The IHK module provides the voltage nec- BMW Test system Multimeter

essary to support communication. The volt-

Freeze image

2.814 V
age level of the M-bus is 5V, but because sta- Minimum
Maximum

tus communication occurs at an average 50%

duty cycle the observed voltage is approxi-
-5.0 0 5.0
System voltage
Measurement Voltage Resistance Temperature Rotation speed
Function V Ohm o
C

mately 2.5V. The presence of 2.5V means Current

2A
Current
50A
Current
1000A
Diode test
-|>|-
Pressure
bar 2nd
measurement

that communication is occurring. Measurement

Connection
MFK 1 MFK 2 Current probe Pressure
Sensor
Temperature
Sensor

Measurement
Kind = Effective value

automatic ± 5V Stimulate
Measurement
Range

Multimeter Counter Oscilloscope Stimulators Preset

setting measurements

64400104
28
Bus System Troubleshooting
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.

Print Change End Services Help

BMW Measuring system Oscilloscope display

Freeze Image
Cursor 1 Memory Cursor 2
A [V] B [V] V
Channel B

T
8
6
16
12
8
6
r Zoom
V High
i 5V
4 8 4 g Amplitude

2 4 2
g Channel A

e
0 0 0 r
-2 -4 -2
Amplitude
Channel B
V Low
l 0.2V
-4 -8 -4 e
Time value
-6 -12 -6
v
e
-8 -16 -8 l

-200 -100 0.0 100 200

-150 -50 50 150
US Stimulate

Multimeter Counter Oscilloscope Stimulators Preset

setting measurments

64400103

Example of correctly operating M-bus

Communication on the M-bus occurs continuously with an average Period duration of 50
ms. 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.

29
Bus System Troubleshooting
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 step-
per motors. If the M-bus is interrupted then the control unit will store a fault for every step-
per motor on the bus.

In order to determine if a stepper motor is at fault for the lack of communication, discon-
nect 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:
Print Change End Services

• Fault/symptom driven test modules BMW Diagnosis Test information

Air supply and defrost Air distribution
A11
Air distribution
Heater/air conditioner control unit
Air distribution for windshield, face and footwell is controlled by buttons on

• Diagnosis request (flap position)

the operating panel and the 3 bipolar stepper motors via the M-bus.
Manual or automatic air distribution can be selected. The motors are coded
and can only be installed in the designated position. It is not possible to
interchange the motors.
1 3 X613
The LED in the AUTO button lights during the automatic program. The
opening angles are set automatically dependant on the inside, outside and
X18836
set temperature.

The air distribution can be set individually in manual mode. The setting is

• Component activation (flap activation) SM+

0.35
RT
Mi-BUS
0.35
GE
SM+

0.35
MI-BUS
0.35
B1214 M0CAN/Message 08
Check lines between the following components Yes
SM+
RT GE
1 3
(component 1, component 2, signal names ):
1 3

M38

Ventilation flap
A11 Control unit for heating/air conditioning No
motor system
All flap motors
2
X661
2
X662
MI-BUS MI-bus flap motors
SM-
SM+ Supply, MI-bus motors
SM-
0.35
SM-Ground MI-bus motors
0.35
BR BR

Are the lines OK?

Note

Function Documents Test Schedule TIS Measuring Control unit

Selection System Functions

07400101

30
Bus System Troubleshooting
Review Questions

1. What should be the voltage preset at a CAN line if checked with a multi-meter? Is the
voltage the same on both lines?

2. 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?

3. Explain the differences of CAN-High and CAN-Low? How can they be distinguished
from one another?

4. 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.

5. What is the minimum voltage required at the D-bus?

6. Why is checking a bus signal with an oscilloscope a practical option?

7. Describe some quick tests that can help to determine if a bus line is currently
operating.

31
Bus System Troubleshooting