CAN-BUS

CAN-BUS

Basic Concepts

Contents

1. Introduction;

2. CAN-BUS Network Development;

3. CAN-BUS Concepts;

4. CAN-BUS Operating Principle;

5. CAN-BUS Network Advantages;5. CAN-BUS Network Advantages;

6. CAN-BUS Network Diagnosis;

6.1. Diagnosis Procedures

7. Communication Failure in the CAN-BUS Network;

7.1. Failure in the CAN-BUS Lines;

7.2. Failure in an ECU;7.3. Tension or Ground Failure in an ECU;7.4. Interference in the CAN-BUS Lines;7.5. Failure in the CAN-BUS Internal Resistance;

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1. Introduction

• More and more, there is the need to have a larger interaction between the

different vehicle systems, and so the number of required functions also

increases;

• Higher need for safety and security (driving and anti-theft);

• In order to reach these goals, the wiring harness quantity in the car was

becoming larger and larger, and so it became necessary to create another way

to reach those goals: enter the CAN-BUS;

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2. CAN-BUS Network Development

• The communication protocol CAN (Controller Area Network) was developed by

INTEL and BOSCH in 1988, to be used in the automotive industry, in order to

create a reliable and low-cost normalized communication line between the

electronic devices of the car, allowing to reduce the vehicle's wiring harness size

and number;

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3. CAN-BUS Concepts

• BUS Line: a communication line with bi-directional data transmission capacity.

This means that, in a car, the Electronic Control Units (ECUs) are linked in

parallel, implying that all information sent by one ECU is also received by the

others.

In practical terms, a BUS line operates like a closed ring, where all units send

and receive information, constantly communicating between them;

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3. CAN-BUS Concepts

• CAN-BUS: is a local area network of communication, through which all of the

car's ECUs connected to that network (via BUS lines), send and receive

information.

Every information sent by an ECU (rpm, engine temperature, air flow, etc.) has

an exclusive ID code, throughout all CAN network. In this way, all ECUs will

receive that signal, but only the ECU determining the signal is important to its

operation will react to the signal. The other ECUs will simply ignore the signal.

The image on the next page illustrates the CAN-BUS network for the new Jazz;

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3. CAN-BUS Concepts

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4. CAN-BUS Operating Principle

• Each ECU receives information from the sensors connected to the ECU and

sends information to the actuators, via the CAN-BUS network;

• So that the ECUs are able to communicate between them, each ECU has an

integrated digital Encoder/Decoder for the CAN information;

• In this way, the logic In and Out signals are converted into digital information,

compatible with the CAN communication protocol;

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• The F-CAN (Fast-CAN) line connects the ECUs of the dynamic systems

(ABS/VSA, EPS, SRS, ECM/PCM), through two intertwined copper wires, each

one of them carrying an opposing signal with the same information: CAN-High

and CAN-Low respectively, with a data transfer rate of 500 kbps;

• The purpose of using tow BUS lines (CAN-H and CAN-L) is to prevent any

parasitic voltage pulses that might interfere with the information sending and

receiving;

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• The wires are intertwined in order to create an equal and constant distance

between them and any electromagnetic fields in the vicinity;

• The B-CAN (Body-CAN) line connects the ECUs of the car body systems

(MICU, Immobiliser, A/C, Headlights Adjuster), through a copper wire, with a

data transfer rate of 33,33 kbps;

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• The F-CAN B-CAN Gateway is integrated in the indicator control module

(tachometer) and allows for the dynamic systems (ABS, SRS, EPS, etc.) and

the body systems (MICU, Immobiliser, A/C, etc.) to communicate between them;

• In practical terms, this Gateway is a communication portal between the F-CAN

and the B-CAN, where there is a “conversion” of the information circulating at

the B-CAN line, so that they can be received by those ECUs connected to the F-

CAN line and vice versa;

Note: the CAN-BUS network is only for the communication between the ECUs and

their peripherals, that is, through the CAN lines, there's only information and

commands circulating. This means that the vehicle power circuit and ground

points are still necessary. 11


• The ECU are connected to the CAN network via the integrated

Encoders/Decoders;

• The information gathered by the sensors circulate in the CAN network and is

recognized as valid for the operation of each ECU, by means of its digital

encoding. The information is only received by the ECU to which it is intendedencoding. The information is only received by the ECU to which it is intended

after this recognition;

1 ECU;

2 Microprocessor;

3 Encoder/Decoder;

4 CAN-BUS network

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• As we know, every information circulating in the CAN-BUS network has a digital

encoding.

