the can data bus

Self-Study Program

The CANData Bus

Design and function

earningransformation

Volkswagen of America, Inc.Learning & TransformationPrinted in U.S.A.Printed 3/99Part# WSP-521-419-00

All rights reserved. All information contained inthis manual is based on the latest productinformation available at the time of printing.The right is reserved to make changes at anytime without notice. No part of this publicationmay be reproduced, stored in a retrievalsystem, or transmitted in any form or by anymeans, electronic, mechanical, photocopying,recording or otherwise, without the priorwritten permission of the publisher. Thisincludes text, figures and tables.

Always check Technical Bulletins and theVolkswagen Worldwide Repair InformationSystem for information that may supersedeany information included in this booklet.

i

Table of Contents

Introduction .............................................................................................................................. 1

CAN data bus ............................................................................................................................ 2

Data transfer ............................................................................................................................. 8

Function .................................................................................................................................. 10

CAN data bus in the convenience system............................................................................ 15

CAN data bus in the drive train ............................................................................................. 22

The Self-study Program is not a Workshop Manual!Precise instructions for testing, adjustment and repair can be found in the appropriateWorkshop Manual

Important/Note New

1

Introduction

The requirements relating to drivingsafety, driving comfort, exhaustemissions and fuel economy arebecoming ever more stringent.

This entails more intensive informationexchange between control units. A well-engineered solution is necessary toensure that the electrics/electronics inthe vehicle still remain manageable anddo not take up too much space.

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The CAN data bus by Bosch is such asolution. It was developed specially forautomobiles and is used by bothVolkswagen and Audi. CAN stands forController Area Network and means thatcontrol units are networked andinterchange data.

In this Self-study Program we willexplain to you the design and functionof the CAN data bus.

A CAN data bus can be compared to apassenger bus.

While the passenger bus transports alarge number of persons, the CAN databus transports a large volume ofinformation.

2

CAN Data Bus

Data Transfer

What are the possible options for data

transfer in vehicles at present?

• Option No. 1:

Each item of information is exchangedover a separate wire.

The figure below shows you option

No. 1, where each item of information

is transferred along a separate wire.

A total of five wires are required for datatransfer in this case.

Conclusion:

A separate wire is required for each itemof information.

As the volume of additional informationincreases, so does the number of wiresand the number of pins on the controlmodules.

Motronic Engine ControlModule (ECM) J220

• Option No. 2:

All information is exchanged betweencontrol modules along a maximum oftwo wires: the CAN data bus.

Transmission ControlModule (TCM) J217

Therefore, this data transfer mode isonly suitable for exchanging a limitedvolume of information.

Engine speed

Fuel consumption

Throttle valve position

Engine intervention

Upshift/downshift SSP 186/04

3

CAN Data Bus

In contrast to option No. 1, allinformation is tranferred along two wiresin the CAN data bus.

The same data is transferred along thetwo bidirectional wires of the CAN databus.

You will find further information in thisSelf-study Program.

Conclusion:

With this data transfer mode, allinformation is transferred along twowires regardless of the number ofparticipating control modules and thevolume of information involved.

Engine speed

Fuel consumption

Throttle valve position

Engine intervention

Upshift/downshift

Motronic Engine ControlModule (ECM) J220

Transmission ControlModule (TCM) J217

Data transfer with the CAN data buswould therefore make sense if a largevolume of information is exchangedbetween control units.

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4

The CAN Data Bus

is a type of data transfer between controlmodules. It links the individual controlmodules to form an integrated system.

The more information a control modulehas regarding the state of the overallsystem, the better it can coordinate theindividual functions.

The following components in the drive

train form an integrated system:

• the Engine Control Module (ECM)• the Transmission Control Module

(TCM)• the ABS Control Module (w/EDL)

The following components in the

convenience system form an integrated

system:

• the central control module and• the door control modules

Benefits of the Data Bus:

• If the data protocol is extended toinclude additional information, onlysoftware modifications are necessary.

• Low error rate through continuousverification of the transmittedinformation by the control modules aswell as additional safeguards in thedata protocols.

• Fewer sensors and signal lines throughthe multiple use of a sensor signal.

• High-speed data transfer is possiblebetween control units.

• More space available through smallercontrol modules and smaller controlmodule plugs.

