CAN-BUS
Transcript of 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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4. CAN-BUS Operating Principle
• 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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4. CAN-BUS Operating Principle
• 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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4. CAN-BUS Operating Principle
• 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
4. CAN-BUS Operating Principle
• 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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4. CAN-BUS Operating Principle
• 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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4. CAN-BUS Operating Principle
• 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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4. CAN-BUS Operating Principle
• 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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4. CAN-BUS Operating Principle
• 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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6. CAN-BUS Network Diagnosis
• 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;
8 Defined by the manufacturer;
9 Defined by the manufacturer;
10 BUS – line ;
11 Defined by the manufacturer;
12 Defined by the manufacturer;
13 Defined by the manufacturer;
14 CAN-L;
15 L line or 2nd K line;
16 Battery +;
6. CAN-BUS Network Diagnosis
• 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;
6.1. Diagnosis Procedures
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;
6.1. Diagnosis Procedures
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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).
6.1. Diagnosis Procedures
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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:
6.1. Diagnosis Procedures
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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