Part of this encoding comprises a field dedicated to the priority level of the

information, so, if two or more ECUs attempt to transmit information at the same

time through the CAN-BUS network, the ECU whose information has the lowest

level of priority does not send the information at that moment;

• The CAN-BUS network is a system that consumes electrical power, and so it also

includes Wake-Up and Sleep functions to avoid draining the battery too fast;

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• In “Sleep” mode, the MICU stops working, that is, the CAN network

communication stops and the power consumption drops from 200 mA to 30 mA,

but there is still power consumption!!!;

• The “Sleep” mode will not function if there is one door open or if the key is in the

ignition switch;

• When the ignition is turned off and the driver's door is opened, and then closed

using the power door lock, there will be a delay of about 40 seconds before the

network goes from “Wake-Up” mode to “Sleep” mode;

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• If we turn ignition off and open the driver's door, there will be a time delay of

about 10 minutes before the network goes from the “Wake-Up” mode into the

“Sleep” mode;

• The difference between the time periods needed to change from “Wake-Up”

mode into “Sleep” mode, when the power door lock is, or not, activated, is

explained because the "door closed" signal "tells" the MICU that the vehicle

systems no longer need to remain in operation;

• When the "closed door" signal is not transmitted, the MICU does not "know" if the

systems still need to be "live" and that's the reason why it "waits" longer

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• The “Wake-Up” mode is activated the moment any action is requested to the

vehicle. Opening a door is enough to take the CAN network out of “Sleep” mode

and immediately resume its operation;

• F-CAN line is:

=> activated ("Wake-Up" mode) by turning the ignition switch ON;=> activated ("Wake-Up" mode) by turning the ignition switch ON;

=> deactivated (“Sleep” mode) by turning the ignition switch OFF;

• B-CAN line is:

=> activated ("Wake-Up" mode) by turning the ignition switch ON oropening a door;

=> deactivated (“Sleep” mode) by turning the ignition switch OFF, closingthe vehicle doors and waiting 40 seconds;

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5. CAN-BUS Network Advantages

• All ECU have access to all information circulating through the CAN network;

• Possibility of more complex functions, without the need to further increase the

wiring harness number;

• Software updates for the vehicles' ECUs without need to replace any ECUs;

• Better self-diagnosis capacity;

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6. CAN-BUS Network Diagnosis

• The CAN network diagnosis is complex and lengthy because all units operate

within the network, communicating to each other;

• The K line is used by the HDS to communicate with the ECU via the DLC

connector;

• HDS allows reading the ECU in terms of communication loss, Diagnostic

Troubleshoot Codes (DTCs) and input and output signals (for some ECUs);

• The DLC (or OBD) connector is a 16 pin connector to connect the HDS

diagnostic equipment;

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• The function of each of the 16 pins at the DLC connector is defined according to

one of the several international norms ruling the car building activity.

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1 Ignition +;

2 BUS + line;

3 Defined by the manufacturer;

4 Body ground;

5 Signal ground;

6 CAN-H;

7 K line;



10 BUS – line ;




14 CAN-L;

15 L line or 2nd K line;

16 Battery +;


• The DTC codes memorized at the car also obey to international norms

regulating its structure;

DTC: X Y Z K W (ex: P1101)

Type of failure (what is not functioning correctly in the affect sub-system)

affected sub-system:

1 Fuel and Air Metering (MAF); 5 Vehicle Speed Control and Idle

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1 Fuel and Air Metering (MAF);

2 Fuel and Air Metering (Injection Circuit);

3 Ignition Systems and Misfire;

4 Emissions Control (Catalyst);

5 Vehicle Speed Control and Idle

Control System;

6 Microprocessor;

7 Transmission;

8 Transmission;

DTC Type (0 According to the SAE norm; 1 Defined by the manufacturer);

Affected System:

P Engine;

B Body (Interior);

C Body (Exterior);

U Not Assigned;


Even after the DTCs have been deleted from the different units where they were

memorized, the DTCs reoccurred;

Before starting any complex diagnostics, you should start by checking and

confirming that all fuses, relays, ground points relating to the ECUs with DTCs and

their wiring harnesses connectors are all in good working order;their wiring harnesses connectors are all in good working order;

Check whether all fuses and relays are OK, all ground points are tight and

making good electrical contact and all wire harness connectors are correctly

connected and installed and their terminals are free from moisture and/or

corrosion;

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Moisture at the electrical contacts and subsequent corrosion increases the wiring

electrical resistance and this causes some of the problems normally attributed to

damages ECUs;


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Fuse Checking:

Use a multimeter (set it to resistance measurement/continuity check and touch the

probes to the visible fuse terminals) or a test lamp (connect it to a ground point of

the vehicle and check for continuity between ground and each terminal of each

fuse).


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Relay Checking:

Use a multimeter (set it to resistance measurement/continuity check and touch the

probes to the visible relay terminals). It must be said that the relays fitted to our

cars are basically from two different types:


Connect the power (BAT+) to terminal 3 and ground

to 4 and check for continuity between the terminals 1and 2;

With the power disconnected, there should be no

continuity between terminals 1 and 2;

Connect the power (BAT+) to terminal 3 and ground

to 5 and check for continuity between the terminals 1and 2;

With the power disconnected, there should be

continuity between terminals 1 and 4; 24

7. Communication Failure in the CAN-BUS Network;

• The communication failure in the CAN-BUS network can have several

causes:

CAN-BUS lines (F-CAN e B-CAN) failure;

ECU (Electronic Control Unit) failure;

Tension or Ground Failure in an ECU;

Interference in the CAN-BUS Lines;

Failure in the CAN-BUS Internal Resistance;

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7.1. Failure in the CAN-BUS Lines;

• The CAN-BUS communication lines can display the following failures:

* CAN-H/CAN-L line damaged;

* CAN-H/CAN-L line shorted to the battery power;

* CAN-H/CAN-L line shorted to ground;

* CAN-H line shorted to CAN-L line;

* Poor contact at the connections (damage, corrosion or poor fitting).