• The CAN data bus conforms tointernational standards and thereforefacilitates data interchange betweendifferent makes of control unit.

CAN Data Bus

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ABS Control Module(w/EDL)

Door Control Module

Central ControlModule

Engine ControlModule (ECM) Transmission Control

Module (TCM)

5

CAN Data Bus

The Principle of Data Transfer

Data transfer with the CAN data busfunctions in much the same way as atelephone conference.

A subscriber (control module) “speaks”data into the line network while the othersubscribers “listen in“ to this data.

Some subscribers will be interested inthis data and will utilize it. The othersubscribers will choose to ignore thisdata.

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Control Module 3

Control Module 1 Control Module 2

Control Module 4

Data Bus Line

6

CAN Data Bus

What Components Make Up a

CAN Data Bus?

The CAN data bus comprises acontroller, a transceiver, two data busterminals and two data bus lines.

Apart from the data bus lines, thecomponents are located in the controlmodules. The functions of the controlmodules are the same as before.

They have the following tasks:

The CAN Controller

receives the transfer data from themicrocomputer integrated in the controlmodule. The CAN controller processesthis data and relays it to the CANtransceiver.Likewise, the CAN controller receivesdata from the CAN transceiver,processes it and relays it to themicrocomputer integrated in the controlmodule.

The CAN Transceiver

is a transmitter and receiver in one. Itconverts the data which the CANcontroller supplies into electrical signalsand sends this data over the data buslines.Likewise, it receives data and convertsthis data for the CAN controller.

The Data Bus Terminal

is a resistor. It prevents data sent frombeing reflected at the ends and returningas an echo. This would corrupt the data.

The Data Bus Lines

are bidirectional and transfer the data.They are referred to as CAN High andCAN Low.

SSP 186/03

Transmission Control Module(TCM) J217 with CAN Controllerand CAN Transceiver

Motronic Engine Control Module(ECM) J220 with CAN Controller andCAN Transceiver

Data Bus Terminal

Data Bus Line

Data Bus Terminal

7

CAN Data Bus

The data bus does not have a designatedreceiver. Data is sent over the data busand is generally received and evaluatedby all subscribers.

Data Transfer Process:

Supplying the Data

The control module provides data to theCAN controller for transfer.

Sending Data

The CAN transceiver receives data fromthe CAN controller, converts it intoelectrical signals and sends them.

Receiving Data

All other control modules networkedwith the CAN data bus become receivers.

Checking Data

The control modules check whether theyrequire the data they have received fortheir functions or not.

Accepting Data

If the received data is important, it isaccepted and processed. If not, it isignored.

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AcceptData

CheckData

ReceiveData

ProvideData

SendData

ProvideData

SendData

AcceptData

CheckData

ReceiveData

Data Bus Line

Control Module 1 Control Module 2 Control Module 3 Control Module 4

8

Data Transfer

What Does the CAN Data Bus Transfer?

It transfers a data protocol between thecontrol modules at short intervals.It is subdivided into seven areas.

The Data Protocol:

comprises a long string of bits. Thenumber of bits in a data protocoldepends on the size of the data field.

The diagram below shows the format ofa data protocol. This format is identicalon both data bus lines.For simplicity’s sake, only one data busline will be shown in this Self-studyProgram.

A bit is the smallest unit ofinformation (one circuit stateper unit of time). Inelectronics, this informationcan only have the value “0”or “1”, i.e. “yes” or “no”.

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Start Field (1 bit)

Status Field (11 bits)

1 bit = unusedData Field (max. 64 bits)

Confirmation Field (2 bits)

Check Field (6 bits)Safety Field (16 bits)

End Field (7 bits)

9

Data Transfer

The Seven Areas:

The start field

marks the start of the data protocol. A bitwith approx. 5 Volts (depending onsystem) is sent over the CAN High Lineand a bit with approx. 0 Volts is sent overthe CAN Low Line.

The status field

defines the level of priority of the dataprotocol. If, for instance , two controlmodules want to send their data protocolsimultaneously, the control module withthe higher priority takes precedence.

The check field

displays the number of items ofinformation contained in the data field.This field allows any receiver to checkwhether it has received all theinformation transferrerd to it.

In the data field,information is transferred to the othercontrol modules.

The safety field

detects transfer faults.

In the confirmation field,the receivers signal to the transmitterthat they have correctly received the dataprotocol. If an error is detected, thereceivers notify the transmitter of thisimmediately. The transmitter then sendsthe data protocol again.

The end field

marks the end of the data protocol. Thisis the last possibility to indicate errorswhich lead to a repeat transfer.

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10

In principle, the CAN data bus functionsin exactly the same way.

The transceiver

can also generate two different bit states.

Status of bit with the value “1”

• Transceiver open, switches to 5 Volts inthe convenience system (drive train:approx. 2.5 Volts)

• Voltage applied to data bus line:approx. 5 Volts in the conveniencesystem (drive train: approx. 2.5 Volts)

Function

How is a Data Protocol Produced?

The data protocol comprises a string ofseveral bits. Each bit can only havestatus or value “0” or “1”.

Here is a simple example to explain how

a status with the value “0” or “1” is

generated:

The light switch

switches a light on or off. This meansthat the light switch can have twodifferent states.

Status of the light switch with the

value “1”

• Switch closed• Lamp on

Status of the light switch with the

value “0”

• Switch open• Lamp is not on

Status of the bit with the value “0”

• Transceiver closed, switches to ground• Voltage applied to data bus line:

approx. 0 Volts

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CANtransceiver

CANtransceiver

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5 Volts

0 Volts

5 Volts

0 Volts

11

Function

The table below shows you howinformation can be transferred with twoconsecutive bits.

With two bits, there are four possiblevariations.One item of information can be assignedto each variation and is binding for allcontrol modules.

Example:

If bits 1 and 2 are transmitted with0 Volts, this indicates that the electricwindows are in motion and that thecoolant temperature is 10°C.

The table below shows you how the number ofitems of information increases with eachadditional bit.

The higher the number of bits, the more items ofinformation can be transferred.

The number of possible items of informationdoubles with each additional bit.

Possible 2nd bit 1st bit Graphic Electric window status Information on

variation information coolant temperature

One 0 Volts 0 Volts in motion 10°C

Two 0 Volts 5 Volts not moving 20°C

Three 5 Volts 0 Volts within range 30°C

Four 5 Volts 5 Volts upper stop recognition 40°C

Bit variants Possible Bit variants Possible Bit variants Possible

containing 1 information containing 2 information containing information

bit bits 3 bits

0 Volts 10°C 0 Volts, 0 Volts 10°C 0 Volts, 0 Volts, 0 Volts 10°C

5 Volts 20°C 0 Volts, 5 Volts 20°C 0 Volts, 0 Volts, 5 Volts 20°C

5 Volts, 0 Volts 30°C 0 Volts, 5 Volts, 0 Volts 30°C

5 Volts, 5 Volts 40°C 0 Volts, 5 Volts, 5 Volts 40°C

5 Volts, 0 Volts, 0 Volts 50°C




12

Function

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Data Bus Line

CAN Data Bus Allocation

If more than one control module wantsto send its data protocol simultaneously,the system must decide which controlmodule comes first.The data protocol with the highestpriority is sent first. For safety reasons,the data protocol supplied by the ABSControl Module (w/EDL) for safetyreasons is more important than the dataprotocol supplied by the TransmissionControl Module (TCM)

How Are Allocations Made?

Each bit has a value, and this value isassigned a weighting. There are twopossibilities: high weighting or lowweighting.

How is the Priority of a Data Protocol

Recognized?

A code comprising eleven bits isassigned to each data protocoldepending on its priority in the statusfield.

The priorities of three different dataprotocols are shown in the table below.

Priority Data protocol Status field

1 Brake I 001 1010 0000

2 Engine I 010 1000 0000

3 Transmission I 100 0100 0000Bit with Value Weighting

0 Volts 0 highweighting

5 Volts 1 low weighting

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Function

All three control modules start sendingtheir data protocol simultaneously. Atthe same time, they compare the data biton the data bus line.

If a control module sends a lowweighting bit and detects a highweighting bit, the control unit stopssending and becomes a receiver

Example:

Bit 1:

- ABS Control Module (w/EDL)transmits a high weighting bit.

- Motronic Engine Control Module (ECM)also transmits a high weighting bit.