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7.2. Failure in an ECU;

• Each ECU connected to the CAN-BUS network integrates a communication

module which allows that ECU to send and receive data within the CAN-BUS

network.

Normally, a failure in one ECU originates DTCs in other units communicating

with that ECU via the CAN-BUS network.

• In order to isolate the failing ECU, the ECU can be disconnected, one at a time,

while the CAN network status is monitored with a multimeter or an oscilloscope.

This procedure can be complemented by erasing the DTCs from the other units

still connected and re-reading those units for DTCs.

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7.2. Failure in an ECU;

• If the ECU disconnected from the CAN-BUS network is the one with the failure,

then the DTCs found after a new health check will only be "communication

failure with that ECU" DTCs.

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7.3. Tension or Ground Failure in an ECU;• The CAN-BUS network tension is split in 5 V to the F-CAN and 5 V to the B-

CAN.

• Because the F-CAN is further split into two lines (CAN-H and CAN-L), then we

have the 5 V split as 2,7 V to the CAN-H (measured between the CAN-H and

ground – IGN ON) and 2,3 V to the CAN-L (measured between the CAN-L and

ground – IGN ON).

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CAN-HGround

CAN-L BAT+

K-Diag

• U=2,7 V CAN-H tension OK

CAN-HGround

CAN-L BAT+

K-Diag

• U=2,3 V CAN-L tension OK

7.3. Tension or Ground Failure in an ECU;

• Because the CAN-H and CAN-L lines read this tension figures does not mean

that the CAN line being tested if free from failures. It only means that there is

tension available for data communication.

• A battery voltage slowly decreasing or a dead battery can cause occasional

communication failures in several ECUs connected to the CAN-BUS network.

So, the battery must ALWAYS be checked for full charge and the charging

system (ACG or IMA) must be free from problems, before we do any diagnosis

to the CAN-BUS network.

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7.3. Tension or Ground Failure in an ECU;

• Because not all ECUs stop communicating at the same tension level, if the

battery voltage drops below a point when one or several ECU are deprived from

the power they need to operate, then there may be DTCs memorized.

• This can happen, for example, when the starter is cranking the engine. With a

fully charged good condition battery, at the cranking time, there is a voltage drop

of about 4 V.

• The poorer the battery condition, the bigger the voltage drop will be, thus the

higher the possibility to occur an occasional failure within the CAN-BUS

network.

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7.4. Interference in the CAN-BUS Lines;

• A faulty alternator or electronic devices such as cellular phones, audio amplifiers

or portable DVD players, might induce voltage pulses into the CAN-BUS

network and, occasionally, deprive the network from the necessary voltage, up

to a point where all communications might be temporarily interrupted.

Many times, this type of interruption is intermittent and only some ECUs might

memorize DTCs, and such DTCs are, normally, hard to reproduce.

• First, we should make sure that there's no problem with the CAN-BUS network

wiring harnesses and check the ACG (alternator or IMA system) for proper

operation. Disconnect all extra electronic equipment installed after the vehicle

leaves the factory and check for further DTCs.

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7.5. Failure in the CAN-BUS Internal Resistance;

• There is a resistor of about 120 Ω fitted to two ECUs, between the CAN-H and

CAN-L lines (F-CAN), in order to absorb electromagnetic "echoes" that might

cause communication failures between the ECUs;

• The F-CAN line has its ECUs parallel-connected, so the real resistance value

shall be about 60 Ω (IGN OFF);

CAN-HGround

CAN-L BAT+

K-Diag

• R=60 Ω F-CAN circuit OK;

• 60 Ω<R<120 Ω Poor contact at

the F-CAN circuit connectors;

• R=120 Ω F-CAN circuit wiring

harness damaged;

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7.5. Failure in the CAN-BUS Internal Resistance;

• The F-CAN circuit is quite stable and will continue to allow the communication

between the ECUs, even if the resistance is not within the normal specifications.

Nevertheless, there might be temporary DTCs memorized related to

communication failures;

• Any ECU whose internal resistance is not 120 Ω can be checked by• Any ECU whose internal resistance is not 120 Ω can be checked by

disconnecting it from the harness and measuring the resistance between the

CAN-H and CAN-L terminals; this measurement shall fall within 2,4 kΩ and 2,6

kΩ;

• In the next images, we can see the measurement results for some models'

ECUs:

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Date post:	02-Dec-2014
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