- Transmission Control Module (TCM)transmits a low weighting bit anddetects a high weighting bit on the databus line. Thus, it loses its priority statusand becomes a receiver.

Bit 2:

- ABS Control Module (w/EDL)sends a high weighting bit.

- Motronic Engine Control Module (ECM)transmits a low order bit and detects ahigher weighting bit on the data busline. This, it loses its priority status andbecomes a receiver.

Bit: 3:

- ABS Control Module (w/EDL)has the highest priority and thusreceives the allocation. It continues tosend its data protocol until it ends.

After the ABS Control Module (w/EDL)has finished sending its data protocol,the other control modules try again totransmit their data protocol.

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ABS Control Module (w/EDL)

Motronic Engine ControlModule (ECM)

Transmission Control Module(TCM)

Data bus line

low weighting

high weighting

Transmission Control Module(TCM) loses priority status

Motronic Engine ControlModule (ECM) priority status

▲

▲

14

Function

Sources of Interference

Sources of interference in the vehicle arecomponents which produce sparks or inwhich electric circuits are open or closedduring operation.

Other sources of interference includemobile telephones and transmitterstations, i.e. any object which produceselectromagnetic waves. Electromagneticwaves can affect or corrupt data transfer.

To prevent interference with the datatransfer, the two data bus lines aretwisted together. This also preventsnoise emission from the data bus line.

The voltage on both lines is opposed.

That Means:

If a voltage of approx. 0 Volts is appliedto one data bus line, then a voltage ofapprox. 5 Volts is applied to the otherline and vice versa.

As a result, the total voltage remainsconstant at all times and theelectromagnetic field effects of the twodata bus lines cancel each other out.

The data bus line is protected againstreceived radiation and is virtually neutralin sending radiation.

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approx. 0 Volts

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approx. 5 Volts

15

CAN Data Bus in the Convenience System

The CAN Data Bus in the Convenience

System

In the convenience system, the CAN databus currently connects the controlmodules of the convenience system.

These are- a central control module and- two or four door control modules.

The Structure of the CAN Data Bus in

the Convenience System

The lines of the control modulesconverge at one point in a star pattern.The advantage: if one of the controlmodules fails, the other control modulesare still able to send their data protocols.

The following functions of theconvenience system transfer data:

• Central locking

• Electric windows

• Switch illumination

• Electrically adjustable and heateddoor mirrors

• Self-diagnosis

SSP 186/21

What Are the Advantages of the CAN

Data Bus in the Convenience System?

• Fewer lines are routed via the doorconnections.

• In the event of a short circuit to ground,to positive or between lines, the CANdata bus goes to emergency runningmode and changes over to single-wiremode.

• Fewer diagnosis lines are required,because self-diagnosis is handledentirely by the central control module.

16


The Features of the CAN Data Bus

in the Convenience System

• The data bus comprises two linesalong which information is sent.

• To avoid electromagnetic interferenceand radiation emission, the two databus lines are twisted together.

• The data bus operates at a speed of62.5 kbit/s (62500 bits per second). Thismeans that it lies in a speed range(low speed) from 0 from 0 - 125 kbit/s.A data protocol transfer takes approx.1 millisecond.

• Each control module tries to send itsdata at intervals of 20 milleseconds.

• Order of priority:1. Central control module ➜2. Control module on driver’s side ➜3. Control module on front passenger’s side ➜4. Control module on rear left ➜5. Control module on rear right

Since the data in the comfort system canbe transferred at a relatively low speed,it is possible to use a transceiver with alower power output.

The advantage is that it is possible tochange over to single-wire mode if adata bus line fails. The data can still betransferred.

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20 ms 20 ms 20 ms

▲ ▲ ▲

▲▲▲

17


Information in the Convenience System

The information relates to states of theindividual functions.For example, information about whichradio-wave remote control was operated,current status of central locking, doerrors exist, and so on.

The table shows you part of the datafield of the driver’s door control moduleby way of an example.

Example showing a possible bit order

You can see how and what informationregarding the status of the centrallocking and the electric windows istransferred.

SSP 186/27

Bit order Value Voltage applied to Meaning of information

data bus line

3 to 1 101 5 Volts, 0 Volts, 5 Volts The central locking is unlocked

5 to 4 10 5 Volts, 0 Volts The window is located in the zone between the upper stop(completely closed) and 4 mm below the window seal

1 = 5 Volts

0 = 0 Volts

Bit 5 Bit 4 Bit 3 Bit 2 Bit 1

Function Information Bit order Value of

status Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 bits

Central Basic status 0 Volts, 0 Volts, 0 Volts 000locking Safe 0 Volts, 0 Volts, 5 Volts 001

Locked 0 Volts, 5 Volts, 0 Volts 010Door unlocked 0 Volts, 5 Volts, 5 Volts 011The central locking is unlocked 5 Volts, 0 Volts, 5 Volts 101Door locked 5 Volts, 0 Volts, 0 Volts 100Signal error, input sensors 5 Volts, 5 Volts, 0 Volts 110Status error 5 Volts, 5 Volts, 5 Volts 111

Electric In motion 0 Volts, 0 Volts 00windows Not moving 0 Volts, 5 Volts 01

Within range 5 Volts, 0 Volts 10Upper stop recognized 5 Volts, 5 Volts 11

18


Networking of Control Units in the

Convenience System

Control Units:

J386 Door control module, driver side

J387 Door control module, passenger side

J388 Door control module, rear left

J389 Door control module, rear right

J393 Central control module for comfortsystem

Fuses:

S6 Fuse, terminal 15 - central control moduleS14 Fuse, terminal 30 - central control moduleS37 Power Window Fuse, terminal 30S238 Fuse, terminal 30 - central locking

Color Coding:

Input signal

Output signal

Positive

Ground

Data bus line High/Low

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S37 S238

J386

J386

31

3015X31

19


J393 S6 S14

KJ387

J389

3015X31

31

20


The Self-Diagnosis of the CAN Data Bus

in the Convenience System

Self-diagnosis can be performed withV.A.G. 1551/52 or with VAS 5051 underthe following address word:

46 “Convenience system”

During self-diagnosis andtroubleshooting, all controlmodules which interchangeinformation with the CANdata bus must be regardedas an integrated system.

The following functions are relevant tothe CAN data bus:

Function 02 - Interrogate fault memory

In the fault memory, two faults areindicated specially for the CAN data bus.

Convenience data bus

This fault is set if data transfer betweentwo or more control units fails.

Possible fault causes are:- Defective control modules- Open circuit in both data bus lines or

in plug and socket connections

Convenience data bus in emergency

running mode

This fault is indicated if the CAN data bushas entered emergency mode.

Possible fault causes are:- Open circuit in one data bus line or

in a plug and socket connection

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VAS 5051

Printout on

V.A.G. 1551 printer

01329

Convenience data bus inemergency running mode

Printout on

V.A.G. 1551 printer

01328

Convenience data bus

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Function 08 - Read measured value block

Display group number 012 - Central

control unit - displays four display fieldsrelevant to the data bus.

Display field 1: Check bus

This field indicates whether the data busis OK or faulty (e.g. fault in single wire).

Display field 2: Equipment front

This field indicates which front controlmodules are fitted and participate in datatransfer.

Display field 3: Equipment rear

This field indicates which rear controlmodules are fitted and participate in datatransfer.

Display field 4: Accessories

This field indicates whether the seat andmirror adjustment memory system isfitted. Both systems (conveniencesystem and memory system)interchange data.

Direct CAN data transfercurrently cannot be checkedusing the available workshopfacilities.

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Display group 012 - Central control unit

Read measured value block 12 Display on monitor

Display fields Setpointxxx xxx xxx xxx

1 2 3 4Accessories

Equipment rear

Equipment front

Check Bus

Memory / empty1)

RLRL and RR

RRempty1)

DriverDriver and FP

FPempty1)

Bus OKBus NOK

➛ ▲▲

22

CAN Data Bus in the Drive Train

The Data Bus in the Drive Train

The CAN Data Bus Links the Following:

• The Motronic Engine Control Module(ECM)

• The ABS Control Module (w/EDL)

• The Transmission Control Module(TCM)

At the moment 10 data protocols aretransferred.Five from the Motronic Engine ControlModule (ECM), three from the ABSControl Module (w/EDL) and two fromthe Transmission Control Module (TCM).

What Special Advantage Does the CAN

Data Bus Have in the Drive Train?

• A high data transfer rate, with theresult that the control modules are verywell-informed about the momentarystate of the overall system and canexecute functions optimally.

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Data bus(with external node)

Motronic Engine ControlModule (ECM)

Transmission ControlModule (TCM)

23

10 ms 10 ms 10 ms


The Features of the CAN Data Bus

in the Drive Train

• The data bus comprises two linesalong which informaton is transferred.

• In order to avoid electromagneticinterference and radiation emission,the two data bus lines are twistedtogether. Note the twist length.

• The data bus operates at a speed of500 kbit/s (500,000 bits per second).This means that it lies in a speed range(high speed) from 125 - 1000 kbit/s.A data protocol transfer takes approx.0.25 milliseconds.

• Each control module (depending ontype) tries to send its data at intervalsof 7 - 20 milliseconds.

• Order of priority:1. ABS Control Module (w/EDL) ➜2. Motronic Engine Control Module (ECM) ➜3. Transmission Control Module (TCM) ➜

In the drive train, it must be possible totransfer the data very quickly so that itcan be fully utilized.For this purpose, a high-performancetransceiver is required.

This transceiver facilitates data transferbetween two ignition systems.This means that the received data can beused for the next ignition impulse.

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▲ ▲ ▲

▲▲▲

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The Information in the Drive Train

What Information is Transferred?

The information in question is veryimportant for the tasks of the individualcontrol modules. For safety reasons inthe case of the ABS Control Module(w/EDL), for reasons of controlling the

In the table below you can find examplesof the format of an individual item ofinformation. On account of the sheernumber of items of information whichhave to be transferred, only one part isdisplayed.

The current position of the throttle valveis transferred with 8 bits, giving apossible 256 bit permutations.Thus, throttle valve positions from 0° to102° can be transferred at 0.4° intervals.

ignition and quantity injected in the caseof the Engine Control Module (ECM) andfor reasons of driving convenience in thecase of the Transmission Control Module(TCM).

Bit order Throttle valve position

0000 0000 000.0° Throttle valve opening angle



• • • • • •


• • • • • •


Order of Data protocol form Examples of information

priority

1 ABS Control Module (w/EDL) - Request for engine braking control (EBC)- Request for Traction Control System (TCS)

2 Engine Control Module (ECM), data - Engine speedprotocol 1 - Throttle valve position

- Kickdown

3 Engine Control Module (ECM), data - Coolant temperatureprotocol 2 - Vehicle speed

4 Transmission Control Module (TCM) - Gearshift- Transmission in emergency mode- Selector lever position

25


Networking of the Control Units

in the Drive Train

J104 ABS Control Module (w/EDL)J217 Transmission Control Module

(TCM)J220 Motronic Engine Control Module

(ECM)

In contrast to the convenience system,only a part of the overall system isdisplayed in the drive train.In this case, only the networking of thecontrol modules is shown.

The node is usually located outside thecontrol module (in the wiring harness).

In exceptional cases, the node may belocated in the Engine Control Module(ECM).In the illustration below, you can see thenode at which the wires in the EngineControl Module (ECM) converge.

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Motronic EngineControl Module(ECM)

TransmissionControl Module(TCM)

CAN data bus (with node in MotronicControl Module (ECM)

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J217 J104

J220

Node

26


Self-Diagnosis of the CAN Data Bus

in the Drive Train

Self-diagnosis can be performed withthe V.A.G. 1551/52 or VAS 5051 underthese address words:

01 for engine electronics02 for transmission electronics03 for ABS electronics

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VAS 5051

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Data bus terminal

Data bus terminal

All control modules whichinterchange informationmust be regarded as anintegrated system duringself-diagnosis andtroubleshooting.

Th following function is relevant to theCAN data bus:

Function 02 - Interrogate fault memory

A fault is stored in the control modulesif data transfer between the controlmodules is disturbed:

• Open circuit in one or more data buslines.

• Short circuit between data bus lines.

• Short circuit to ground or positive in adata bus line.

• One or more control modules aredefective.

WSP-521-419-00

WE ENCOURAGE PROFESSIONALISM

THROUGH TECHNICIAN CERTIFICATION

the can data bus

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